intro to color vision

spectral composition

a light source made of certain amounts of light at one or many wavelengths - sunlight

metamer

stimuli appear same but physically different

making a match

ability to make two stimuli appear identical.

what indicates a color defect with making a match

indicates a defect in color vision if fewer than normal number of primaries needed to make a match

peak absorption of L cones?

557nm

peak absorption of M cones?

530nm

peak absorption for S cones?

426nm

peak absorption for rods?

507nm

what cones are not present in central fovea?

S cones are not found in central 0.3 degrees (20arcminutes) of fovea

wavelength visual system is most sensitive to?

yellow-green, 550-620nm

lamda function/photopic spectral sensitivity

curve represents addition of L and M cone absorption spectra

which photoreceptors contribute to lambda function?

M and L cones

2 theories of color vision?

Trichromatic and Color Opponent

trichromatic theory

color is coded by relative activities of three different types of cones

opponent color theory

herring. states certain color pairs are not seen simultaneously.

observed chromatic stimulus produces an afterimage of complementary color

hue is predicted from?

color opponent cells

brightness is predicted from?

non color opponent cells

saturation is predicted from?

ration of opponent to non opponent cell activity

univariance

once a photopigment absorbs quantum of light, all info of wavelength is lost. (forget)

primary

light (stimulus wavelengths) combined to make a match to a standard reference light.

monochromat

only need one primary to match all other lights in a color matching experiment.

all cone receptors have same pigment, no color discrimination based on wavelength alone

dichromat

two primaries to match color. have two different photopigments with overlapping abs spectra, limited wavelength discrimination

trichromat

three primaries to match color. most humans are trichromats with overlapping abs spectra, superior wavelength discrimination for all wavelengths

monochromat example

rod monochromic

blue (s) cone monochromacy

fully dark adapted (only rod vision)

how do monochromatic name colors correctly?

use brightness information

how is color vision wavelength discrimination in dichromate compared to monochromat

they have superior wavelength discrimination.

why is it misleading to refer to cones as blue/red/green receptors

all receptors are light receptors and are sensitive to some degree across the entire visible spectrum. respond to wavelengths.

normal trichromatic when viewing wavelength longer than 520?

S cones become so insensitive compared to L and M cones. behaves like a dichromat

anomalous trichromat

normal

how does anomalous trichromat become like dichromate?

-light spectrum is limited to long and middle

-tiny light stimulus falls within 0.3 degrees of central fovea (s cones absent)

when does anomalous trichromat become monochromat?

fully dark adapted and vision governed by rods

wavelength discrimation ability of normal trichromat

varying ability to distinguish wavelength across spectrum.

better distinguish one wavelength from another in certain regions of spectrum compared to others

why is discrimination curve called w curve of color discrimination?

plot forms w shape

regions of best wavelength discrimination of normal trichromatic?

495 (B/G) and 590 (orange)

two stimuli are identified as different hues even if wavelength are separated by one nanometer

region of spectrum of pure hue/wavelength discrimination ?

short wavelength region - 410nm can be perceived as same hue as 415 nm

how does region of best wavelength discrimation relate to cone absorption spectra sensitivity?

color discrimination best where slopes of cone absorption spectra change most rapidly with respect to another.

How is color processes by bipolar cells according to hering's color opponent theory?

color processed by bipolar hue channels.

R/G channel and Y/B channel can only signal one at a time.

at bipolar cells in primates, three class of cones are wired together to produce specially opponent neurons (very early in retina)

how does red and blue chromatic stimulus elicit afterimage?

when one component of an opponent pair like red is simulated, its opponent green is inhibited. prolonged stimulation leads to adaptation/fatigue caused rebound where inhibited opponent mechanism becomes more active when stimulus is removed, perception of complementary color as afterimage

evidence electrophysiolocal study demonstrate to color opponent?

in retina and LGN, reveals receptors information (trichromacy) encoded in opponent fashion at post receptor levels

where are color opponent neurons present in visual system

retina and LGN

What channels to L and M cones contribute to?

Red-Green opponent pathway, carried through midget bipolar and midget ganglion cells

channel S cone contributes to?

Blue-Yellow cells, carried through S con bipolar and bistratified ganglion cells (S-L + M)

saturation in terms of bipolar cells?

ratio of activity of opponent color to noncolor opponent cells

double duty hypothesis?

opponent color neurons ode both hue and brightness information

color information parvo cells manifest?

red-green

color information kongo cells manifest?

yellow-blue

color information mango cells manifest?

noncolor opponent, primarily luminance

color opponent channel if polar has excitatory on center L cone and inhibitory off surround M cone feed?

Red-Green opponent channel

color opponent channel if on center M cone and Off surround L cone feed?

Red-Green opponent channel

color opponent channel if on center S cone and off surround (L+M) cone feed?

blue-yellow opponent channel

CIE

Commission Internationale de L'Eclairage

what is CIE based on?

trichromacy of vision

what does CIE diagram specify

amounts of imaginary primaries necessary to match a color sample

are CIE real?

they are imaginary. mathematical transformation of real primaries. ensures all wavelengths can be matched with positive quantities of primaries. overcome issue of negative quantities of primaries.

RGB system of color space?

uses real primaries. n

negative aspect of RGB

to make color match, negative quantities of primary required for wavelength between 450 - 550 nm

XYZ?

Tristimulus values

xyz?

chromaticity coordinates

why z not plotted?

x+y+x = 1, z can be derived from x and y

tristumulus values obtained how?

have observers view one half of a test reference, and other half projects three primary colors (wavelengths) intensities of three primaries adjusted to match wavelengths

tristumulus value and chromaticity relationship

chromaticity coordinates are derived from tristimulus values by dividing tristumulus values by their sum

cie chromaticity diagram constructed from tristimulus values?

convert tristimulus values into chromaticity coordinates and plot x and y coordiantes

spectral hues in CIE?

along arc of perimeter of diagram

non spectral hues in IE?

straight light connecting 380 and 700 nm

chromaticity coordinate of equal energy white in CIE?

CENTER OF DIAGRAM.

planckian locus

various standard illuminants considered variants of white fall on this arc

how CIE determine color mixtures?

resulting color will lie on straight line that connects chromaticity coorinates of two colors

how are dominant wavelength determined given chromaticity coordinates of color?

Draw a straight line from the white point (W) through the color's point (M) and extend it to the spectral locus (perimeter). The wavelength at the intersection is the dominant wavelength.

how are excitation purity determined from chromaticity?

This is the relative distance of the color sample along the line from white (W) to the spectral locus. It's calculated as the ratio of the distance from W to the sample (segment 'a') to the total distance from W to the spectral locus through the sample (segment 'a+b').

MacAdam discrimation ellipses?

perceptual areas within CIE where all colors appear the same to the human eye. represent JND for changes in hue and saturation for a color.