Imaging System for Mesopic Vision

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Imaging System for Mesopic Vision
CHIBA UNIV.

• H.Yaguchi, Y.Ushio, K.Kikuchi, D.K.Thahn, J.Shin,
  and S.Shioiri
• Chiba University and Tohoku University

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Introduction
Human visual system covers Illuminance range
from 10-3 lx to 105 lx.
Rod and cone Mesopic vision
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How does color appearance change with illuminance
?
To predict color appearance in mesopic vision
from the photopic image.
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Outline

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

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Color appearance in mesopic vision
Haploscopic color matching technique
1000 1000 lx
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Color appearance in mesopic vision
Stimuli
Test field 48 Munsel color chips chromatic
45,achromatic 3 Matching field 21 CRT display
(SONY Multiscan G500) Controlled by VSG (15
bits color, Cambridge System) Stimulus size
10?10º
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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.

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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.

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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.

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Results Achromatic Lightness

• The Stevens effect Perceived lightness range is
  reduced with decreasing illuminance.

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Correlation between perceived lightness at 0.01
lx and photopic and scotopic luminance
Photopic luminance
Scotopic luminance
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A Color Appearance Model in Mesopic Vision

• Luminance channel LM
• Red/green opponent-color L-2M
• Yellow-blue opponent-color LM-S

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Luminance as a function of illuminance
A(E) ?(E)100 ((LpMp) / (LpMp)w)?(E) 78.4 (Y
/ Y w)? Lp , Mp , Sp cone outputs at
photopic condition Y scotopic luminance
factor (LpMp)w , Yw each output of
luminance channels for white (being 100 Kw) ?
(E), ? (E) Weighting coefficients for functions
of illuminance E
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Change of cone- and rod-signal as a function of
illuminance
A(E) ?(E)100 ((LpMp) / (LpMp)w)?(E) 78.4 (Y
/ Y w)?
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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-2Mp) ?(E) Y y/b(E)
m(E)(LpMp-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
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Comparison between Experimental Results and
Prediction Hue and Chroma
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Comparison between Experimental Results and
Prediction Lightness
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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
  opponent-color channel.

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Performance of a model

• Average ?Eab in mesopic range is around 3.

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Imaging System for Mesopic Vision
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Imaging System for Mesopic Vision
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To obtain XYZ and Y from Digital Camera RGB
Predicted Value
Measured Value
X
Camera model
Y
Minimize error
Z
Y
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Calibration of Camera
Color SampleMacbeth Color Checker
and GretagMacbeth ColorCheker
Digital CameraCanon,EOS1-Ds
Image size 40642704 pixels Quantization RGB
12 bits
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Camera Model
Coefficients a, b,,,t are obtained by the
pseudo-inverse method.
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Camera performance as a colorimeter
Average color difference ?Eab 2.23 Maximum
color difference ?Eab 9.03
Original Macbeth Color Checker
Predicted by a Camera Macbeth Color Checker
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Prediction of scotopic luminance Y by camera RGB
Predicted value
Measured value
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Examples of mesopic color image
Original