unit 2 chpt 5 perception of color

three steps to color perception

1. Detection: Wavelengths of light must be detected in the first

place.

2. Discrimination: We must be able to tell the difference between

one wavelength (or mixture of wavelengths) and another.

3. Appearance: We want to assign perceived colors to lights and

surfaces in the world and have those perceived colors be stable

over time, regardless of different lighting conditions.

most of the light we see is_

reflected

color

-does not exist in the physical world (exists only in our brain)

-not a physical property but a psychophysical property

-how brains interpret and differentiate light info

-perception of color is generated internally

-has evolutionary benefits (see animals in jungle, see ripe berries)

benhams disk

moves so fast it make ur brain think there are colors

three types of cone photoreceptors

• S-cones detect short wavelengths (“blue” range).

• M-cones detect medium wavelengths (“green” range).

• L-cones detect long wavelengths (“red” range).

photoreceptors

we have 4 photoreceptors, but 3 contribute to seeing color (the 3 cone photoreceptors)

most sensitive to wavelength. we can differentiate the most diff shades of _.

mid, green

photopic

Light intensities that are bright enough to stimulate the cone

receptors and bright enough to “saturate” the rod receptors to their

maximum responses

-provides color info

ex: sunlight and bright indoor lighting

scotopic

Light intensities that are bright enough to stimulate the rod

receptors but too dim to stimulate the cone receptors.

ex: moonlight and extremely dim indoor lighting

duplex vision

as we go more peripheral we lose the concept of color because there are less cones and as it gets darker we also lose color perception bc our cone system is less sensitive in the dark

the principle of univariance (said this is important)

An infinite set of different wavelength and intensity

combinations can elicit exactly the same response from

a single type of photoreceptor.

• Therefore, one type of photoreceptor cannot make color

discriminations based on wavelength.

as you can see in this picture with one photoreceptor 450 and 625 are different colors and wavelengths but elicit the same response (we are unable to discriminate)

rods are sensitive to _

scotopic light levels

rhodopsin

all rods contain the same photopigment molecule. Therefore, all rods have the same sensitivity to each individual wavelength of light (only one type of rod, all rods are same). Every rod will react the same way to 500 nm of light. Rods obey the principle of univariance and cannot sense diffs in colors.

trichromacy (trichromatic theory of color vision) *says this is important

The theory that the color of any light is defined in our visual

system by the relationships of three numbers, the outputs of

three receptor types now known to be the three cones.

– Also known as the Young-Helmholtz theory

3 cones

_ makes able to differentiate between colors and see colors. most sensitive to green

thomas young and helmholtz

credited for first proposing the trichromatic theory of color perception in humans

metamers

Different mixtures of wavelengths that look identical; more generally, any pair of stimuli that are perceived as identical in spite of physical differences.

james maxwell

developed a color-matching technique that is still being used today

primary colors of light

red, green, and blue

additive color mixing

A mixture of lights

– If light A and light B are both reflected from a surface to the eye, in

the perception of color, the effects of those two lights add together. lights coming together

subtractive color mixing

A mixture of pigments.

– If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of color. absorbing the light, only thing not getting absorbed is what we see. ex. mixing paints, seeing blue shirt in the sun the blue wavelength on the shirt is getting reflected. black absorbs light

lateral geniculate nucleus

has cells that are maximally stimulated by spots of light

cone opponent cell

a neuron whose output is based on a difference between sets of cones

there are color opponent ganglion cells (he said to know)

the antagonistic red-green or blue-yellow centre-surround

color space

3D space that describes all colors.

RGB color space

defined by the outputs of long, medium, and short wavelength lights

HSB color space

defined by hue, saturation, and brightness

hue

the chromatic color aspect of light (what color it is)

saturation

the chromatic strength of a hue

brightness

the distance from black in color space

nonspectral colors

some colors that we see do not correspond to a single wavelength of light (purple)

ewald hering

noticed that some color combinations are “legal” while other are “illegal” (ie we can have bluish green (cyan), but you cant have reddish green or bluish yellow)

opponent colors

red and green/ blue and yellow

opponent color theory

the theory that perception of color depends on the output of three mechanisms, each of them based on an opponency btw two colors: red-green, blue-yellow, and black-white

keeps saying opponent color theory is important

ewald hering made this happen

hue cancellation experiments

used to determine the wavelengths of unique hues

unique hue

any of four colors that can be described with only a single color term: red, yellow, green, blue

afterimages

a visual image seen after a stimulus has been removed

negative afterimage

an afterimage whose polarity is the opposite of the original stimulus. colors are complementary

achromatopsia

loss of color vision from brain damage

v1, v2, and v4

all involved in color perception, but not exclusively

cultural relativism

in sensation and perception, the idea that basic perceptual experiences may be determined by the cultural environment

linguistic relativism

ppl have diff names for the colors they see

color anomalous

a term that is usually called color blindness, most can still make discriminations based on wavelength

deuteranope

type of color anomalous, due to absence of m cones

protanope

type of color anomalous, due to absence of l cones

tritanope

type of color anomalous, due to absence of s cones

cone monochromat

has only one cone type, truly color blindre

red monochromat

has no cones of any type; turly color blind and very visually impaired in bright light

anomia

inability to name objects or colors in spite of the ability to see and recognize them. typically due to brain damage

color anomia

see color but cannot name it

synethesia

when one stimulus evokes the experience of another stimulus that is not present

color contrast

color perception effect in which the color of one region induces the opponent color in a neighboring region

color assimilation

a color perception effect in which 2 colors bleed into each other, each taking on some of the chromatic quality of the other

unrelated color

a color that can be experienced in isolation

related color

a color such as brown or gray which is seen only in relation to other colors

ex: a grey patch in complete darkness appears white

color constancy

the tendency of a surface to appear the same color under fairly wide range of illuminants

illuminant

the light that illuminates a surface