physics measurement norm perception

Claudio OleariUniversità degli Studi di Parma

Dipartimento di Fisicaclaudio.oleari@fis.unipr.it

Glossiness & colour of a transparent glass:

Light TRANSMISSION and REFLECTION

are considered in relation to the perception in order to open a debate useful to define the

most meaningful specification of the

APPEARANCE of a transparent glass.

Transmitted and reflected lights are present

at the same time in a glass object.

In practice:How to judge visually

the appearance of a glass?

In science:How to specify and measure

the appearance of a glass?

1) OpticsFor an optical characterization of a non scattering glass

i) appearance characterization

spectral transmittance () spectral reflectance ()

ii) optical characterization spectral refraction index n() absorption coefficient k()

VISIBILE SPECTRUM

violet blue green yellow orange red380 430 490 560 580 620 780 nm

gamma X radio ray ray UV IR microwave wave long radio wave

10-14 10-12 10-10 10-8 10-6 10-4 10-2 1 102 104 106 108 m

Colour stimulusColour stimulus

- Snell laws (geometrical properties of light)- Fresnel laws (energetic properties of light)

reflectance & transmittance

Refraction index reflection & refraction

n()

1

From Wikipedia

Snell laws

geometrical optics

Fresnel laws

Surface reflectance

What geometry for illumination and light collection?

Biconical Biconical TransmittanceTransmittance and and ReflectanceReflectance spectral Distribution spectral Distribution Function (BTDF) (BRDF)Function (BTDF) (BRDF)

i

i

i

i

i

iiiii

iiiiiii

iiE

Ef

L

dcos),,(

ddcoscos),,(),,,,(

),,(

2) CIE norm(Commission Internationale de l’Éclairage)

for transmittance measurement(a standard reflectance measurement of a transparent medium is not defined)

Norms for light-modulation measurementCIE publication No. 15:2004, Colorimetry, 3rd Ed.

specimen

10°

10°

to the spectrometer

CIE geometry (0°:0°)

specimen

10°

2°

to the spectrometer

CIE geometry (di:0°), (de:0°)

specimen

=2

to the spectrometer

CIE geometry (d:d)

Incidentlight

reflectedlight

refractedlight

22

airglass

airglass0 1)(

1)(

)()(

)()()(

n

n

nn

nn

skesi)(),(

absorption

Transparent glass

CIE geometry (0°:0°)

scs

ssi

)( 10)0(

)(),(

,

,

Absorption and Internal transmittanceLambert-Bouguer and Beer laws

,(s=0) ,(s)

(s)

s

skescae )()(

Total spectral transmittance

Total spectral reflectance

nair()nglass()

≈ 0

i

tr 2

0

20

),()(1

)(1),(

)(

)(),(

s

ss

i

i

i

t

Physical quantities directly related to the perception

Measured by spectrophotometer

2

0

20

20

0),()(1

)(1),()()(

)(

)(),(

s

ss

i

i

i

r

For an (approximate) complete optical characterization

n(), k()

(), ()

Transmittance for orthogonal incidence

2

0

20

),()(1

)(1),(),(

s

ss

i

i

skescaescsi)()( )( 10),(

2

0 1)(

1)()(

n

n

Approximate equation

)0,(),()(1

)(1),(),(

20

20

sss

s ii

Zero-thickness transmittance

1)(

)(2

)(1

)(1)0,(

220

20

n

ns

For a complete optical (approximate) characterization

From measures of transmittance for two different thicknesses

)0,()exp()0,(),(),( 111 skssss i )0,()exp()0,(),(),( 222 skssss i

1)(

)(2

)exp(

),(

)exp(

),()0,(

22

2

1

1

n

n

ks

s

ks

ss

),(

),(ln

1)(

2

1

12 s

s

ssk

absorption coefficient

)0,(11)0,(

1)( 2

s

sn

refraction index

For an (approximate) complete characterization of

appearance

(), ()

For an (approximate) complete characterization of appearance

refraction index

),( sTotal transmittance is measured

Total reflectance is function of n() and k(), obtained by approximation from two transmittance measurements

2

0

20

20

0),()(1

)(1),()()(),(

s

ss

i

i

3) Colour perception & specification of transmitted and reflected lights

•CIE observers•CIE colorimetric systems•CIE colorimetric computation

MACULA LUTEAMACULA LUTEA

Macula luteaFoveaBlind point

400 500 600 700 nm lunghezza d’onda

Macular absorbance [a. u.]

