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Glossiness & colour of a transparent glass:. physics measurement norm perception. Claudio Oleari Università degli Studi di Parma Dipartimento di Fisica [email protected]. - PowerPoint PPT Presentation
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physics measurement norm perception
Claudio OleariUniversità degli Studi di Parma
Dipartimento di [email protected]
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