Jean Paul Freyssinier - Lighting Research Center · © 2010 Rensselaer Polytechnic Institute. All...

© 2010 Rensselaer Polytechnic Institute. All rights reserved.11

Introduction to ColorimetryIntroduction to Colorimetry

Jean Paul FreyssinierJean Paul Freyssinier

Lighting Research Center, Rensselaer Polytechnic InstituteLighting Research Center, Rensselaer Polytechnic InstituteTroy, New York, U.S.A.Troy, New York, U.S.A.

sponsored by

www.lrc.rpi.edu/programs/solidstate/assist

IES NY

Issues in Color Seminar

February 26, 2011

© 2010 Rensselaer Polytechnic Institute. All rights reserved.

AcknowledgmentsAcknowledgments

NYC IES organizers of NYC IES organizers of Issues in Color SeminarIssues in Color Seminar, , especiallyespecially›› Jason Livingston, Wendy Jason Livingston, Wendy LuedtkeLuedtke, Dan Rogers, and , Dan Rogers, and

Meg SmithMeg Smith

LRC faculty, staff, and studentsLRC faculty, staff, and students

Sponsors of ASSIST ProgramSponsors of ASSIST Program

22

©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.33

Radiometry


Radiometry

Detection and measurement of electromagnetic energy› Purely physical – no consideration of how it stimulates

the eye

Unit of measurement: watt› The watt is a unit of power› Power is the rate of energy; energy per time

• 1 watt = 1 joule/second


Radiometry: Geometry and units

The geometry of how radiant energy is produced, emitted, propagating, defines the units of measurement

Description Quantity Unit

Energy per time Power W

Incident on a surface Irradiance W/m2

Leaving a surface Exitance W/m2


Sources of radiance

High Medium Low

Sun

1.6x109 1.2x107

to 3.9x107

3.0x104 1.4x104

Approximate luminance, cd/m2

1.5x106

to 1.0x109

30

Electroluminescent


Daylight 5700 K

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer

Incandescent

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer

Spectrum: Radiation as a function of wavelength

The electromagnetic spectrum can be divided into smaller and smaller bands, or expressed as a continuous function of wavelength (or frequency)

Units: W/nm

curveunder area0

dPPtotal

©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

350 450 550 650 750

Wavelength(nm)

Rel

ativ

e En

ergy

88

0

1

390 440 490 540 590 640 690 740Wavelength (nm)

Spectra of typical light sources

Incandescent Fluorescent High pressure sodium

0.0

0.2

0.4

0.60.8

1.0

1.2

350 450 550 650 750

Wavelength(nm)

Rel

ativ

e En

ergy

0

1

2

3

4

5

6

350 450 550 650 750

Wavelength(nm)

Rel

ativ

e en

ergy

Light emitting diodes


PhotometryPhotometry


What is photometry?

A simple, mathematically precise system of measuring and specifying light agreed to by an international community involved with its commerce and specification


Why is photometry important?Why is photometry important?

Promotes international tradePromotes international trade Provides a quantitative language for Provides a quantitative language for

communicating between stakeholderscommunicating between stakeholders


Light

IESNA Definition: Radiant energy capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 780 nanometers. CIE defines it over 360 to 830 nm.

Official (CIE) definition: radiant energy weighted by the photopic luminous efficiency function, V().

Based on flicker photometry.

300 350 400 450 500 550 600 650 700 750 8000

0.2

0.4

0.6

0.8

1

Wavelength, nm

Lum

inou

s ef

ficie

ncy

V() - Photopic

300 350 400 450 500 550 600 650 700 750 8000

0.2

0.4

0.6

0.8

1

Wavelength, nm

Lum

inou

s ef

ficie

ncy

V() - PhotopicV() - Scotopic


What does flicker photometry mean?

Related to response of photoreceptors in central fovea› L and M cones› 2L + 1M V()

Cone Fundamentals and V( )

0

0.2

0.4

0.6

0.8

1

400 500 600 700

wavelength (nm)

rela

tive

valu

e

photopicL coneM coneS cone


Light: Calculation of luminous flux

Fluorescent lamp, 4100 K (F32T8/841)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer




0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer

1616




0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

360 410 460 510 560 610 660 710 760

Wavelength (nm)

Relativ

e po

wer

1717




Flux Luminousfunction efficiency luminous Photopic

WPower Wlm683

nm830

nm380

VP

k

dVPk


Photometry

Description Quantity UnitLight Luminous flux Lumen

Amount incident per surface area

Illuminance Lumen/m2

Amount leaving per surface area

(Luminous) Exitance Lumen/m2

In a particular direction (range of directions)

