[BS 1041-5-1989] -- Temperature measurement. Guide to selection and use of radiation pyrometers.pdf

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BRITISH STANDARD BS 1041-5:1989

Temperaturemeasurement

Part 5: Guide to selection and use ofradiation pyrometers

UDC 536.5 + 536.52


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BS 1041-5:1989

This British Standard, havingbeen prepared under thedirection of the IndustrialProcess Measurement andControl Standards Policy

Committee, was publishedunder the authority of theBoard of BSI and comesinto effect on30 March 1990

BSI 02-2000

First published January 1943

Second edition March 1972

Third edition March 1990

The following BSI referencesrelate to the work on thisstandard:

Committee reference PCL/1

Draft for comment 87/21033 DC

ISBN 0 580 17777 7

Committees responsible for thisBritish Standard

The preparation of this British Standard was entrusted by the Industrial

Process Measurement and Control Standards Policy Committee (PCL/-) toTechnical Committee PCL/1, upon which the following bodies wererepresented:

British Coal Corporation

British Gas plc

British Pressure Gauge Manufacturers Association

Department of Energy (Gas and Oil Measurement Branch)

Department of Trade and Industry (National Weights and MeasuresLaboratory)

Energy Industries Council

Engineering Equipment and Materials Users AssociationGAMBICA (BEAMA Ltd.)

Health and Safety Executive

Institution of Gas Engineers

Coopted members

Amendments issued since publication

Amd. No. Date of issue Comments


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Contents

Page

Committees responsible Inside front cover

Foreword iii0 Introduction 1

1 Scope 1

2 Definitions 1

3 Principles of radiation pyrometry 2

3.1 Introduction 2

3.2 The laws of radiation 2

3.3 Emissivity 3

3.4 The emissivity of materials 3

3.5 Radiation exchange and emissivity errors 4

3.6 Colour temperature and ratio pyrometers 4

3.7 Effective wavelength 54 Advantages and limitations of radiation pyrometers 5

4.1 Introduction 5

4.2 Advantages 6

4.3 Limitations 6

5 Classification of radiation pyrometers 6

5.1 Introduction 6

5.2 Classification by wavelength selection principle 7

5.3 Classification by method of operation: direct-reading andcomparison pyrometers 7

6 Principal types of radiation pyrometer 8

6.1 Total and broadband pyrometers 8

6.2 Narrow band, or spectral, pyrometers 8

6.2.1 Photoelectric pyrometers 8

6.2.2 Visual pyrometers 9

6.2.3 Ratio pyrometers 10

6.3 Emissivity and background compensation 11

6.3.1 Emissivity compensated pyrometers for specularlyreflecting surfaces 11

6.3.2 Emissivity compensation using a hemispherical reflector 11

6.3.3 Two-sensor pyrometry 11

6.3.4 The use of cavities 11

7 Detectors 12

7.1 General 12

7.2 Detector characteristics 12

7.2.1 Properties 12

7.2.2 Sensitivity and noise 12

7.2.3 Stability 13

7.2.4 Response time 13

7.3 Types of detector 13

7.3.1 The eye as a detector 13

7.3.2 Thermal detectors 13

7.3.2.1 General 13

7.3.2.2 Pyroelectric detectors 14

7.3.3 Photon detectors 14

7.3.3.1 Photoemissive detectors 14

7.3.3.2 Semiconductor detectors 15


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Page

8 Optical systems 15

8.1 Introduction 158.2 Aperture optics 16

8.3 Lens optics 16

8.4 Mirror optics 16

8.5 Fibre optics 16

8.6 Sighting 17

9 Signal treatment 17

10 Calibration 18

10.1 Introduction 18

10.2 The ITS-90 18

10.3 Blackbody sources 18

10.4 Standard lamps 1910.5 Transfer standard pyrometers 19

10.6 Uncertainties and procedures 20

11 Precautions necessary in the use of radiation pyrometers 21

11.1 Factors affecting indicated temperature 21

11.2 Process monitoring 21

11.3 Installation 21

11.4 Absorption by the intervening medium 23

11.5 Emissivity 23

Figure 1 Spectral radiance of a blackbody as a function ofwavelength and temperature 27

Figure 2 Schematic arrangement of a pyrometer D, and aradiating surface R in surroundings S 28

Figure 3 Optical system for disappearing filament optical pyrometer 28

Figure 4 The visibility function,A, and the transmission of atypical red glass,B 28

Figure 5 Relative spectral sensitivities of some photon detectors 29

Figure 6 Diagram of an aperture optical system 30

Figure 7 Diagram of a lens optical system 30

Figure 8 Diagram of a single mirror optical system 31

Figure 9 Diagram of a Cassegrain optical system 31

Figure 10 Fibre optic pyrometer sighted on an inaccessible target 32

Figure 11 Fibre optic pyrometer in a hostile environment 32Figure 12 Corrections for wavelength changes in lampcalibration at 650 nm 33

Table 1 Transmission of lens and window materials 24

Table 2 Typical emissivity values (for guidance only) 25

Table 3 Variations of radiance temperature with pyrometereffective wavelength and with emissivity 26

Table 4 Errors in temperature arising from an error of 10 % inemissivity at three effective wavelengths 26

Publications referred to Inside back cover


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Foreword

This Part of BS 1041 has been prepared under the direction of the IndustrialProcess Measurement and Control Standards Policy Committee. It is a revisionof BS 1041-5:1972 which is withdrawn. It should be noted that the title has been

restyled for consistency with other Parts of BS 1041.

Compared with earlier editions of BS 1041-5, less prominence is given to visualpyrometers as these are not so widely used as hitherto. Measuring circuits areexplained in less detail as this guide is intended to assist the user in the selectionof the appropriate measurement system rather than the designer of instruments.

A British Standard does not purport to include all the necessary provisions of acontract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunityfrom legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i to iv,

pages 1 to 34, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have hadamendments incorporated. This will be indicated in the amendment table on theinside front cover.


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0 Introduction

Radiation pyrometers measure temperature by

radiation methods. The characteristics of theavailable types of pyrometer, the detectors andoptical systems employed, signal treatment,calibration, the advantages and limitations ofradiation pyrometers, and some precautionsnecessary in their application, are explained in thisguide. Conditions of temperature measurementvary so widely that it is not possible to provide strictguidance but it is hoped that a user will findsufficient information in the guide to assess theavailable options and ultimately, in discussion witha supplier, to choose the most appropriatemeasurement system.

1 Scope

This Part of BS 1041 gives guidance on the selectionof radiation pyrometers for use in scientific andindustrial environments.

NOTE The titles of the publications referred to in this guide arelisted on the inside back cover.

2 Definitions

NOTE Only basic definitions are given. For example the termsemissivity, reflectance and absorptance are defined, but thespecial cases of the hemispherical, directional, diffuse, specularand total have been left to be qualified as the occasion demands.

For these terms the definitions are not complete unless thegeometric and spectral conditions are included. Generalmetrological definitions are given in BS 5233.

For the purposes of this Part of BS 1041, thefollowing definitions apply.

2.1thermal radiation

electromagnetic radiation emitted by an object byvirtue of the thermal motion of the atoms andmolecules of which it is composed

2.2radiation pyrometer

generic name for an instrument for measuring thetemperature of an object by means of the thermalradiation emitted by the object

NOTE An alternative synonymous term used is radiationthermometer.

2.3full radiator or blackbody

thermal radiator which absorbs completely allincident radiation, whatever the wavelength, thedirection of incidence or the polarization, and whichemits, for any wavelength, the maximum spectralconcentration of radiation for a thermal radiator in

thermal equilibrium at a given temperature. Itsemissivity is unity

NOTE Other synonymous terms used are black body andPlankian radiator.

2.4grey body

thermal radiator whose spectral emissivity is lessthan unity but is independent of wavelength overthe range considered

NOTE An alternative synonymous term used is non-selectiveradiator.

2.5radiant flux,

power emitted, transferred or received in the form ofradiation

2.6radiant exitance (at a point of a surface,M)

quotient of the radiant flux leaving an element ofthe surface containing the point, by the area of theelement

NOTE The expression thermal radiant exitance indicatesthat the flux considered is produced by thermal radiation.

2.7full radiator temperature

temperature of a full radiator at which it has thesame thermal radiant exitance as the radiatorconsidered

NOTE An alternative synonymous term used is full radiationtemperature.

2.8

radiant intensity (of a source, or an element ofa source, in a given direction), I

quotient of the radiant flux leaving the source, orthe element of the source, propagated in an elementof solid angle containing the given direction, by theelement of solid angle

2.9radiance, L

the quotient of the radiant intensity in the givendirection of an infinitesimal element of the surfacecontaining the point under consideration, by thearea of the orthogonal projection of this element on

a plane perpendicular to the given direction

2.10spectral radiance (L2)

the quotient of the radiance L, taken over aninfinitesimal range of wavelength, 2, by the range;i.e.

