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GEK1532
Color perception
Thorsten WohlandDep. Of Chemistry
S8-03-06Tel.: 6516 1248
E-mail: [email protected]
http://webvision.med.utah.edu/
Nassau, Fig. 1-16
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Common problems
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Nearsightedness/Myopia
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
1. Too much refractive power, f too small2. Distant objects (> 2f) are imaged
between f and 2f, but f lies in front of theretina
3. To image on the retina one would have to
increase f but that is not possible with thelens (try to do it with your finger, butcarefully)
4. Near objects are imaged farther awayfrom f. At a certain point that is near enough finally the image will fall on theretina.
5. For all objects nearer than that your eyecan then accommodate by decreasingthe focus.
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f
f xx
Normal
2f x
2f x
Retina
f
f xx
Nearsighted
2f x 2f x
Retina
Not clear
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Common problems
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Farsightedness/Hyperopia
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
1. Too little refractive power, f too large
2. Distant objects (> 2f) are imagedbetween f and 2f; the eye can adjust bydecreasing the focus
3. Close objects though give an image thatlies so far behind the retina that the eyecannot adjust enough (there are limits tothe lens curvature) and the picture getsfuzzy.
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f
f xx
2f x
2f x
Retina
f
f xx
2f x 2f x
Retina
Normal
Farsighted
You can only increase the refractive power and thus only decrease the focal length
f
f xx
2f x 2f x
Retina
Not clear
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Common problems
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Astigmatism
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Camera Obscura
http://brightbytes.com/cosite/what.html
Aristotle is the first person weknow who used a cameraobscura.
http://www.rleggat.com/photohistory/history/cameraob.htm
Object
Image
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Simple eyes: the pinhole eye
The pinhole eye . These eyes are a pigmented pit or cup. The best examples of this eye type is from the molluscs, in the cephalopod Nautilus and the abalone
Haliotis . The image produced by these eyes is either extremely dim or extremelyblurred.
www.maayan.uk.com/evoeyes1.html
For a nice discussion and some nice explanatory pictures:
http://www.dcs.uky.edu/~jaynes/classes/636/Lectures/2001/optical-properties.pdf
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Simple eyes: the camera eye
The positive lens, also known as the camera eye , is found in vertebrates as wellas invertebrates (e.g. humans, some aquatic animals, spiders). This provides a
bright picture, with high optical quality.
www.maayan.uk.com/evoeyes1.html
The lens can be spherical, or it can beformed from a thickening of an outer layer (exoskeleton).
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The superposition eye
www.maayan.uk.com/evoeyes1.html
Every sensor on the retina gets itslight from several ommatidia.
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Focusing
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
This means the eye can only increase the curvature of the surface and thus decreasethe focal length.
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Image Formation
f f xx
Object
Image
f f xx
Object
Image
2f x
f f xx
Object
Image
2f x
2f x
2f x
2f x
2f x
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Processing types
CAR
1. Color processing -> recognition of colors in a picture
2. Spatial processing -> recognition of shapes in a picture
3. Temporal processing -> recognition of movements in a picture
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The Retina
http://webvision.med.utah.edu/
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Blind spot
http://ourworld.compuserve.com/homepages/cuius/idle/percept/blindspot.htm
http://ourworld.compuserve.com/homepages/cuius/idle/percept/blindspot.htmhttp://ourworld.compuserve.com/homepages/cuius/idle/percept/blindspot.htm -
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Rods and Cones
Cones: S (short ), M (medium),and L (long) wavelength
All figures: http://webvision.med.utah.edu/
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The sensitivity curves of Rods andCones
A sensitivity curve describes how sensitive a cell (cone or rod) responds to alight source.
a
b
1: cell a is much more sensitivethan cell b in the ratio 4:1
2: cell b is much more sensitivethan cell a in the ratio 6:1
6
1
4
Example:
1) 1000 photons arrive at one of thecells at the wavelength shown at 1.Cell a absorbs 600 photons. Thencell b would absorbs only 150photons.
2) 1000 photons arrive at one of thecells at the wavelength shown at 2.Cell a would absorbs only 150photons. Cell b would then absorb6 times as much, namely 900
photons.
sensitivity
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The sensitivity curves of Rods andCones
Nassau, Fig. 1-16
Maximum sensitivity of
S cones ~ 440 nm
M cones ~ 530 nm
L cones ~ 570 nm
Rods ~ 500 nm
Note: 1) The rods are much moresensitive than the cones between ahundred (M and L) and several thousandtimes (compared to S)
2) The S cones are the least sensitive inhuman vision
3) There is a strong overlap between thesensitivity curves, and light of onewavelength can possibly excite all threecones.