(Luminous) Intensity Lumen/srcd

In a direction, the amount emitted per surface area

Luminance Lumen/(m2 sr)cd/m2, nit


Photocell and photopic response

415 0.0022 420 0.393194 3.85E+02 4.0420 0.004 430 0.407356 3.90E+02 4.3425 0.0073 440 0.419748 3.95E+02 4.7430 0.0116 450 0.434304 4.00E+02 5.0435 0.0168 460 0.446499 4.05E+02 5.3440 0.023 470 0.463415 4.10E+02 5.5445 0.0298 480 0.476397 4.15E+02 5.7450 0.038 490 0.487215 4.20E+02 5.9455 0.048 500 0.500787 4.25E+02 6.1460 0.06 510 0.512982 4.30E+02 6.3465 0.0739 520 0.52439 4.35E+02 6.5470 0.091 530 0.536389 4.40E+02 6.7475 0.1126 540 0.548387 4.45E+02 7.0480 0.139 550 0.560386 4.50E+02 7.2485 0.1693 560 0.571597 4.55E+02 7.4490 0.208 570 0.583202 4.60E+02 7.6495 0.2586 580 0.594611 4.65E+02 7.8500 0 323 590 0 606412 4 70E+02 8 1

CIE Photopic Luminous Efficiency Function and Silicon Photocell Spectral Response

0.0

0.2

0.4

0.6

0.8

1.0

350 450 550 650 750 850 950 1050Wavelength (nm)

CIE PhotopicSilicon


Errors applying V()

Filters work well for broadband, “white” light sources, but not for narrowband sources

400 450 500 550 600 650 7000

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Rel

ativ

e re

spon

se

Illuminance MeterCIE Photopic

440 450 460 470 480 490 5000

0.05

0.1

0.15

0.2

0.25

0.3

Wavelength (nm)

Rel

ativ

e re

spon

se

Illuminance MeterCIE Photopic

Blue LED


ColorimetryColorimetry


What is color?What is color?

Perception Perception –– opponent color theoryopponent color theory›› Red vs. greenRed vs. green›› Blue vs. yellowBlue vs. yellow›› HueHue›› Saturation (chroma)Saturation (chroma)›› Lightness (brightness)Lightness (brightness)

Color matching Color matching –– trichromatictrichromatic color theorycolor theory›› Any light can be perfectly matched with a combination of Any light can be perfectly matched with a combination of

just 3 standard lightsjust 3 standard lights Specification of the light stimulusSpecification of the light stimulus

›› Color matching functionsColor matching functions›› Equivalent to photometryEquivalent to photometry


Human color perception

Trichromatic vision› 3 cone photoreceptors› Overlapping spectral

sensitivity› A lot of not completely

understood neural processing both at the retina and within the visual cortex of the brain


Opponent color encoding


Metamers

Lights of the same color appearance can be made up of different spectral power distributions as seen in the diagram at the right.

Sources with the same color appearance, but different spectral power distributions will render colors differently.

Broad spectral power distributions are more likely to produce better color rendering

These three spectra can produce the same color perception


Metamers

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1x 10

-4

Wavelength, nm

Spec

tral P

ower

, W/n

m

Yellow-filtered incandescentYellow LEDYellow LED Yellow-filtered white source


CIE colorimetric system

Based on color matching – not color perception Principle of univariance

› Once a photon is absorbed by a photoreceptor all wavelength information is lost

› Photoreceptor response is determined by the number of photons absorbed

› Color information is contained in the relative strength of the signals from each type of photoreceptor

Matching done under very particular and controlled conditions› 2° observer and 10° observer› Bipartite field

Reference field

Matching field


CIE 1931 System

The basic CIE (Commission Internationale de L’Eclairage)system was developed in 1931.› Cartesian graph of chromaticity coordinates (x,y)

• Chromaticity coordinates describe the color of the source or thelight reflected from a surface under given lighting conditions.

• Set of 3 chromaticity coordinates, (x,y,z) represent the proportional amounts of 3 established primary colors that must be added together to form the test color.

• The coordinate z can be calculated if x and y are known.


Introduction to tristimulus values

Tristimulus values, R,G,B, or X,Y,Z show the absolute amounts of the three primaries required to make a match being specified

Tristimulus values are psychophysical quantities› Based on functions derived from averaged data of multiple

observers› Do not correspond to perceptual color› Y describes luminance

CIE system is for specifying difference or equivalence of light stimuli


Color matching functions

XYZ System employing imaginary primaries

400 450 500 550 600 650 7000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength, nm

Tris

timul

us V

alue

x()y()z()

ZYXYy

ZYXXx

dzPZ

dyPY

dxPX


X tristimulus value calculation

400 450 500 550 600 650 700 7500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)

spectral power spectral power distributiondistribution3500 K T83500 K T8

x color x color matching matching functionfunction

weighted weighted spectral spectral powerpower

X tristimulus X tristimulus value (area)value (area)


Y tristimulus value calculation

400 450 500 550 600 650 700 7500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)


y color y color matching matching functionfunction


Y tristimulus Y tristimulus value (area)value (area)


