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GEK1532
Interlude: Painters Color and
Color deficiencies
Thorsten Wohland
Dep. Of Chemistry
S8-03-06
Tel.: 6516 1248
E-mail: [email protected]
Claude Monet, Japanese Bridege over
water Lily Pond, 1926
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The Anomalouscope
From Scientific Amrican, Special on
Color (German Version)
a) Normal vision
b) No red
c) No green
d) Red anomalous
e) Green anomalous
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Lightness and Color Constancy
Webers law states that we see brightness in logarithmic scale.
However, we know as well that we perceive something white always as
white, no matter how bright the illumination is. This phenomenon is called
Lightness constancy.
Lightness constancy thus means that we see objects always in relationto the surrounding. So when the illumination changes, the brightness
(absolute intensity) changes, but not the lightness (the ratio of different
brightnesses).
Good illumination Darker illumination
http://www.purveslab.net/seeforyourself/
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Lightness changes not uniformly
everywhere
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The rod system
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Kurt Nassau, Fig. 1-16
Scotopic vision: night vision, based on rods;
maximum sensitivity: around 500 nm
Photopic vision: day vision, based on cones;
maximum sensitivity around 550 nm
Mesopic vision: transition from photopic to
scotopic vision, both systems operate (e.g. at dusk)
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Scotopic vs Photopic Vision
http://www.cquest.utoronto.ca/psych/psy280f/ch3/purkinje/ps.html
Purkinje shift
Scotopic vision: max sensitivity ~500 nm Photopic vision: max sensitivity ~550 nm
Mesopic vision: Humans have characteristics of tetrachromat
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Painters and use of color
Rembrandt (1606-1669)Leonardo
da Vinci (1452-1519) Charles-Joseph
Natoire (1700-1777)
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Joseph Mallord William Turner
(1775-1851 )
1819 Colour Beginning
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Joseph Mallord William Turner
(1775-1851 )
1835 grand-canal
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Joseph Mallord William Turner
(1775-1851 )
~1845 sunrise with sea-monsters (detail)
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Pointillism
Georges Seurat 1884-6 Un dimanche aprs-midi l'Ile de la Grande Jatte
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Pointillism
Paul Signac Harbour at Marseille 1906
Camille Pissaro 1897 Boulevard-Montmartre-Afternoon-Sun-1897
Paul Signac The bridge at Asniere 1888
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Pointillism
http://www.state-of-entropy.com/points.htm
Add NoiseSharpenReduce Color depth to 2
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Claude Monet (1840-1926) -
Impressionism
1872/3: Impression Soleil levant (sunrise)
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MonetThe port of the Cathedral at Rouen
Morning Mist 1893 Sunlight 1894 Sunlight 1892-4
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Monet
Japanese Bridge over
Water Lily Pond 1894
Japanese Bridge over
Water Lily Pond 1899
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Color vision defects
http://www.psych.ucalgary.ca/pace/va-lab/Brian/acquired.htm
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Monet
House seen from the
rose garden 1924
House seen from the
rose garden 1924
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The organization of the retina
Rods (R) and Cones (C)
Bipolar Cells (B)
Ganglions Cells (G)
Amacrine Cells (A)
Horizontal Cells (H)
Falk: Fig. 7.2
To optic nerve
Light
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The organization of the retina
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The rod system (scotopic vision):
The experiment of Hecht, Schlaer
and Pirenne from 1942
T.N. Cornsweet, Fig. 2.2
http://hyperphysics.phy-astr.gsu.edu/
hbase/hframe.html
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Dark adaptation
Dark adaptation is complete within 40 minutes.
T.N. Cornsweet, Fig. 2.1
T.N. Cornsweet, Fig. 2.6
The threshold is defined as the
intensity at which a subject perceives
60 % of flashes.
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Spatial summation
T.N. Cornsweet, Fig. 2.5
1st spot: only few rods on average
2nd spot: smaller than summation area
3rd
spot: larger than summation area
Sensitivity constant
Sensitivity decreases
Illuminate spots on the retina of different size
and determine the number of photons
needed before the spot can be seen
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Temporal summation
Adapted form T.N. Cornsweet, Fig. 2.5
time (ms)
0 10 20
How many photons have to arrive
in a certain time interval so that the
eye sees a flash?
time (ms)
0 10 20
time (ms)
0 10 20
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Temporal summation
Adapted form T.N. Cornsweet, Fig. 2.5
time (ms)
0 10 20
How many photons have to arrive
in a certain time interval so that the
eye sees a flash?
time (ms)
0 10 20
time (ms)
0 10 20
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Spectral sensitivity of rods
J19
106.3 }P
R hchE !!9
834
10550
103106.6
v!
We just saw that the eye does not need more than 2 photons to perceive a
flash (a rod can actually be activated by one photon alone).
How many photons do we get from a 20 W [=20J/s] light bulb?
Therefore a light bulb gives us
sphotonsJ
sJ/106.5
106.3
/20 1919
!
56.000.000.000.000.000.000 photons
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Lateral Inhibition
+ +
- -
These two cones/rods inhibit each
other, that is the higher the signal for
cone/rod A, the more will be the
signal for cone/rod B diminished and
vice versa.
A B
+ +
- -
A =0.5 B = 0.5
A=B
+ +
- -
A =0.5 B = 0.25
A - B
+ +
- -
A =0.75 B = 0.5
A - B
With lateral inhibition we are much more sensitive to differences than absolute values.
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Lateral Inhibition
STL Fig. 7.12
rest
excitation
inhibition
No difference -> rest
Strong excitation
No difference -> rest
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Lateral Inhibition
STL Fig. 7.8
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If edge information is missing
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Spatial frequency and tilt
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AfterimagesYou can have negative and positive afterimages.
The effect comes from the fact that when a cone/rod is stimulated for a long time
it desensitizes.
1) The cones perceiving the black square are not excited, the cones perceiving the
white surrounding are excited and desensitize with time.
2) When looking at the white surface on the right, the desensitized cones are lessexcited than the rested cones in the middle and thus you see a white square.
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Afterimages
Positive afterimages.
You can sensitize your retina by closing your eyes and resting your cones
(remember when you close eyes a long time and open them you seem to be
blinded first).
When you open your eyes shortly (seconds) and look at some bright object the
cones get excited.
When you close your eyes again the cones will not desensitize and will stay
stimulated longer and give you a positive afterimage.
See the TRY IT on page 194 of STL.
http://www.michaelbach.de/ot/
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Summary
Spatial summation in the eye
Temporal summation
Lateral Inhibition Edge discrimination
Afterimages
Are there non-trivial constraints on colour categorization?
B.A.C. Saunders, J. van Brakel
Behavourial and Brain Sciences (1997), 20, 167-179