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Alternative diagrams showing thesame sensitivity curves
T.N. Cornsweet, Fig. 8.9
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Distribution of Rods and Cones
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Macula Lutea: The central area inthe retina where we have clear vision (high cone densities).
Fovea centralis: The central partof the macula lutea. There are norods, and thus dim objects cannotbe seen. But there are many conesand vision is best in this part
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The Retina
Blind Spot: At the position where the optic nerveenters the eye, there are no rod or cone cells and weare blind at this spot.(http://www.tedmontgomery.com/the_eye/index.html)
Optic nerve : There are more than millionnerve cells in an optic nerve. The opticnerve transmits the signal from the retina
to the brain. And the nerve itself is muchmore like brain tissue than nerve tissue.
http://webvision.med.utah.edu/
Macula Lutea: The central area inthe retina where we have clear vision (high cone densities).
Fovea centralis: The central partof the macula lutea. There are norods, and thus dim objects cannotbe seen. But there are many conesand vision is best in this part
http://webvision.med.utah.edu/http://faculty.washington.edu/chudler/chvision.html
http://faculty.washington.edu/chudler/chvision.htmlhttp://faculty.washington.edu/chudler/chvision.html -
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How do we get all colors encodedby 3 cones?
Cones: S (short ), M (medium),and L (long) wavelength
All figures: http://webvision.med.utah.edu/
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Monochromats
T.N. Cornsweet, Fig. 8.2
Absorption
probability for 500 nm light is10% (or 0.1)
Absorptionprobability for 575 nm light is1.5% (or 0.015)
At 500nm:
1000*0.1=100
At 575 nm
1000*0.015=15
At 575 nm
6667*0.015=100
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Cones with different sensitivities:Dichromats
Exception: Two cones withdifferent sensitivities but equalrelative sensitivities
T.N. Cornsweet, Figs. 8.4 & 8.6
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remember metamerism?T.N. Cornsweet, Fig. 8.7
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T.N. Cornsweet, Fig. 8.8
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GamutThe GAMUT of a set of colors are all thecolors that can be mixed with this set of colors.
Example: 3 cannot be mixedwith 1 and 2 in the previousexample. Thus 3 is not in the gamutof with 1 and 2 However: 2 can be mixed with 1 and 3. Thus 2 is in the gamut of the set of 1 and 3
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A Color Matching Experiment
This does not work in the case we saw before (negative intensity for 3)
1
2
3 2+ 3
Matching field:Left eye only Reference
field: Righteye only
The subject has to manipulate the wavelength for the matching field until itshows the same (or closest) color as the refrence field.
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A Color Matching Experiment
but this configuration works.
1
2
3
1+
3
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Dichromats (2 cones)
Reference field
Matched field
The two colored light sources from the left have to be mixed to match in hue,saturation and brightness the color in the reference field (which can be anypossible color).
1
2
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Trichromats (3 cones)
Reference field
Matched field
The three colored light sources from the left have to be mixed to match in hue,saturation and brightness the color in the reference field (which can be anypossible color).
R
G
B
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A cure for color blindness?
Assume you have glasses that absorblight differently for the left and right eye
T.N. Cornsweet, Figs. 8.24 and 8.25
then the cones in the two eyes wouldsee different colors differently.
Suppose you have a monochromat, i.e. a subject with only one sort of cones.
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Cone sensitivity and color
For a dichromat:
In a 2D plot we can find for everywavelength one point which representshow much each cone is excited by light
of this wavelength. The two axisrepresent the excitation of the two cones.
The curve seen here represents thepure spectral hues (only onewavelength) of a standard constant
intensity and how they are perceived bytwo different cones of a dichromat.
The dashed line represents light of 620 nm at different intensities.
T.N. Cornsweet, Fig. 8.12b
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Cone sensitivity and color What happens when we have light of two wavelength?
T.N. Cornsweet, Figs. 8.13a and e
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Coming back to gamut
T.N. Cornsweet, Fig. 8.17
Note: With light of twowavelength we can mix almost all colors for the dichromat.
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Sensitivity space color spaceFor a trichromat we can create a similar plot, but now in 3D.
T.N. Cornsweet, Fig. 8.18
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Sensitivity space color space
Here again we see that given three wavelength (pure spectral hues) which we
can vary in intensity, we can reproduce any other color within the pyramid (thegamut) they describe with the origin.
T.N. Cornsweet, Fig. 8.19
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Sensitivity space color spaceNow let us describe planes where the sum of the absorbed photons of all three cones is always constant.
T.N. Cornsweet, Fig. 10.2
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Summary
Retina (rods, cones and their distribution) Cone sensitivities (3 different cones) The number of different cones determines the
number of different wavelength we need at leastto mix all colors The gamut determines which colors can actually
be mixed by a certain colors set
Colors can be graphically depicted in a 3Ddiagram with the axis denoting the conesensitivities