1. Sensory Information Processing
Color space
Perception and reproduction of color

2. Light and Color Light : physical color：psychological
Light is a electromagnetic wave
Physically, there is no concept of “color” (distribution with respect to the wavelength)
color：psychological
One aspect of perception of the light
Relative intensity of three ranges of wavelength
 To learn about color, we must learn about human

3. eyeball diameter 24mm Flexible lens (crystalline) Iris = aperture
Focal length = 17mm
Retina : film or CCD
Fovea

4. Fovea
Center of the image
Highest resolution

5. Retina Two types of photo receptor
Rod : monochromatic, high response, work in dark environment
Cone : three types (L, M, S) : color low response, work only at bright environment

6. Spectral sensitivity of cones
measured neurophysiology

7. How to identify the color perception?
How to measure the relationships between wavelength and color perception
Measuring signals of neurons
Not easy
Perception is not directly related to signal
Subjective tests
Comparing very single spectral light with additive color mixture
Ambiguity of coordinate transform remains

8. Color matching experiment(1)
Negative response
Adjust the levels of R, G, B so as to match to monochromatic light
R : 700nm　G : 546.1nm　B : 435.8nm

9. Color matching (2) Negative value means the light at opposite side
All in positive
ith negative values
Negative value means the light at opposite side

10. Why we have “negative values”?
No negative response at our retina
Actual spectral response(unknown)
sensitivity
M(λ)
S(λ)
S=∫S(λ)・i(λ)dλ
L(λ)
wavelength ×
Sample stimulus L
intensity
i(λ) λ
change M
wavelength
Smooth curve in positive region

11. Why we have “negative values”?B G R S
i=B
i=G L
i=R i λ M
R>0, G=B=0 when i = 700nm(R)
RGB space is smaller than the original space with our actual sensitivity. Therefore, between the wavelength of B and G, R should be negative

12. rg chromaticity coordinateB G R L S M R B G
i=B r g
Linear trransform
i=G
i=R λ
rg : chromaticity : Relative values of RGB
r = R/(R+G+B), g = G/(R+G+B)
Single wavelength passes three points (r,g)=(0,0), (0,1), (1,0)

13. rg chromaticity and RGB color matching curve
All three values have negative regions

14. XYZ color space To avoid the negative values of RGB color space
New coordinate system which includes the spectral curve in positive area Three points, X, Y, Z, is outside of spectral curve, so, it does not physically exist called as”imaginary color”

15. XYZ color matching function
CIE1931 XYZ color matching function

16. xy chromaticity Relative value of XYZ x = X/(X+Y+Z), y = Y/(X+Y+Z)
Black curve : black body radiation (related to the color temperature)

17. xy chromaticity and brightness
White light source
intensity
Object with various color
Reflectance is limited to 1.0 or below
White is brightest
Intensity of the boundary of spectral curve is 0
Called MacAdam limit

18. Color reproduction and gamut (1)
Range of additive color is limited in the triangle which verteces are primal colors
Called gamut
left figure indicates the range of RGB tristimulus

19. Display and color reproduction
Range of the color depends on the primal colors
“RGB” of PC is not CIE RGB
Wrong use of color space causes the color mismatch s

20. examples Color space of digital camera Adobe Photoshop Apple ColorSync
Display

21. Printing and color Printing is subtractive color (CMY)
Inside the convex hull which defined by 6 points of C, M, Y, CM,CY, MY
RGB and CMY have different gamut

22. White material (paper)
print
cyan
green
black
blue
magenta
red
yerrow
white
yellow=R+G
yellow=R+G
magenta=R+B
cyan=G+B
White material (paper)
・spectral reflectance of green ≒ yellow ・ cyan
　→spectral reflectance of green is linearly independent from yellow and cyan
・represented by halftone dot
　→mixture of 8 colors above
Halftone dot
Convex hull of 6 primal colors

23. Color space matched for perception
HSB color space
Polar coordinate
HSB (Hue, Saturation, Brightness)
xy color space
Perceptually not uniform
MacAdam ellipse

24. L*a*b* color space
Perceptually uniform color space by nonlinear transform

25. Metamerism Visually same color but different spectral curve
Depends on not only the object but also the light source
Light 1 ×
Object 1
Perception R
same
`erception R
Perception G
same
`erception G
Perception B
same
Light 2 ×
Object 2
Perception B

26. Spectral sensitivity and evaluation of color
If the spectral sensitivity of the sensor is different from the human beings, color matching fails
If the spectral curve of light source is special, color matching fails (object with different colors seems same)
（fluorescent light is not good as light bulb)

27. Psychology vs. Physics Psychology physical Marginal
System which works as same as human
Color quality control
physical
Color marker (distinguish objects with color)
Recognition of material using color
Marginal
QC of papers using color (visual, or other?)
QC of agricultural products