GEk1532 Receptors

8/6/2019 GEk1532 Receptors

1/43

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


2/43


3/43

Common problems

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


4/43

Nearsightedness/Myopia


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.


5/43

f

f xx

Normal

2f x

2f x

Retina

f

f xx

Nearsighted

2f x 2f x

Retina

Not clear


6/43

Common problems



7/43

Farsightedness/Hyperopia


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.


8/43

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


9/43

Common problems



10/43

Astigmatism


11/43

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


12/43

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


13/43

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.


The lens can be spherical, or it can beformed from a thickening of an outer layer (exoskeleton).


14/43

The superposition eye


Every sensor on the retina gets itslight from several ommatidia.


15/43

Focusing


This means the eye can only increase the curvature of the surface and thus decreasethe focal length.


16/43

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


17/43

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


18/43

The Retina



19/43

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


20/43

Rods and Cones

Cones: S (short ), M (medium),and L (long) wavelength

All figures: http://webvision.med.utah.edu/


21/43

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


22/43

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.


23/43

Alternative diagrams showing thesame sensitivity curves

T.N. Cornsweet, Fig. 8.9


24/43

Distribution of Rods and Cones


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


25/43

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.


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


26/43

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/


27/43

Monochromats


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


28/43

Cones with different sensitivities:Dichromats

Exception: Two cones withdifferent sensitivities but equalrelative sensitivities

T.N. Cornsweet, Figs. 8.4 & 8.6


29/43

remember metamerism?T.N. Cornsweet, Fig. 8.7


30/43



31/43

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


32/43

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.


33/43

A Color Matching Experiment

but this configuration works.

1

2

3

1+

3


34/43

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


35/43

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


36/43

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.


37/43

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


38/43

Cone sensitivity and color What happens when we have light of two wavelength?

T.N. Cornsweet, Figs. 8.13a and e


39/43

Coming back to gamut


Note: With light of twowavelength we can mix almost all colors for the dichromat.


40/43

Sensitivity space color spaceFor a trichromat we can create a similar plot, but now in 3D.



41/43

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.



42/43

Sensitivity space color spaceNow let us describe planes where the sum of the absorbed photons of all three cones is always constant.



43/43

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

GEk1532 Receptors

Documents

Transcript of GEk1532 Receptors