L2= dL/d2

NOTE An alternative synonymous term used is spectralconcentration of radiance.

2.11radiance temperature (of a thermal radiator

for a wavelength)the temperature of a full radiator for which theradiance at the specified wavelength has the samespectral concentration as for the radiator considered


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NOTE Use of the term brightness temperature isdiscouraged.

2.12emissivity,

ratio of the thermally emitted component of theradiant exitance of the radiator to that of a fullradiator at the same temperature

2.13reflectance,

ratio of the reflected radiant or luminous flux to theincident flux

2.14transmittance,

the ratio of the transmitted radiant or luminous fluxto the incident flux

2.15absorptance,

the ratio of the absorbed radiant or luminous flux tothe incident flux

2.16filter

a device which is used to modify by transmission theradiant or luminous flux, the spectral distribution,or both, of the radiation passing through it

NOTE A distinction is made between selective filters and

non-selective (neutral) filters according to whether they do ordo not alter the relative spectral distribution of the radiation.

2.17comparison pyrometer

pyrometer establishing the equality of the radiationfrom the source whose temperature is required withthat from a comparison source at a knowntemperature

2.18visual pyrometer

comparison pyrometer in which the detector is thehuman eye

2.19spectral luminous efficiency, or visibilityfunction (V2)

relative spectral sensitivity of the standard eye(see Table 1, BS 4727-4:Group 01 and Figure 4 ofthis guide)

2.20objective pyrometer

radiation pyrometer which employs a detector otherthan the human eye

2.21total radiation pyrometer

radiation pyrometer which measures all thethermal radiation received from the source

2.22ratio or two-colour pyrometer

a pyrometer which determines temperature bymeasuring the ratio of the spectral radiances of thesource at two distinct wavelengths

3 Principles of radiation pyrometry

3.1 Introduction

All bodies emit radiant energy as a result of atomic

and molecular agitation. The temperature of a bodyis a measure of the mean kinetic energy of itscomposite particles in their various modes ofmotion, and the radiant energy arising from themotion of the particles and emitted from the surfacecan therefore be used to measure the temperature ofthe body.

In practice radiation pyrometry can be a convenientmeans of determining temperature in bothlaboratory and industrial environments. One of themost important advantages of the method is thatthe pyrometer need not be brought into the hot zoneand so it is free from the effects of heat and chemical

attack that often cause other measuring elements todeteriorate in use. There is no theoretical limit tothe temperature that can be measured. In practice,considerable problems arise above about 4 000 K, asall materials are then gaseous and opticallytransparent at most wavelengths.

Radiation pyrometers are more expensive thanthermocouples, so tend to be used only incircumstances which can justify the extra cost; forexample, in furnaces in which frequent replacementof thermocouples would be necessary, where thetemperature of a moving surface is required, orwhere high accuracy has to be attained. It should be

noted that, in general, a correction has to be madefor the emissivity of the hot surface if the truetemperature is needed.

3.2 The laws of radiation

The radiation within an isothermal cavity exists ina state of dynamic equilibrium with the atomicparticles that form the cavity wall. The level ofradiation at which this equilibrium occurs dependsonly on the temperature of the cavity and not at allon its composition, nor on its shape. The radiationwithin a uniform temperature enclosure is calledfull or a blackbody radiation.


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Planck showed that the radiation emitted per unitsolid angle from unit area in a uniform temperature

enclosure in a direction normal to that area, in therange of wavelengths between 2and 2+ d2, is L2d2where:

L2is called the spectral radiance of the blackbody.The two constants c1and c2, the first and secondradiation constants, may be written as 2;hc2andhc/k, respectively, where his Plancks constant, cisthe velocity of light, and kis Boltzmanns constant.The value of cis 3.7418 1016Wm2The value of c2is specified in the International Temperature Scaleof 1990 to be 0.014388 metre kelvin.

The Wien formula, in which the 1 in thedenominator of equation (1) is omitted, ismathematically more convenient to use:

If 2T< 3 103metre kelvin the Wien formulaapplies with less than 1 % deviation from Planckslaw and can therefore be used for most purposes.

The spectral distribution of thermal radiation isshown in Figure 1, in which the spectral radianceand the wavelength are both plotted on logarithmicscales. The curves are all of closely similar shape,reaching maxima such that 2max.T= 2 8984mK.This is Wiens displacement law, and it is indicatedby the dotted line in Figure 1.

The total radiance is obtained by integrating L2d2over all wavelengths, which leads to Stefans law:

where

is the Stefan-Boltzmann constant.

3.3 Emissivity

In all opaque bodies the space within and between

the atoms is occupied by full radiation. However, asthe radiation passes out of the surface some of it isreflected back into the interior, so that all surfacesemit less than a full radiator would at the sametemperature. The fraction of full radiation that isactually emitted is called the emissivity of thematerial. Since the reflectance of a surface is thesame for radiation approaching the surface fromeither side, the emissivity 2at a wavelength 2is related to the reflectance2by the simplerelation:

If the absorptance is 2, then also:

Consequently 2and 2are equal, which isKirchhoffs law.

The emissivity of a surface depends partly on thenature of the material of which the surface iscomposed and partly on the smoothness of thesurface. If there are no indentations in the surface ofa size comparable with or greater than thewavelength of the radiation considered, theemissivity would be entirely related to the materialof the surface. A surface that is not smooth and flatwill emit more radiation than one that is, becausethe emitted radiation will be augmented by

radiation reflected from other parts of the surface.

A general definition of emissivity is given in 2.12.The emissivity at a particular wavelength is calledspectral emissivity and when all wavelengths aretaken into consideration the term total emissivityis used.

The emissivity of a surface varies with the angle ofemission but the variation is small for angles notexceeding 45to the normal to the surface and it isgenerally adequate to use the normal emissivitywithout further consideration. The values ofemissivity quoted in the context of pyrometry are

usually values of normal emissivity. Ifmeasurements are made at glancing angles itshould be remembered that emissivity may differfrom normal emissivity, and that the radiation willprobably be strongly polarized. The emissivityaveraged over all directions is called thehemispherical emissivity. It is this value that hasto be used in heat loss calculations.

3.4 The emissivity of materials

The emissivity is a function of the optical propertiesof the substance and for a non-conductor it is afunction of the refractive index n:

The refractive indices of most inorganic compoundslie in the range 1.5 to 4.0 and metallic oxidesusually fall within the range 2.0 to 3.0.Consequently, many industrial surfaces haveemissivities in the range 0.96 to 0.64. Non-metallicsubstances have emissivities that generally do notvary greatly with temperature or with wavelength,except that some are transparent to visible andnear-infrared radiation and therefore emit verylittle at these wavelengths.

L2= c125[exp(c2/2T) 1]

1 (1)

L2= c125exp( c2/2T) (2)

L= T4 (3)

2+ 2= 1 (4)

2+ 2= 1 (5)

2= 4 n2/(n2+ 1)2 (6)


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Metals behave quite differently from non-metallicsubstances. The emissivity of a clean metal surface

is always low; many metals have emissivities of 0.35to 0.40 for red light. Metals such as copper, silver,gold and aluminium, that are very good electricalconductors, may have much lower emissivities. Thespectral emissivity of metals becomes lower atlonger wavelengths and nearly all metals haveemissivities less than 0.10 at 104m. The spectralemissivities of metals in the visible and the nearinfrared do not change greatly with temperature.The total emissivity of metals increases withincreasing temperature because more of theradiation is of shorter wavelengths at highertemperatures. Transparent substances such as

silica and alumina (in the near infrared) have totalemissivities that decrease with increasingtemperature, because larger proportions of theradiation lie within the transparent region.

The most difficult surface to measure is a metalsurface covered by a layer of oxide so thin as to bepartially transparent. Such surfaces occur onfreshly heated steel, but more persistently onnon-ferrous metals. The most difficult of all isprobably aluminium.

Values of emissivity for some important surfaces arediscussed further in 11.5.

3.5 Radiation exchange and emissivity errorsTo illustrate the problems introduced by theexchange of radiation between objects, and theemissivities of radiating surfaces, consider apyrometer consisting of a detector D at atemperature Toin an enclosure at the sametemperature. The enclosure defines a field of viewfor the pyrometer, as indicated in Figure 2, which isfilled by a radiating surface R at a temperature T.The total radiant exitance of this surface is the sumof the emitted thermal radiation and that reflectedfraction of the radiation from the surroundings S,which are at a temperature Ts. At a particular

wavelength, the radiant exitance is:

For thermal detectors, the response is determinedfrom the difference between this received radiationand that radiated by the detector, which, with theenclosure, may be taken to act as a blackbody, sothat:

Substituting for 2from equation (4) andrearranging gives:

The temperature of the surface measured by thepyrometer will not be the true surface temperature

unless either its emissivity is unity, i.e. it acts as afull radiator, or the temperature Tsof thesurroundings is the same as that of the surface, inwhich case the whole system acts as a full radiator.In both cases:

and for ToT, the term L2(To) may be neglected. Inall other cases, an error is introduced. If Tsis small,the temperature measured is low, because 2is lessthan unity. As Tsincreases, the value obtainedincreases, tending, in the limit, to Tswhatever thesurface temperature.

