CHIBA UNIV Imaging System for Mesopic Vision H

CHIBA UNIV. Imaging System for Mesopic Vision H. Yaguchi, Y. Ushio, K. Kikuchi, D. K. Thahn, J. Shin, and S. Shioiri* Chiba University and Tohoku University*

Introduction Human visual system covers Illuminance range from 10 -3 lx to 105 lx. Rod and cone Cone Mesopic vision Photopic vision　　　　　 Rod Scotopic vision

How does color appearance change with illuminance ? To predict color appearance in mesopic vision from the photopic image.

Outline • Experiment of the haploscopic color matching. • A mesopic color appearance model. • A imaging system for mesopic color appearance. • Examples of mesopic color images.

Color appearance in mesopic vision • Haploscopic color matching technique 1000 0. 01 10 1 0. 1 1000 lxlx

Color appearance in mesopic vision • Stimuli Test field 48 Munsel color chips …chromatic: 45，achromatic: 3 Matching field 21” CRT display (SONY Multiscan G 500) Controlled by VSG (15 bits color, Cambridge System) Stimulus size… 10� 10º

Results: Hue and Chroma • Chroma reduces continuously with decrease of the illuminance level until 0. 01 lx. • The loci of matching color on the a*-b* diagram are not straight for many test color chips, indicating that hue shifts with the change in illuminance level.

Results: Chroma • Reddish and yellowish color (Y, YR, R, RP, P)： Chroma rapidly decreases from 100 to 1 lx, and constant below 0. 1 lx. • Greenish and bluish color (PB, B, BG, G, GY): Chroma rapidly decreases from 1000 to 1 lx, does not change below 1 lx.

Results: Lightness • Reddish and yellowish color: Lightness gradually reduces from 10 to 0. 01 lx. • Greenish and bluish color: Minimum lightness is observed around 10 to 1 lx.

Results: Achromatic Lightness • The Stevens effect: Perceived lightness range is reduced with decreasing illuminance.

Correlation between perceived lightness at 0. 01 lx and photopic and scotopic luminance Photopic luminance Scotopic luminance

A Color Appearance Model in Mesopic Vision • Luminance channel: L+M • Red/green opponentcolor: L-2 M • Yellow-blue opponentcolor: L+M-S

Luminance as a function of illuminance A (E ) = � (E)100 ((Lp+Mp) / (Lp+Mp)w)+� (E) 78. 4 (Y ’ / Y ’w) Lp , Mp , Sp: cone outputs at photopic condition Y’ : scotopic luminance factor (Lp+Mp)w , Y’w : each output of luminance channels for white (being 100 = Kw) �(E), �(E) : Weighting coefficients for functions of illuminance E

Change of cone- and rod-signal as a function of illuminance A(E) = (E)100 ((Lp+Mp) / (Lp+Mp)w)+ (E) 78. 4 (Y ’ / Y ’w)�

Red/green- and yellow/blue opponent channel as a function of illuminance Outputs of r/g and y/b channels at illuminance level of E r/g(E) = l(E)(Lp-2 Mp) + (E) Y ’ y/b(E) = m(E)(Lp+Mp-Sp) + (E) Y ’ Lp , Mp , Sp: cone outputs at photopic condition Y’ : scotopic luminance factor l (E), (E) and m (E), (E) : Weighting coefficients for functions of illuminance E

Comparison between Experimental Results and Prediction: Hue and Chroma

Comparison between Experimental Results and Prediction: Lightness

A Color Appearance Model in Mesopic Vision • Chromatic components decrease with decreasing illuminance. • Hue shifts are predicted by introducing a different process for red/green and yellow/blue opponentcolor channel.

Performance of a model • Average E*ab in mesopic range is around 3.

Imaging System for Mesopic Vision

To obtain XYZ and Y’ from Digital Camera RGB Predicted Value R G B Camera model Measured Value Xp Yp Zp Y p’ Minimize error X Y Z Y’

Calibration of Camera Color Sample・・・Macbeth Color Checker and Gretag. Macbeth Color. Cheker Digital Camera・・・Canon, EOS 1 -Dｓ・Image size　　4064× 2704 pixels ・Quantization　　RGB 12 bits

Camera Model Xp Yp = Zp Y p’ a 1 a 2 a 3 a 4 b 1 b 2 b 3 b 4 c 1 ・・・・ r 1 c 2 ・・・・ r 2 c 3 ・・・・ r 3 c 4 ・・・・ r 4 s 1 s 2 s 3 s 4 t 1 t 2 t 3 t 4 Coefficients a, b, , , t are obtained by the pseudo-inverse method. R 3 G 3 B 3 R 2 G R 2 B ・・・ BR R G B １

Camera performance as a colorimeter Average color difference　 ⊿E*ab = 2. 23 Maximum color difference ⊿Eab = 9. 03 Original Predicted by a Camera Macbeth Color Checker

Predicted value Prediction of scotopic luminance Y’ by camera RGB Measured value Coefficient of Correlation : 0. 998 Slope : 1. 000

Examples of mesopic color image Original 1(lx) 100(lx) 0. 1(lx) 10(lx) 0. 01(lx)