CIE 1931CIE 1931

CIE 1964CIE 1964

CIE observers

aperture modeaperture mode typical of the psycho-physical and psycho-metric

colorimetry

CIE colorimetric systems:

•(X, Y, Z) linear vector space (tristimulus space)

•Luminance factor, d dominant wavelength, purity

•CIELAB metric space (L*,a*,b*), (L*, hab, C*ab) + colour-difference formulae

•CIELUV metric space (L*,u*,v*), (L*, huv, C*uv)

•Luminance factor, whiteness, tint

CIELAB – CIELUV

A colour specification close to the perception

•unique hues (red, yellow, green, blue)unique hues (red, yellow, green, blue)

•binary huesbinary hues

•colour opponencycolour opponency

Binary Binary hueshues

L*

a*

b*

hab

C*ab

Colorimetric computation:

Colour specification depends on

the observer (CIE 1931 or CIE 1964) the spectral transmittance/reflectance the illuminant (A, D65, F11)

(X, Y, Z)

Few warnings

TRISTIMULS COMPUTATION according to CIE[1] and ASTM [2]TRISTIMULS COMPUTATION according to CIE[1] and ASTM [2]

RAW SPECTRAL DATA IF 1 nm THEN ELSE OR

CHOICESobserver (CIE 1931, CIE 1964,., …)illuminant (A, B, C, D65,…, F11, …)

deconvolution

interpolation

ASTM 1996WeightingFunctions

ASTM 1985WeightingFunctions

1 nm CIEWeightingFunctions

TRISTIMULUS VALUES (X, Y, Z)

1 nm CIEWeightingFunctions

[1]Publication CIE N° 15:2004, Colorimetry, 3rd edition, Central Bureau of the CIE, A-1033 Vienna, P.O. BOX 169 Austria.[2] ASTM E 308-96 Standard Practice for Computing the Colors of Objects by Using the CIE System, Annual book of ASTM Standard, American Society for Testing and Materials, Philadelphia, USA.

What means

“COLOR CALCULATION ACCORDING TO CIERECOMMENDATION (D65/10° - CIE 15:2004)”

?

RECOMMENDATIONS CONCERNING THE CALCULATION OF TRISTIMULUS VALUES AND CHROMATICITY COORDINATES Calculation of tristimulus values

The CIE Standard (CIE, 1986) on standard colorimetric observers recommends that the CIE tristimulus values of a colour stimulus be obtained by multiplying at each wavelength the value of the colour stimulus function () by that of each of the CIE colour-matching functions and integrating each set of products over the wavelength range corresponding to the entire visible spectrum, 360 nm to 830 nm. The integration can be carried out by numerical summation at wavelength intervals, , equal to 1 nm.

(7.1)

In the above equations () denotes the spectral distribution of the colour stimulus function, i.e. () = d()/d, see CIE International Lighting Vocabulary item 845-01-17 (CIE,1987). X, Y, Z are tristimulus values, X(), Y(), Z() (or ) are colour-matching functions of a standard colorimetric observer, and k is a normalising constant defined below. Each of these may be specified for the CIE 1931 standard colorimetric system by being written without a subscript, or for the CIE 1964 standard colorimetric system by the use of the subscript 10.

The fundamental colorimetric tables are the 1 nm tables in CIE standards. All rigorous calculations should use these 1 nm tables. For most practical purposes, the summation may be approximatedapproximated by using wavelength intervals, equal to 5 nm over the wavelength range 380 nm to 780 nm. Values of the CIE colour-matching functions at 5 nm intervals suitable for use in summation over this range of wavelengths are given in Tables T.4 and T.5. In case measurement have been made at smaller intervals than 5 nm, the appropriate values from the tables in the standards should be used.

)(Z)(kZ

)(Y)(kY

)(X)(kX

)(Z)(kZ

)(Y)(kY

)(X)(kX

10

10

10

1010

1010

1010

What about

“WHITENESS and TINT ”

?

The evaluation of whitenessTo promote uniformity of practice in the evaluation of whiteness of surface colours, it is recommended that the formulae for whiteness, W or W10, and for tint, Tw or Tw,10, given below, be used for comparisons of the whiteness of samples evaluated for CIE standard illuminant D65. The application of the formulae is restricted to samples that are called "white" commercially, that do not differ much in colour and fluorescence, and that are measured on the same instrument at nearly the same time. Within these restrictions, the formulae provide relative, but not absolute, evaluations of whiteness, that are adequate for commercial use, when employing measuring instruments having suitable modern and commercially available facilities.

W = Y + 800(xn – x) + 1700(yn – y)W10 = Y10 + 800(xn,10 – x10) + 1700(yn,10 – y10) (9. 11)Tw = 1000(xn – x) – 650(yn – y)Tw,10 = 900(xn,10 – x10) – 650(yn,10 – y10)

where Y is the Y-tristimulus value of the sample, x and y are the x, y chromaticity coordinates of the sample, and xn, yn are the chromaticity coordinates of the perfect diffuser, all for the CIE 1931 standard colorimetric observer; Y10, x10 , y10, xn,10 and yn,10 are similar values for the CIE 1964 standard colorimetric observer.

Conclusion:

deep physical specification refraction index n() {measured, approximate from 2 transmittances}

absorption coefficient k() {approximate from 2

transmittances} appearance physical specification spectral transmittance (,s) {measured by spectrophotometer}

spectral reflectance (,s) {measurement?} {approximate from 2 transmittances}

Colorimetric specification of the appearance- physical specification observer: CIE 1931 illuminants: A, D65, F11 colorimetric system: CIELAB (L*, C*ab, hab)

About the VISUAL JUDGEMENT

of the appearance

?

Thank you for your attention

Claudio Oleari