It is clear that in general an assessment has to bemade of the surface emissivity for a hot surface incold surroundings. The uncertainty in temperatureintroduced by an uncertainty%2in the emissivity 2of the surface may be estimated using the Wienapproximation as:

The sensitivity of the pyrometer to uncertainties inthe emissivity and in the measured spectralradiance is dependent on the magnitude of thefactor c2/2T(which has to be substantially greater

than unity for the Wien approximation to be valid,see 3.2. It may be increased by working at shorterwavelengths, but in practice this possibility isstrictly limited by the concomitant rapid decrease inthe spectral radiance, see Figure 1). For example, ata wavelength of 650 nm and a temperatureof 1 500 K (1 227C), the factor is 15 and the error%Tproduced by an error of 10 % in the emissivity, orin the pyrometer output, would only amount toabout 0.7 %, or 10 K.

3.6 Colour temperature and ratio pyrometers

The temperature of a full radiator can be measured

by determining the ratio of the radiances at twodifferent wavelengths. A pyrometer operating onthis principle is called a ratio pyrometer or atwo-colour pyrometer, see 6.2.3. A ratio pyrometersighted on a grey body (i.e. a body whose emissivitydoes not vary with wavelength) also measures itstrue temperature without any emissivity correction.It is emphasized that few real surfaces are evenapproximately grey in this sense.

If the radiances of a full radiator at a temperature Tand at wavelengths 21and 22are L1and L2respectively, we can apply Wiens formula andwrite:

2L2(T) + 2L2(Ts) (7)

L2= 2L2(T) + 2L2(Ts) L2(To) (8)

L2= 2L2(T) L2(To) + (1 2) L2(Ts) L2(To) (9)

L2= L2(T) L2(To) (10)

%T/T= (c2/2T)1%2/2 (11)

L1/L2= (22/21)5exp {(c2/T) (1/22 1/21)} (12)


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If the ratio L1/L2is plotted against temperature thecurve will be simply proportional to that of a

monochromatic pyrometer measuring radiation ofwavelength 2, where:

If the surface is not a full radiator nor a grey bodybut has emissivities 1and 2at wavelengths 21and22, then the apparent temperature Tc(which iscalled the colour temperature) is defined by theequations:

and

Hence

This equation may be compared with thecorresponding equation for a radiance pyrometerthat measures at one wavelength 21and indicatesan apparent temperature T1given by:

Since the emissivity of a metal decreases withincreasing wavelength it will be seen that the colourtemperature of a hot metal is normally greater thanits true temperature while its radiance temperatureis always lower than the true value.

3.7 Effective wavelength

In the application of the simple Planck and Wienlaws to the determination of the temperature of a

source, it is assumed that the optical bandwidth isnegligible. For many practical pyrometers,especially in those applications requiring goodaccuracy, this assumption is not valid.

In fact, all pyrometers measure radiation in a finiteband of wavelengths. Instead of analyzing theresponse of a pyrometer by integrating over thisband, it is more convenient to determine a weightedmean value called the effective wavelength, suchthat the response is the same as that of a pyrometerwith the same physical characteristics, but sensitiveonly to radiation at that wavelength.

The response of a pyrometer is determined by thetransmittance E2of the optical system, principally

derived from the optical filter, and the spectralsensitivity s2of the detector. In a visual pyrometerthe visibility function of the eye takes the place ofthe spectral sensitivity of the detector.

For a pyrometer measuring the ratio of intensitiesbetween temperatures T1and T2, the mean effectivewavelength 2mis that for which:

If T2is made to approach T1very closely we find a

limiting value of the effective wavelength 2ewhichapplies to a single temperature T, and which isgiven by:

The effective wavelength is a function oftemperature, decreasing slowly with increasingtemperature. The variation is greater if theresponse band is broad.

The effective wavelength should be used inequations derived with the assumption of narrow

bandwidth. For example, from the Wien law theresponse characteristics of a pyrometer that has aneffective wavelength 2ecan be expressed as:

where

A is a constant.

This formula is of course applicable only topyrometers operating at sufficiently shortwavelengths for Wiens formula to be valid.

4 Advantages and limitations of

radiation pyrometers4.1 Introduction

It has been shown that the temperature of a bodycan in principle be deduced from the amount ofradiation it emits. Operationally radiationpyrometers are clearly very different from contactthermometers, and some of their advantages andlimitations are enumerated in 4.2and 4.3.

(13)

L1/L2= (1/2) (22/21)5exp {(c2/T) (1/22 1/21)} (14)

L1/L2= (22/21)5exp {(c2/Tc) (1/22 1/21)} (15)

(16)

(17)

(18)

(19)

ln L2= A c2/2eT (20)


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4.2 Advantages

The advantages of radiation pyrometers are as

follows.a) Since a non-contact method is employed, nopart of the instrument need be brought into thehot zone. As a result, radiation pyrometers arecapable of measuring temperatures far beyondthe range of all but the hardiest thermocoupleswithout suffering deterioration through heatingor chemical attack.

b) Good sensitivity can be achieved even atmodest temperatures. At long wavelengths(e.g. 84m to 144m) measurements can be madedown to 50C or even lower. The operating

wavelength or waveband of a pyrometer, which isdetermined by the optical components anddetector, can be chosen to suit a particulartemperature range or application (see 6.2.1).

c) The source may be large or small, physicallyinaccessible or otherwise not susceptible tocontact measurement. Since contact is avoidedthe temperature of the object undermeasurement is relatively unaffected by themeasurement process.

d) Moving objects, for example on a productionline, can be viewed.

e) Portable, hand-held instruments are available.

f) Fast response can be achieved, and hence rapidtemperature changes can be monitored.

g) In common with contact thermometers, anelectrical output is available which can belinearized, recorded or used for control purposes.

4.3 Limitations

The limitations of radiation pyrometers are asfollows.

a) The radiation emitted by an object isdependent on the surface emissivity, and this isin general dependent on temperature,

wavelength, angle of view and surface condition.If the emissivity is not known well enough,special techniques may have to be employed.

b) A considerable contribution to the detectedsignal may be due to radiation reflected from thetarget surface. This is usually unfortunate,although some emissivity compensatedpyrometers make use of reflected radiation, andthe Kirchhoff law (see 3.3), in their operation.

c) Radiation pyrometers should be calibratedeither using a source whose radiance in the

relevant wavelength range is known, or bycomparison using a calibrated pyrometer of thesame effective wavelength.

d) The signal may be dependent on the nature ofthe medium between the source and the detector.Water vapour, carbon dioxide, haze, smoke anddust, for example, can cause errors by partiallyabsorbing or reflecting the radiation. Similarlythe transmission of any window or othercomponent between the target and the pyrometerwill have to be taken into account, and the targetshould fill the field of view.

e) First costs are often higher than for an

equivalent contact measurement system, and aswith all temperature measurement, a certainskill and training of operators is needed. Thechoice of the most appropriate pyrometer for aparticular purpose has to be carefully made.

f) Changes in ambient temperature may affectthe measurement, and in industrialenvironments water or air may be needed forcooling the instrument or purging theatmosphere.

Some of the precautions which should be observed inusing radiation pyrometers are discussed in

clause 11.

5 Classification of radiationpyrometers

5.1 Introduction

Because there are many types of radiationpyrometer, each with its own advantages and oftendesigned for a specific purpose, it is useful toattempt a classification before proceeding todetailed descriptions. Classifications based oncontinuously varying parameters such astemperature and wavelength are likely to be of only

transitory value, since they would requiremodification as optical design and detectorperformance improve. The two main classificationswhich are followed here are based on thewavelength selection principle and on method ofoperation. These alone will not be sufficient tospecify the instrument required for a givenapplication, which would of course depend ontemperature and wavelength ranges, accuracy andresolution, signal treatment, whether theinstrument is to be portable or fixed, and any otherspecial requirements.


