Sensory Information Processing Color space Perception and reproduction of color

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

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

Fovea Center of the image Highest resolution

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

Spectral sensitivity of cones measured neurophysiology

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

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

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

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

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

rg chromaticity coordinate B 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)

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

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”

XYZ color matching function CIE1931 XYZ color matching function

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)

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

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

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

examples Color space of digital camera Adobe Photoshop Apple ColorSync Display

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

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

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

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

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

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)

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