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TRANSCRIPT
Psychophysical fundamentals of colorimetry
The concept of colour
Colour can be perception – psychological phenomenon colour stimulus – psychophysical
phenomenon radiometry - physics photometry – one dimension of colour
stimulus description
Colorimetry
Additive colour mixture
Colour mixing
Additiv subtractiv colour mixing
Grassmann’s lawsGrassmann’s laws
To specify a colour match three independent variables are necessary and sufficient
For additive mixture of colour stimuli, only their tristimulus values are relevant, not their spectral composition
In additive mixtures of colour stimuli, if one or more components are gradually changed, the resulting tristimulus values also change gradually
Colour equation - 1
Positive additivity C R G B B R G
Colour equation - 2
External colour mix - negative additivity C R G B R G B
Additiv colour matching
Additivity: IfC1R1(R)+G1(G)+B1(B)
C2R2(R)+G2(G)+B2(B)
thenCR(R)+G(G)+B(B),
where R= R1+ R2, G= G1+ G2, B= B1+ B2,
Additiv colour matching
ProportionalityIfC1R1(R)+G1(G)+B1(B)
thenaC1aR1(R)+aG1(G)+aB1(B)
The stronger form of trichromatic generalisation - 1
1/ symmetry law: If colour stimulus A matches colour stimulus B, then colour stimulus B matches colour stimulus A.
2/ transitivity law: If A matches B and B matches C, then A matches C
3/ proportionality law: If A matches B, then aA matches aB, where a is any positive factor.
The stronger form of trichromatic generalisation - 2
4/ additivity law: If A, B, C, D are any four colour stimuli, then if any two of the following three conceivable colour matches hold:
A B C D A C B D , , ( + ) ( + )
then so does the remaining match
C) (B D) (A
Composit spectrum - 1
Sum of colour stimuli: the power radiated in the single spectrum bands
6 0 04 0 0 5 0 0
r e l . i n t .
w a v e l e n g t h , n m7 0 0
Composed spectrum - 2
For one wavelength band
Composed colour stimulus:
Colour matching
R = SR()
G = SG()
B = SB()
drSkR )(
dgSkG )(
dbSkB )(
Colour equation
Integral form:
with three t values
Three real primaries with L = 1,000R + 4,5907G + 0,601B
T k ti i enm
780nm
d 380
( ) ,
R, G, B colour space
RGB - XYZ transformation
wavelength, nm
rel.
sen
s.
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
1,80
350 400 450 500 550 600 650 700 750 800 850
x2(lambda)
y2(lambda)
z2(lambda)
A new system of colorimetry?
TC 1-36: Physiologically significant ...
-8
-7
-6
-5
-4
-3
-2
-1
0
1
350 450 550 650 750
wavelength, nm
log
co
ne
ac
tio
n s
en
sit
ivit
y
L-cone
M-cone
S-cone
Improved colorimetry?Improved colorimetry?
Luminance-brightness discrepancy
Additivity problemsNew colour matching functions?Individual variationsHighly metameric matchesColour appearance models
TC 1-36: Physiologically significant ...
L, M, S primaries based on psychophysical measurements:
König hypothesis - congenital dichromats
Photopigment spectral absorbance
Selective absorption by the lensSelective absorption by the
macular pigment
Further issues to be described
Lightness contrast
Colour contrast
Assimilation