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5.2 Classification by wavelength selectionprinciple

Classifications by wavelength selection principleare as follows.

a) Total radiation pyrometers. These are sensitiveto the major part of thermally-emitted radiation.Their response may be characterized by Stefanslaw (see 3.2)

b)Broadband radiation pyrometers. This type issensitive to radiation over a broad band ofwavelengths, but the response cannot be relatedto either Stefans or Plancks law (see 3.2).Instead the relationship between the outputsignal and the temperature of the source has to be

established by calibration. The spectral range isusually restricted by the short wavelength cut-offof the Planck function and the long wavelengthcut-off of the detector or window.

c) Narrow-band or spectral radiation pyrometers.The response of this type of pyrometer is limitedto a single narrow band of wavelengths by theinclusion of an optical filter, and hence Planckslaw can be applied. This class includes several ofthe more popular instruments, including thevisual or disappearing filament pyrometer, andthe wide range of photoelectric pyrometers.

d) Two-colour or ratio pyrometers. These

instruments attempt to compensate for errorswhich arise from the emissivities of real surfacesby measuring the ratio of the spectral radiancesin two narrow bands at different wavelengths. Itis, however, necessary to assume that the sourcebehaves as a grey body or to assume a value forthe ratio of the spectral emissivities at the twowavelengths.

e) Multi-wavelength pyrometers. These extendthe principle of the previous class to severalwavelength bands. It is necessary to makefurther critical assumptions about the behaviour

of the source emissivity with wavelength andtemperature. A small computer may be needed toanalyse the signals from each spectral band.

5.3 Classification by method of operation:direct-reading and comparison pyrometers

As the name implies, direct-reading pyrometersrelate to the output signal from the radiationdetector directly to the temperature of the radiatingsource. They consist essentially of an optical system,a detector and an amplification and display system.They are relatively simple and robust.

However, the temperatures measured are onlyaccurate if the detector is stable and its output a

function only of the incident radiation intensity.Many detectors do not satisfy these requirements,and have to be used in the comparison mode, inwhich the unknown source temperature is obtainedby comparison with a stable reference source. Thisis often a small tungsten filament lamp, placedinside the pyrometer itself. The alternative, ablackbody reference source, is usually toocumbersome, has too long a response time andrequires too much power for general use. Blackbodysources are found, external to the pyrometer, inresearch and industrial applications whereaccurately predictable spectral radiances are

essential. One example of a comparison pyrometerwith an internal lamp is the disappearing filamentvisual pyrometer.

There are many advantages in employing a nulldetection system, giving zero output when the testand reference sources match. The balance point isindependent of the characteristics of that part of thepyrometer common to both sources, including thedetector. Also, changes in the gain of the amplifier,and other electronic effects, cancel out. In addition,the test and reference sources are often compared bychopping the radiation from them so that they eachalternately illuminate the detector. The

out-of-balance signal is therefore approximately asquare wave, and a.c. amplification and filteringtechniques, with phase-sensitive detection, may beused to reduce the effects of thermal noise,interference and drift.

Occasionally, the difference between the outputsfrom two detectors, separately illuminated by thetwo sources, is measured and nulled. This techniquedoes not, however, possess the same freedom fromcommon errors as that employing the a.c. nulldetection technique.

Non-linear amplifiers may prevent saturation and

loss of the a.c. signal when the sources are far frombalanced. Nulling may be achieved manually, byminimizing the output from the a.c. detectionsystem. Alternatively, the pyrometer may be madeautomatic by feedback from the phase-sensitivedetector. If this is taken to the reference source, forexample the current supply to the filament lamp,the pyrometer follows the source temperature. If thesignal is taken to the electrical supply to the source,it is possible to control the source to follow a fixed orprogrammed temperature determined by thereference lamp.


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6 Principal types of radiationpyrometer

6.1 Total and broadband pyrometers

The design of total radiation pyrometers is generallybased upon the combination of a mirror opticalsystem with a thermal detector (see clause 7). Thecharacteristic feature of the thermal detector is thepotentially wide spectral range, and a pyrometerusing this type of detector is therefore particularlysuitable for sources at comparatively lowtemperatures, at which most of the emittedradiation is beyond the range of uncooled photondetectors. The advantage of far infra-red response isto some extent offset by the low intrinsic sensitivity

of thermal detectors, and their susceptibility toextraneous thermal effects, draughts and changesin atmospheric pressure. The effect of backgroundradiation may also be significant. If the pyrometer iscompletely non-selective, with no window, theresponse will be proportional to (T4 To

4), where Tois the background temperature.

Broadband pyrometers fall into two main classes:those, like total radiation pyrometers, with thermaldetectors but whose spectral response issignificantly limited by an absorbing window orother optical component, and those with photondetectors in which the waveband is limited by the

response of the detector or an optical component. Inboth cases, if the Toterm can be neglected, anempirical equation of the form:

L=A Tn

can usually be used to relate the output of thepyrometer to the source temperature. It is, however,necessary to establish different values forAand ntocover different temperature ranges, and it is betterin practice to relate the detector output totemperature using a series, preferably including anexponential term.

6.2 Narrow band, or spectral, pyrometers

6.2.1Photoelectric pyrometers. This group of

instruments covers a wide variety of combinationsof different detectors, lens and mirror opticalsystems, and operation in both the direct-readingand comparison modes. In addition, it is usuallypossible to select an operating wavelengthappropriate to the particular application. Forexample, many photoelectric pyrometers operate atabout 655 nm in order to calibrate standard lampsfor use with visual pyrometers, which are normallyrestricted to this wavelength. As may be seenfrom Figure 1, the sensitivity of a spectralpyrometer is greater the shorter the wave-length,because of the factor c2/2T. This factor should be 10

or greater (see 3.5). Thus spectral pyrometers areintrinsically more sensitive than total or broadbandpyrometers, where the equivalent factor may onlybe 4. Wavelengths in the visible range are suitablefor accurate measurements of high temperatures,above about 800C. At lower temperatures, down toambient, it is necessary to utilize infra-redwavelengths in order to obtain a satisfactorysignal-to-noise ratio.

The operating wavelength is normally establishedby the inclusion of an optical filter, which may takethe form of a coloured glass or an interference ormesh filter, placed at a convenient point in theoptical path. It is of course necessary to select adetector with reasonable sensitivity at the chosenwavelength. The spectral response curve of thedetector may play a useful part in reducing thecontribution to the output signal from radiation atwavelengths far from the operating wavelength,which could otherwise give rise to sizeable errors inthe measurement of sources with varyingemissivity.


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If the transmission characteristics of the opticalfilter and the response characteristics of the

detector are known, it is possible to measure theradiance temperature of a source by measuring theratio of the spectral radiance of this source to that ofa second source at a known or reference radiancetemperature, by application of the Planck law. If thesource is a blackbody, or its emissivity at the chosenwavelength is known, then the true temperaturecan be calculated. Photoelectric pyrometers are usedas the standard instruments for measurements ofthe highest accuracy, and the application of thePlanck law is specified as the means of realizing theInternational Temperature Scale of 1990 (ITS-90)for temperatures above the freezing point of

silver (961.78C), using as the reference source ablackbody at the freezing point of silver,gold (1064.18 C) or copper (108.62 C) according tochoice. A reproductibility of 0.01C or better can beachieved at these points. Photoelectric pyrometryhas also been used successfully at lowertemperatures, especially at wavelengths in the nearinfra-red, and good consistency with the ITS-90(here realized using platinum resistancethermometers) can be achieved.

While photomultipliers are often employed in thistype of instrument, they are too cumbersome anddelicate for industrial equipment. For this, solid

state detectors are suitably robust. Siliconphotodiodes, which are sensitive to radiation atvisible wavelengths and up to 14m, may form thebasis of pyrometers with a working range rathersimilar to that of visual pyrometers. Forapplications at lower temperatures, pyrometerscontaining the various infra-red detectors areavailable. In some cases, the operating wavelengthmay be selected to coincide with a region of highemissivity from the particular surface whosetemperature is being measured. Forexample, 3.434m corresponds to the C-H stretchingfrequency in organic materials, so that this

wavelength is often chosen to measure the surfacetemperature of plastic objects or films.

Similarly, when measuring the temperature ofglass, it is possible to measure either the bulk or thesurface temperature by choosing the appropriateinfra-red wavelength.

6.2.2 Visual pyrometers. Visual pyrometers of thedisappearing filament type are widely used in

industry and in laboratories. An image of thefilament of an internal reference lamp issuperimposed upon that of the source. The currentthrough the filament is varied until a luminancematch is obtained, and the section of the filamentimage against the source disappears. The humaneye, used here as a detector, suffers from a numberof non-linear and time-dependent defects, theresults of which are minimized by this directcomparison method.

A typical optical system of a high precisiondisappearing filament optical pyrometer is shownin Figure 3.