Z tristimulus value calculation

400 450 500 550 600 650 700 7500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)


z color z color matching matching functionfunction


Z tristimulus Z tristimulus value (area)value (area)


X, Y, and Z

400 450 500 550 600 650 700 7500

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)400 450 500 550 600 650 700 750

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)400 450 500 550 600 650 700 750

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Wavelength (nm)

X = 20.88X = 20.88 y = 19.85y = 19.85 z = 9.99z = 9.99


Spectrum Locus

Purple Boundary

Blackbody Locus

CIE 1931 chromaticity space

Fluorescent lamp, 3500 K


Properties of the CIE chromaticity diagram

Gamut of possible colors using these three LEDs


Limitations of 1931 CIE Chromaticity Diagram

There is no luminance level. Sources may have identical chromaticity coordinates, but

SPD will be different and colors can be rendered differently with these sources. (Metameric)

Color space is not represented in a uniform fashion. The visual difference between two points separated by a particular distance on the diagram varies with the position of the colors.

Other color systems have been developed which represent more uniform color space.


Perception of chromaticity differences

The distance between the end points of each line segment are perceptually the same.

Diagram is not perceptually uniform.


Discriminating differences in chromaticity

MacAdam’s ellipses of chromatic discrimination, plotted 10 times their actual size in the CIE chromaticity diagram.

Lamps with chromaticities within a 3-step ellipse should appear to be the same color by most observers.

ANSI specifies 4-step ellipses for fluorescent lamp chromaticities.


CIE 1976 Uniform Chromaticity Space (UCS)CIE 1976 Uniform Chromaticity Space (UCS)

The CIE 1976 UCS diagram is perceptually uniformThe CIE 1976 UCS diagram is perceptually uniform›› uu’’ = 4= 4X X / (/ (XX + 15+ 15YY + 3+ 3ZZ) = 4) = 4xx / (/ (--22xx + 12+ 12yy + 3)+ 3)›› vv’’ = 9= 9Y Y / (/ (XX + 15+ 15YY + 3+ 3ZZ) = 9) = 9yy / (/ (--22xx + 12+ 12yy + 3)+ 3)


Brightness of saturated colors

Saturated colors,

especially deep reds

and blues, appear

brighter than

photometric

measurements imply

Contours of enhanced brightness factors


Luminance is linear

+ =

L(g) + L(r) = L(y)1.5 + 1 = 2.5


Brightness is nonlinear!

B(g) + B(r) B(y)

+ =

In fact...


Brightness is nonlinear!

B(g) or B(r) > B(y) !!!

+ =


Other ways of specifying color

Many other color spaces have been developed and used for various tasks.› L*, u, v

• Based on CIE 1976 UCS diagram• Basis of CRI calculation (currently uses 1964 version)• Hue, lightness, chroma and saturation

› L*, a, b• Based on CIE 1976 UCS diagram• Hue, lightness, chroma

Generally not used for lighting industry No system is perfect


CCT is CCT is ›› an indication of the color appearance of the light an indication of the color appearance of the light

emitted by a sourceemitted by a source›› applicable to nominally white light sourcesapplicable to nominally white light sources›› derived from the chromaticity of a reference derived from the chromaticity of a reference

(blackbody radiator)(blackbody radiator)

Correlated Color Temperature (CCT)Correlated Color Temperature (CCT)


Correlated Color Temperature (CCT)Correlated Color Temperature (CCT)


Correlated color temperature (CCT)

Isotemperature lines: Lines perpendicular to the CIE 1960UCS defining constant CCT

0 0.2 0.4 0.6 0.8

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

CIE 1931 Chromaticity Diagram with Planktain Locus

xy

0.1 0.2 0.3 0.4 0.5 0.6

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45CIE 1960 Chromaticity Diagram with Planktain Locus

u

v

2000 K3000 K

4000 K∞ K

8000 K

5000 K6000 K

7000 K

2000 K

8000 K


Graphically determining CCT


Limitations of CCTLimitations of CCT


ANSI Tolerance Zones for SSL Lamps

ANSI Tolerance Zones for CCT of Linear ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State SourcesFluorescent and Solid State Sources

ANSI Tolerance Zones for Linear Fluorescent Lamps


ANSI Tolerance Zones for CCT of Linear ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State SourcesFluorescent and Solid State Sources

ANSI Tolerance Zones for Linear Fluorescent Lamps


CCT of Compact Fluorescent LampsCCT of Compact Fluorescent Lamps


0.42

0.47

0.52

0.57

0.15 0.20 0.25 0.30

u'

v'2700 K3000 K

3500 K

4000 K

5000 K

6500 K

Blackbody Locus

CCT of Linear Fluorescent LampsCCT of Linear Fluorescent Lamps


Thank you.Thank you.

5656

Jean Paul Freyssinier - Lighting Research Center · © 2010 Rensselaer Polytechnic Institute. All...

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