The objective lens O forms an image of the source inthe plane of F, the filament of a small lamp. Thefield consisting of the filament F superimposed onthe image of the source is viewed through aRamsden or Huygens eyepiece and the erectinglens G. A Ramsden eyepiece is shown in thediagram. An aperture D1, which is fixed in positionrelative to the filament, is placed between theobjective lens and the filament to define theentrance cone of radiation and is as near to theobjective as possible. The angle !subtended by thisaperture at the filament is known as the entranceangle of the pyrometer. A second aperture D2isplaced between the erecting lens and the eyepiece.The optimum position is coincident with the imageof D1in the lens G: deviations from this position willresult in the brightness of the field falling offtowards the periphery when the aperture is movedaway from the ideal position. The angle "subtendedby the effective diameter of lens G at F is the exitangle of the telescope. The aperture D3is the fieldstop and is placed at the focal point of the eyepiece.The aperture D4is the pupil of the observers eye.

A neutral filter N, which reduces the luminance ofthe image of the source, enables the instrument to

be calibrated in higher temperature ranges. It maybe placed at any convenient position between thefilament and the source. The so-calledmonochromatic screen M is usually a red glass filterand is placed in any convenient position between thefilament and the observers eye.


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The pyrometer lamp has an important influence onthe design and performance of a disappearing

filament optical pyrometer. Round wire tungstenfilaments about 0.04 mm diameter are commonlyused in pyrometer lamps, but high gradeinstruments are fitted with lamps having flat ribbonfilaments about 0.04 mm wide, which have theadvantages of greater uniformity of temperaturealong the length of the filament, with smallerambient temperature effects at low luminosities,small thermal inertia and fast response, low currentconsumption, and larger limiting angles in theoptical system with a consequent gain in fieldbrightness and resolving power. In the more preciseforms of pyrometer the lamp has flat sighting

windows of good quality glass. The part of thefilament at which matching should be effected isindicated by a pointer built into the lamp, or by thebend of a hairpin-shaped or M-shaped filament. Thelamps are usually of the vacuum type. They shouldbe aged at a radiance temperature of about 1 700 Cfor 15 h in order to stabilize them.

The effective wavelength of a disappearing filamentpyrometer is determined by equation 18 with thevisibility function, V2, in place of s2(the spectralsensitivity of the detector) and using transmissionof the red glass filter for E2, see Figure 4. Theinstrument is usually designed so that the effective

wavelength is 650 10 nm and this value isgenerally assumed for the purpose of calibration. If,however, the effective wavelength differs by %2from 650 nm, then the error in calculating thecorrections is given by:

%T TR2ln %2/c2

Thus if %2= 10 nm and = 0.4, %T= 2 C at aradiance temperature of 1 600C (1 873 K). In themost accurate applications it will be necessary totake account of the change in effective wavelengthwith temperature.

Disappearing filament pyrometers may be used

from 700 C to 3 500 C or higher, in two or threeranges. It is sometimes suggested that the rangemay extend downwards by removing the filter.However temperatures measured in this way maybe seriously in error, as they depend more criticallyupon the eye response of the operator and thespectral emissivity of the surface.

6.2.3 Ratio pyrometers. If the spectral emissivity ofan object is constant within a particular wavelength

range, i.e. it acts as a grey body within this range, itis possible to eliminate the error from this source bymeasuring the ratio of the spectral radiances at twoseparate wavelengths, thereby cancelling the effectsof the unknown emissivity. This principle forms thebasis of the ratio or two-colour pyrometer, as wasdiscussed in 3.6. In practice the ratio may beobtained by placing two detectors in the beam, witha different optical filter in front of each, or byswitching the filters alternately in front of a singledetector. Both arrangements are similar to thoseemployed in comparison pyrometers and, in thelatter case, the advantages of a.c. amplification and

detection can be realized. It is also possible to feedback the difference between the two signals in orderto adjust a variable compensating filter in theoptical path. The position of the filter such that thedifference is reduced to zero is a measure of thetemperature of the source.

The separation of the wavelengths of the twochannels of a ratio pyrometer is not critical,although if they are too close the difference betweenthe two signals will approach the detector noiselevel, and if they are too widely separated, thepyrometer becomes sensitive to the differences inthe characteristics of the optical components in

different wavelength regions.

The effectiveness of a ratio pyrometer dependscritically upon the validity of the basic assumptionregarding emissivity. Whereas emissivity errors inmost pyrometers are related to the uncertainty inthe value of the emissivity itself [see equation (11)]in this case they are related to the uncertainty ind&/d2. Moreover, since in two-colour pyrometry oneis measuring the ratio of two signals, both of whichare positive functions of temperature, the intrinsicsensitivity of the method is necessarily less thanthat of a monochromatic pyrometer in the samewavelength range.

That is to say, a given uncertainty in the radiancemeasurement (from whatever cause) will lead to alarger corresponding uncertainty in temperature.However, the method can be of value for aluminiumsurfaces, since the emissivity does not vary greatlywith wavelength and measurements with amonochromatic pyrometer may be complicated byemissivity variations as the surface roughens andoxidises. In the case of many steels d/d2is large,and a monochromatic pyrometer is likely to givebetter accuracy.


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Apart from the possibility of reducing errors due tothe emissivity of the surface, a ratio measurement

may also be useful when the size of source is likelyto vary, or if the sight path may be partiallyobscured, for example by steam, smoke or dust. Theeffect on the measured temperature is largelyremoved if the variation in the radiance isindependent of the wavelength. However, for steam,smoke or dust this is unlikely to be the case if theparticle size is of the same order as the operatingwavelengths of the pyrometer. Similarly, ifemissivity variations occur the reading will beunaffected only if the emissivity ratio at the twowavelengths remains the same.

6.3 Emissivity and background compensation

6.3.1 Emissivity compensated pyrometers forspecularly reflecting surfaces. In these instrumentsthe output of the detector when viewing an internalsource is compared with that when viewing thetarget irradiated by the internal source. Thetemperature of the internal source is varied untilthe two outputs are equal. The temperature isinferred from the power supplied to the internalsource, and since the sum of the spectral emissivityand spectral reflectance of the target surface mustbe unity [equation (4)] the result is independent ofthe emissivity. In practice a chopper is used to cause

radiation from the internal source to be detectedalternately with the combined radiation direct fromthe target and from the internal source after onereflection. The instrument is then used as a nullsystem with a.c. detection, and the performance islargely determined by the stability of the internalsource. The method can only be applied to surfaceswhose diffuse reflectance is low.

6.3.2 Emissivity compensation using ahemispherical reflector. It is possible to makesurface temperature measurements on bothspecularly and diffusely reflecting surfaces that arepractically unaffected by the emissivity of the

surface, provided this exceeds about 0.6, by using apyrometer with a hemispherical reflector. Thepyrometer head has a hemispherical concave mirrorattached to it, the radiation passing through a smallwindow at the apex of the hemisphere to thedetector. The hemispherical mirror, which is goldplated to make it a good reflector of infra-redradiation, is put close to the hot surface. The surfaceis then surrounded by its own reflection in thehemispherical mirror, forming an approximation toa blackbody enclosure.

Full radiation at the temperature of the surfacepasses through the window to the detector. As the

reflected radiation heats the surface under thereflector, the reading must be taken quickly. Themethod can be used for surface temperatures upto 1 300 C, but unlike other techniques it requiresgood access to the surface.

6.3.3 Two-sensor pyrometry. With modernimprovements in microprocessor analysis it hasbecome feasible to adopt a two-sensor system inwhich the principal measurement is made with theradiation pyrometer sighted on the target in theusual way, and a subsidiary measurement is madeof the background temperature using either thepyrometer itself, or a second radiation pyrometer or

a thermocouple.The background reading is used to make acorrection to the principal measurement. Asequation 9 shows, one still needs to know theemissivity, unless T= Ts, and the method istherefore background corrected rather thanemissivity compensated. Its usefulness partlydepends on how well the background temperature ischaracterized by the auxiliary measurement, but ithas been applied successfully under a variety ofindustrial conditions.

6.3.4 The use of cavities. The calibration table

supplied with a radiation pyrometer by themanufacturer, unless otherwise stated, will be formeasurements made when the pyrometer is sightedon to a blackbody. For a surface whose emissivity isless than unity, but known and constant over therequired temperature range, the true temperaturemay be calculated, or indicated directly by theappropriate setting of the emissivity control.

If these conditions cannot be achieved, errors due toreflection and emissivity can be largely eliminatedby sighting the pyrometer into a cavity in the objectwhose temperature is required. Provided that thecavity is deep enough, blackbody conditions will

obtain within it and the radiation pyrometer willthen indicate the true temperature. Therequirement for the ratio of the depth of such cavityto its diameter is dependent on the accuracy soughtand the emissivity of the surface. The depth ofcavity in this context is obviously the length,measured from the closed end, over which thetemperature is uniform.


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The emissivity of a closed-ended cylindrical tubehaving a surface emissivity of 0.4 and ratio of length

to diameter 5, is 0.9931)

. Equation 11 shows thatthe temperature measured by a pyrometer havingan effective wavelength up to 14m sighted into sucha cavity would be in error by less than 0.1 % of thetemperature up to 1 500C because of thisdeparture from blackbody conditions. A totalradiation pyrometer in the same circumstanceswould have an error less than 0.3 % of thetemperature.

As an illustration to satisfy the above conditionsusing a sillimanite target tube 80 mm in diameterin a furnace having a wall thickness of 300 mm, theminimum length of tube required would

be 300 + (5 80), i.e. 700 mm.Some reduction in minimum overall length of atarget tube can be obtained in such circumstances ifthe wall thickness can be effectively reduced in thearea surrounding the tube. It is sometimesnecessary to do this to prevent damage by thechange which could occur if the tube protruded toofar into the furnace.

Ceramic target tubes have lower resistance toimpact and thermal shock than metal tubes andwhen required, should be installed and used withcare. Silicon carbide tubes are less susceptible to

thermal shock than other types of ceramic tubes.

7 Detectors

7.1 General

Detectors used for radiation pyrometers fall into twomain groups:

a) thermal detectors, which utilize thetemperature rise resulting from the absorption ofthermal radiation. Their spectral response maybe made independent of wavelength over a verywide range.

b) photon detectors, whose characteristics are

determined by the absorption of individualphotons. Their response is limited to a relativelynarrow wavelength range, dependent upon theelectronic band structure of the photosensitivematerial involved.

In addition, the human eye is employed in visualpyrometers.

7.2 Detector characteristics

7.2.1Properties. A number of properties of detectors

have to be known in order to design pyrometers andto determine their working range. These aredescribed in 7.2.2to 7.2.4.

7.2.2 Sensitivity and noise. The responsecharacteristic of a detector is normally expressed interms of the electrical output, either voltage orcurrent, obtained under specified conditions foreach watt of incident radiation. For thermaldetectors, the radiation is taken to be the totalradiation from a blackbody at a specifiedtemperature, even though the detector may not besensitive at every wavelength. For photon detectors,the peak or maximum sensitivity for monochromaticradiation is often quoted. The photo-sensitive areaand the field of view are also required in order tomatch the device to the optical system of thepyrometer, and to estimate the electrical output ofthe detector for given source temperatures.

A lower limit to the measurement of temperature isset by the background noise in the output signal. Inmost pyrometric detectors, this arises from thermaleffects either within the detector or from thermalradiation within its field of view. It may therefore bereduced by cooling the detector. This, in fact, isessential for photon detectors operating at infra-red

wavelengths near the maximum of roomtemperature blackbody radiation (104m). If theresidual noise is determined by the level of externalradiation, the detector is said to be backgroundlimited. This noise component may be reduced byplacing cooled apertures or windows in front of thedetector to limit the effective field of view to thatrequired by the pyrometer design.

1) See Quinn, T.J. The calculation of the emissivity of cylindrical cavities giving near black-body radiation,Brit. J. Appl. Phys. 1967, Vol 18, p.1105. The first-order approximation for the emissivity of a cavity, c, made of a material ofdiffuse reflectivity , and with length-to-diameter ratioD, is:

c= 1 /4D2

This expression should only be used to obtain a rough magnitude of cfor values ofDgreater than 5.


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Quantitatively, the detector noise is given as theroot mean square value of the electrical output in a

bandwidth of 1 Hz at a specified frequency andunder typical operating conditions. For white noise,this is proportional to the square root of thebandwidth, so that it may be expressed in units ofvolts (or amperes) per square root bandwidth,i.e. VHz". For practical purposes, the noiseequivalent power (NEP), which is the amount ofradiative energy required to give an output signal ofthe same amplitude as the noise, is a convenientindicator of detector performance. It has been found,both theoretically and in practice, that the NEP isproportional to the square root of thephoto-sensitive area of the detector. A useful figure

of merit for the performance of detectors is thereforethe area-normalized detectivity, denoted byD*,equal to the square root of the detector area dividedby the NEP in cmHz"W1. It is necessary to specifythe conditions under which this has been measured,i.e. the blackbody temperature for thermal detectorsor the wavelength for photon detectors, thefrequency at which the signal and noise aremeasured, and the bandwidth employed. It isconventional to normalize measurements to abandwidth of 1 Hz.

7.2.3 Stability. If the output of a detector isdependent within the required accuracy, only on the

intensity of the radiation incident upon it, thecalibration of the pyrometer may be expressedsimply in terms of the electrical output as a functionof the radiance temperature of the source. Someinfluences, for example that of the ambienttemperature, may be reduced by compensatingmechanisms or electronic circuitry. Others, fatiguein the photodetector for example, can only bereduced or estimated with great difficulty. If theseproduce changes or errors which may exceed thespecified uncertainty, it is necessary to operate thepyrometer as a comparator, that is, as a devicewhich can compare the radiance from the source

with that from an internal or external referencesource, whose calibration is known and stable.

For pyrometers with an electrical output, it may benecessary to know the relationship between theincident intensity and the output voltage or current,for example, in order to apply the equations given inclause 3. In many cases the response is linear, andthe output is directly proportional to the intensity.

7.2.4 Response time. Although the response time ofthe detector is not often a critical factor in

pyrometer design, it is often relevant to the range ofapplications which can be handled by theinstrument. The surface temperature or emissivityof an object, especially if it is moving, may fluctuaterapidly. Some pyrometers are designed to detect themaxima or minima in the measured signals, andthese have to have a response time less than theduration of the fluctuations, otherwise an averagedsignal is obtained. In comparing pyrometers, thetest and reference sources are often compared byswitching the detection system from one to theother. The maximum rate at which this can becarried out is determined by the response time of the

detector, and this limits the total response time ofthe pyrometer. At low frequencies, the NEP tends toincrease as other sources of noise add to the thermalnoise within the detector.

7.3 Types of detector

7.3.1 The eye as a detector. As with any type ofradiation detector the two most important featuresof the eye are its absolute sensitivity to the radiationfalling upon it, and its relative spectral sensitivity.The response curve of the eye, generally known asthe visibility function, is given in Figure 4. Themaximum sensitivity occurs at 555 nm and the

cut-off is at 430 nm in the blue and at 700 nm in thered end of the visible region. The sensitivity of theeye is such that the lower limit of a visual pyrometeris at about 700C, while above about 1 300C it isnecessary to protect the eye from glare and to thisend an absorption filter is used to reduce theapparent intensity of the source.

7.3.2 Thermal detectors

7.3.2.1 General. The operation of thermal detectorsdepends upon the temperature rise produced by theabsorption of thermal radiation. This may bemeasured by conventional temperature sensors,such as thermocouples, thermopiles or resistancethermometers (usually thermistors of a specialdesign) or using other temperature-dependenteffects, such as charge generation as in pyroelectricdetectors. Conventional sensors may be attached tometal discs in order to increase the sensitive area,although the response time will be lengthened as aresult. The absorbing surface may be blackened inorder to decrease the reflectance. Such detectors canpossess a flat spectral response over a very widewavelength range and hence are ideally suited foruse in total radiation pyrometers. Although thespectral sensitivity is much less than the peakvalues achieved by photon detectors, a blackenedthermal detector is sensitive to a much greaterfraction of the total thermal radiation emitted by asource and can therefore be used to measuretemperatures down to room temperature or below.


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The temperature of the detecting element rises untilthe rate of heat loss is equal to the radiant power

absorbed. Heat is lost by gaseous conduction andconvection, by conduction through Wires andsupports, and by re-radiation. Gaseous conductionand convection are usually the major contributors toheat loss, unless the detector is evacuated.Re-radiation losses are generally small.

The response characteristic of the detector isdependent on the temperature rise and on thetemperature sensitivity of the detecting element(and, for a thermopile, on the number of detectingelements). The response time of the detector isdependent on the thermal capacity and diffusivity ofthe detecting element, and is typically long,

especially as compared with photon detectors. Ifheat losses from the element are reduced so as toincrease the sensitivity, this may have a deleteriouseffect on the response time.

To take advantage of the broad spectral bandwidth,thermal detectors are best used with reflectingoptical systems rather than lenses, which tend tocut off in the infra-red where thermal radiation is atits most intense. In practice, the spectral range isoften limited by the need to include windows in thepyrometer to protect sensitive elements of either theoptics or the thermal detector. The results oftruncation of the spectral range are two-fold; first,Stefans law is no longer obeyed, and an arbitraryrelationship between radiated power and blackbodytemperature has to be established. Second, the errorarising from a lack of knowledge of the emissivity ofreal surfaces is much more difficult to analyze andto correct for. Some special designs of pyrometerhave been introduced in an attempt to minimizethese effects.

In order to prevent changes in the ambientconditions interfering with the measurement, mostthermal detectors are operated in a differentialmode. In the case of thermocouple or thermopile

detectors, the reference or cold junctions are placedin thermal contact with the case of the device, andthe temperature difference between this and thethermally isolated absorbing surface carrying themeasuring junctions is measured. Bolometers,which are detectors measuring a resistance changein a sensor, may employ two small flakes ofthermistor material, one exposed to the incidentradiation, the other shielded from it, in a resistancebridge configuration which compensates forfluctuations in the ambient temperature. Ingeneral, good compensation is more difficult toachieve at low levels of incident radiation, and

under these circumstances it may be desirable tothermostat the enclosure.

7.3.2.2Pyroelectric detectors. A pyroelectric detectorconsists of a permanently electrically polarized

dielectric material, often in crystalline form,connected as a capacitor. The heating effect ofabsorbed thermal radiation produces a change inthe distribution of charge which may be detected asa change in the voltage across the device by a highinput impedance amplifier. As excess charge leaksaway gradually, this detector has to be used tomeasure changes in the level of thermal radiation,rather than the absolute level. The incident signal istherefore normally chopped at a lowaudio-frequency to provide a convenient signal fora.c. amplification. Polarized materials in commonuse at present include triglycine sulphate (TGS),

lithium tantalate (LiTaO3), lithium niobate(LiNbO3) and lead zirconate tantalate (LZT).

Pyroelectric detectors are usually sensitive tofluctuations in the atmospheric pressure, i.e. theyare microphonic, but using two elements mayprovide some compensation from this effect as wellas from changes in the ambient temperature. Theabsorbing surfaces of pyroelectric detectors areusually blackened in order to increase the range ofspectral sensitivity.

7.3.3Photon detectors

7.3.3.1Photoemissive detectors. Photons incident

upon certain photosensitive materials will cause theemission of electrons into a vacuum. These may becollected by an anode positively charged withrespect to the emitting material, or photocathode, togive a photocurrent proportional to the number ofphotons striking the photocathode each second. Thissimple device constitutes a vacuum photodiode. Amore common and valuable photodetector thephotomultiplier is obtained when specialelectrodes called dynodes are introduced betweenthe cathode and the anode. These increase thephotocurrent by the process of secondarymultiplication. With potential differences of

about 100 V between adjacent dynodes, a modernphotomultiplier will give an overall gain of up to 106or 107. As the total transit time is small,about 30 ns, it is possible to record the detection ofsingle photons at the photocathode with the fastoutput pulses at the anode.


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The response times of photomultipliers aredetermined by the variations in the transit time for

individual photoelectrons, and arecharacteristically between 1 ns and 10 ns. In manyapplications, the pulses of electrons arriving at theanode are smoothed electronically to produce ad.c. current. The secondary multiplication process isessentially noise-free, and the main source of noisein the output of photomultipliers used for radiationpyrometry originates in the thermionic emissionfrom the photocathode. As the thermionic workfunction is generally between 1 eV and 2 eV, thebackground noise level may be reduced to very lowlevels, about 10 output pulses/s for each squarecentimetre of photocathode area (corresponding

roughly to a photocathode current ofabout 2 1018A), by cooling the photomultiplier toa temperature between 20C and 80 C.

The spectral sensitivity is often characterized by thequantum efficiency of the photocathode, i.e. theratio of photoelectrons emitted to the number ofincident photons. The useful spectral range isdetermined by the electron band structure of thecathode material, and usually shows a sharp cut-offat the long wavelength end corresponding to thebandgap energy. The upper limit to photomultiplierresponse lies at present at 1.44m, cathodes thenbecoming difficult to prepare and are of limited

stability. The sensitivity of the output of aphotomultiplier to changes in its ambienttemperature is determined mainly by the change inthe quantum efficiency of the photocathode. This istypically of the order of 0.1 %/C to 0.3 %/C, butvaries, in sign as well as magnitude, with thephotocathode type, the ambient temperature andthe wavelength of the incident radiation. With somephotocathodes, it becomes very large in the longwavelength cut-off region.

In several respects photomultipliers arecumbersome to use in industrial instruments. Theyrequire a high voltage supply, betweenabout 800 volts and 2 600 volts, and a resistancedivider network for the dynode voltages. They aresensitive to magnetic fields, and have to bescreened, and they may be damaged by vibrationand mechanical shock. They tend to be used only inpyrometers of the highest accuracy required forresearch or calibration laboratories.

7.3.3.2 Semiconductor detectors. In a simplesemiconductor material, it is possible for photons of

sufficient energy to excite electrons from the valenceto the conduction band. These, as well as the holesleft in the valence band, increase the conductivity ofthe material, enabling it to be used as aphotoconductive detector. if the photon absorptionoccurs near a p-n junction, the charge carriers maybe collected by the junction electric field to producea detector photocurrent. In both cases, a longwavelength cut-off occurs at photon energiescorresponding to the bandgap difference betweenthe valence and conduction bands. If it is required todetect radiation at longer wavelengths than ispossible with simple semiconductors, excitation

from levels close to the conduction band which occurin impurity or complex ternary semiconductors maybe employed. To some extent the response of ternaryphotodetectors may be tailored by adjusting therelative proportions of the constituent elements.The major source of semiconductor noise resultsfrom the thermal excitation of electrons into theconduction band. As the spectral response isextended into the infra-red by reducing the photonenergy required, the noise level at a giventemperature will tend to increase rapidly. For agiven photodetector, the noise level roughly doublesfor each 10C rise in its temperature. To achieve

good sensitivity for photon detectors, therefore, itbecomes increasingly important to cool them astheir operating wavelength increases.

It is apparent from the preceding paragraph thatsemiconductor detectors may be eitherphotoconductive or photovoltaic. In fact, manysemiconductor materials may be made in bothforms, although some cannot easily be made intophotodiodes. Relative spectral sensitivities for somephoton detectors, including a type S-20photo-multiplier, are shown in Figure 5. Theambient temperature coefficients of the sensitivityof semiconductor detectors is relatively small, of the

order of 0.1 %/C to 0.3 %/C, and may vary in bothmagnitude and sign with wavelength.

8 Optical systems

8.1 Introduction

The aims of the design of the optical systems, whichare more or less common to all pyrometers, are:

a) to isolate the required band or bands ofwavelengths;

b) for a given source area, to increase theproportion of radiation incident upon the

detector, which is usually small;c) to reduce the effect on the output of changes inthe distance between the source and the detector,and;


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d) to remove interference from radiating sourcesoutside the desired field of view.

8.2 Aperture opticsIf a detector is exposed to the radiation from anextended but finite source, not only does the signalfrom it depend on the distance of separation, but thepresence of other sources in the vicinity may affectthe reading. These effects may be overcome byplacing an aperture stop and a field stop betweenthe source and the detector, as shown in Figure 6, insuch a way that all rays arriving at the detectorarise from some point on the source.

It will be seen from Figure 6 that the extreme raysAB and CD define the extent of the source AC whichis used when the detector has an effective size BD.

For a given aperture and field stop geometry, thedistance R can increase until the limiting rays moveto the edge of the source. If the source is small andthe required range of R has to be large, the aperturestop would have to be small, resulting in a lowoutput and loss of accuracy. This difficulty can beovercome by the use of a lens to image the source onto the field stop.

Aperture optics are only used in pyrometers formeasuring either high temperatures, in which casethe low optical throughput is compensated by thehigh radiance, or the average temperature of a large

surface.8.3 Lens optics

In a lens system, such as that in Figure 7, the pointof intersection of the limiting rays has now movedup to the lens, so that a smaller source can bemeasured without reducing the diameter of theaperture stop. The detector output depends on thesolid angle subtended at the aperture stop by thefield stop, and on the area of the aperture stop.

The focal length of the lens should be such that theimage completely covers the field stop. It is notessential for the image to lie in the plane of the

detector but it is important for all rays reaching thedetector to originate from the hot source. A reflectedradiation stop, or glare stop, is sometimes necessaryin order to prevent diffracted and stray radiation,reflected from the casing of the instrument, fromreaching the detector. The overall sensitivity of theinstrument varies with the angular field of view ofthe pyrometer. At a given distance a smaller sourcemay be focused by using a lens of longer focal length,but the sensitivity is reduced unless the aperture ofthe lens is correspondingly increased. Chromaticand spherical aberrations should always be takeninto account when calculating the minimum target

size required.

Lenses are normally made of glass but othermaterials are used depending on the temperaturerange being measured. As the temperature to be

measured increases, the energy maximum of theemitted radiation moves towards shorterwavelengths, and glass lenses may be used. Forlower temperatures, it is necessary to use lenses ofvitreous silica or other materials. The transmissionsof typical materials are given in Table 1.

8.4 Mirror optics

It is generally the case that lenses do not pass asufficiently high fraction of the thermal radiationfrom them to be used in total radiation pyrometers,which therefore often employ mirror optics. Figure 8and Figure 9 show typical arrangements of single

and Cassegrain mirror imaging systems. Aneyepiece is generally fitted at the rear to facilitatedirecting the pyrometer at the hot body and focusingits image so that it completely covers the detector.

Mirrors reflect a high proportion of the incidentradiation over a wide wavelength range. They havemetallic surfaces chosen for this property and forfreedom from tarnishing. These surfaces may beeither formed on solid metal, or deposited as filmson glass substrates. It is, however, necessary toprotect the mirror surface from dust and fumes. Insome cases, where the atmosphere is relativelyclean and continuous measurements are not

required, a protective cap may be fitted, which isremoved only when measurements are to be made.In dirtier conditions, and where continuousrecording is essential, a protective window shouldbe fitted as shown to make the enclosure dust tight.This should be chosen from the same range ofmaterials as are lenses (see Table 1) and should bemade as thin as possible to minimize the amount ofradiation absorbed.

8.5 Fibre optics

Fibre optics, both rigid and flexible, arecommercially available with adequate transmission

for incorporation in the optical system of a radiationpyrometer. Such pyrometers are particularly usefulin applications where the hot surface cannot beviewed directly because of obstructions, or wherethe pyrometer would otherwise have to be in achemically, thermally or electromagnetically hostileenvironment (see Figure 10 and Figure 11).

The optical head can be built to withstandtemperatures of 200 C to 400 C, much higher thanthose at which detectors can operate. If thesetemperatures would otherwise be exceeded, air orwater cooling may be employed, or alternatively theradiation can be focused on the fibre tip by a lens,both being kept at an acceptable temperature, asin Figure 11. This would also give better targetdefinition compared with the arrangement ofFigure 10.


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The transmission of silica-based fibres becomesprogressively poorer beyond 1.54m, and

applications at temperatures below about 600Chave therefore to employ a fibre of a differentmaterial or some form of optical stimulation.Techniques are evolving in which effects in the fibreitself, such as temperature-dependentback-scattering, are used to produce a distributedtemperature sensor, as well as those in which thefibre is a passive transmitter of, for example,stimulated fluorescent emission from a sensormaterial at the fibre tip. At higher temperaturesfibres made from single-crystal sapphire have beenused to sense temperatures up to 2 000C. Bycoating the fibre tip with an opaque material, a

small blackbody source is produced whose radiationis piped directly to a second, conventional fibreopticsystem.

8.6 Sighting

It is normally possible to have optical sightingthrough a pyrometer system, provided that the lensor window is transparent to at least part of thevisible spectrum. For pyrometers intended for fixedoperation this may not be necessary because theinitial positioning and alignment can usually becarried out with sufficient accuracy. For portablepyrometers it is essential to have a sighting system,

either through the lens, an auxiliary telescope or bypistol sights.

9 Signal treatment

The output signal from a pyrometer is oftenunsatisfactory for indicating, recording or control.This may be due to one or more reasons which aredetailed below. It is usually possible to modify theraw signal to obtain a suitable output. This isnormally done by one or more modules contained ina processor situated remote from the pyrometer.The available modules include the following.

a) Linearizer. The raw signal is always non-linear(often very non-linear) with target temperature.The linearizer produces an output which is linearwith temperature which can then be displayed onan indicator or recorder without the need forspecial scales and charts. If the output is scaledto, for example, 1 mV/C and a suitable offset isadded, the resulting output can be displayed astemperature on a suitable digital voltmeter.

b) Emissivity. Pyrometers are, almost withoutexception, calibrated to read correctly whenviewing a blackbody source. Without correctionfor the emissivity of the surface the pyrometer

reads low. This is corrected by multiplying theoutput by a factor 1/, and most commercialpyrometers incorporate an emissivityadjustment.

c)Averager. The raw signal may showconsiderable short term variations. These may be

real (i.e. due to temperature variations of thesurface viewed) or false, but in either case thesignal is not suitable for indication or control. Theaverager smoothes the signal so that short termvariations are eliminated.

d)Peak-picker. The true signal level may bepresent intermittently either because the targetis not always present or because of periodicalobstructions in the sight path. The peak-pickerholds the output at the level corresponding to thehighest input recently acquired. A slow decay(adjustable to suit the application) is provided toprevent the retaining of the highest input ever

received during a measurement period.

e) Valley-picker. This is the reverse of thepeak-picker. The lowest input is held subject to aslow rise. This circumstance of the lowesttemperature being required is very seldomencountered.

f) Sample-and-hold. The desired temperaturemay not be that of the hottest or the coolestsurfaces seen in a short period. This applies toseveral cyclical processes. A command signalfrom the process instructs the sample-and-holdmodule either to follow the input (when the

desired target is seen) or to hold the signalunchanged (when the desired target is not seen).The held signal does not need to decay since itwill be updated in the next sample period.

g)Alarm. Relay contacts change over when thesignal is above a pre-set high level or below apre-set low level.

h) Output. Extra outputs in current or voltageform are provided for additional display or forcontrol purposes.

i) Hot target detector. This senses the presenceor absence of a hot target and supplies a yes/no

signal.Signal treatment is usually available in the form ofelectronic modules which can be tailored to suit eachtype of pyrometer and combined to give the desiredsystem performance.


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10 Calibration

10.1 Introduction

Radiation pyrometry differs in many respects fromthe more familiar methods of temperaturemeasurement. Factors such as the wide choice ofoperating wavelength and optical bandwidth makethe technique more complex, both in theory and inpractice. However, the situation may be simplifiedin one respect. All pyrometers should be calibratedagainst sources which may be treated asblackbodies. Thus the effects and errors arising inmany applications from the emissivities of realsurfaces are not of concern in calibration.

10.2 The ITS-90

All measurements of temperature which need to bereliable and reproducible should be traceable to theestablished International Temperature Scaleof 1990, ITS-90. This is based upon the assignmentof numerical values to the temperatures of a numberof fixed points; that is, the temperaturescorresponding to phase changes in pure elementsand compounds, such as the triple point ofwater, 0.01 C, and the freezing points of certainpure metals.

Up to the freezing point of silver (assigned thevalue 961.78C, or 1234.93K) the ITS-90 specifiesthe use of platinum resistance thermometers

calibrated at these fixed points to realize the scale.Above this temperature it specifies the applicationof the Planck law using as the reference source ablackbody at the freezing point of silver,gold (1064.18C) or copper (1084.62 C) accordingto choice. Above the silver point the temperature ofa source is determined on ITS-90 by using a(photoelectric) pyrometer to measure the ratio Qofits spectral radiance to that of a blackbody at thechosen reference point. In the Wien approximation(equation 2) the unknown temperature (in kelvins)is calculated from:

where Tois the reference temperature, alsoexpressed in kelvins. The result will be independentof the effective wavelength 2eof the pyrometer,provided that the bandwidth of the pyrometer issufficiently narrow.

Since many photoelectric pyrometers operate withbest accuracy only as comparators, the scale is oftenmaintained on standard sources. These, andtransfer standard pyrometers are now discussed.

10.3 Blackbody sources

Blackbody sources suitable for the calibration of

pyrometric instruments generally consist of a cavityof high emissivity held in a temperature-controlledenclosure, such as a bath of circulating liquid or atubular furnace. This type of source is essential forthe calibration of broadband and total radiationpyrometers, since corrections for the variations inspectral emissivity of other sources cannot be madewith sufficient accuracy. The blackbody calibrationis normally referred to a contact thermometerplaced just behind the radiating surface of the cavityas viewed through the aperture. Modern cavities aregenerally based on cylindrical designs, which aresimple to construct, heat and measure. They should

have roughened or grooved walls and re-entrant orangled end-surfaces. A lower limit to the emissivityof the cavity resulting from its geometry and theemissivity of the material of which it is constructedcan usually be calculated without much difficulty byadopting a simple model for its design. The error %Tproduced by a deviation %from ideal blackbodyconditions is given by:

The apparent emissivity of a cavity will also dependupon the temperature gradient along its length.Again, the effect can be estimated by calculation.

More importantly, if there is a temperature gradientthere will also be a temperature difference betweenthe blackbody and the contact thermometer. Thepresence of a gradient may be detected by plottingthe temperature of the thermometer as a function ofits depth of immersion into the blackbody.Departures from a constant value may originatefrom two causes: from the temperature gradientitself, and from heat flow along the thermometer,which changes the temperature registered by thethermometer at a given point. If the curve obtaineddoes not show a plateau at the operating position ofthe thermometer, the associated errors may be

large. Unfortunately, the converse is not alwaystrue, as the measurement procedure may itself alterthe gradient.

In (Q) = c2(1/To 1/T)/2e (22)

%T= (2T2/c2)% (23)


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For these reasons it may be preferable to calibratethe blackbody source as a co

[BS 1041-5-1989] -- Temperature measurement. Guide to selection and use of radiation pyrometers.pdf

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