Chapter 5

Question 1

3 steps to color perception:

Answer 1

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

Discrimination: We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another

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.

Question 2

Detection

Answer 2

Wavelengths of light must be detected in the first place.

Question 3

Discrimination

Answer 3

We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another.

Question 4

Appearance

Answer 4

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.

Question 5

Color

Answer 5

Not a physical property, but rather a psychophysical property.

Most of the light we see is reflected.

Typical light sources:
Sun, light bulb; emit a broad spectrum of wavelengths 400–700 nm

Question 6

COLOR of a surface depends on….

Answer 6

the mix of wavelengths that reach the eye from that surface.

Question 7

In the electromagnetic spectrum, we perceive light of a wavelength of 700 nm as ___ .

Answer 7

RED

“There is no red in a 700 nm light, just as there is no pain in the hooves of a kicking horse.” - Steven Shevell

Question 8

Mostly red light emitted:

Answer 8

Heatlamp & Candle
(only red)

Halogen, Maglight, & Incandescents
(sloped up towards red end)

Question 9

Basically all wavelengths (colors) of light are emitted in ….

Answer 9

Daylight

Question 10

Mostly green light (intermediate wavelengths) emitted:

Answer 10

Standard Fluorescent
(green with some dark blue)

Lab Fluorescent, LCD
(green with some orange)

Question 11

Only violet light

Answer 11

Blacklight Fluorescent

Question 12

Strong Orange, with a lot of green and blue

Answer 12

Cathode Ray Tube TV

Question 13

The light coming from an object is composed of ….

Answer 13

…a distribution of different wavelengths

Question 14

Reflectance Curve:

Answer 14

Proportion of light at different wavelengths that is reflected from a pigment.

Question 15

Scotopic

Answer 15

Referring to dim light levels at or below the level of bright moonlight.

Moonlight and extremely dim indoor lighting are both scotopic lighting conditions

Rods are sensitive to scotopic light levels

All rods contain same type of photopigment molecule: Rhodopsin

All rods have same sensitivity to wavelength, making it impossible to discriminate light

Question 16

All rods have same sensitivity to wavelength, making it impossible to….

Answer 16

discriminate light

Question 17

Scotopic vision

Answer 17

with rods only:
The moonlit world

moonlit world appearing to be drained of color

Question 18

Photopic

Answer 18

Light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors

Sunlight and bright indoor lighting are both photopic lighting conditions.

Question 19

Cone photoreceptors: Three varieties

Answer 19

S-cones (420 nm): blue cones. Cones that are preferentially sensitive to short wavelengths.

M-cones (535 nm): green cones.
Cones that are preferentially sensitive to middle wavelengths.

L-cones (565 nm): red cones.
Cones that are preferentially sensitive to long wavelengths.

Question 20

blue cones

Answer 20

S-cones
(420 nm):
Cones that are preferentially sensitive to short wavelengths

Question 21

green cones

Answer 21

M-cones (535 nm):

Cones that are preferentially sensitive to middle wavelengths.

Question 22

red cones

Answer 22

L-cones (565 nm):

Cones that are preferentially sensitive to long wavelengths.

Question 23

Never put dark blue text on a dark background.

Answer 23

…

Question 24

Problem of univariance:

Answer 24

An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor.

One type of photoreceptor cannot make color discriminations based on wavelength.

Question 25

Show single photoreceptor’s response to lights of different wavelength. M-cone example.

Answer 25

Say that the AMPLITUde of the response (y-axis) is a function BOTH of the amplitude of stimulation (how much light there is night vs day) and the COMPOSITION of the wavelentgth (which one). Example of response to 625 nm light. Perception orange.

Question 26

Lights of 2 different wavelengths can produce same response from photoreceptor.

Answer 26

PLUS, it is actually an INFINITY of possible stimulations giving rise to the same response!!!

Question 27

Trichromacy | Trichromatic Color Theory

Answer 27

The theory that the color of any light is defined in our visual system by the relationships between a set of 3 numbers, the outputs of 3 receptor types now known to be the 3 cones. (The Young-Helmholtz theory). Thomas Young (1773–1829) and Hermann von Helmholtz (1821–1894) independently discovered the trichromatic nature of color perception.

Question 28

two wavelengths that produce the same response from one type of cone produce different types of responses across three types of cones

Answer 28

Going back to example, now you can see that the two initial lights now have a different “triplet” of response values… these will lead to VERY different color representations.

Question 29

Color Discrimination....

Answer 29

With 3 cone types, we can tell the difference between lights of different wavelengths!

Question 30

Color space

Answer 30

The 3-dimensional space, established because color perception is based on the outputs of 3 cone types, that describes the set of all colors. [S-response M-response L-response] “3-dimensional space” We can perceive as many as 10 million different colors!

Question 31

Color space: A 3-dimensional space that describes all colors. There are several possible color spaces:

Answer 31

RGB color space: Defined by the outputs of long, medium, and short wavelength lights HSB color space: Defined by hue, saturation, and brightness CMYK color Space: Cyan, Magenta, Yellow and Black. Used by printers.

Question 32

RGB color space

Answer 32

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

Question 33

HSB color space

Answer 33

Defined by hue, saturation, and brightness Hue: The chromatic (color) aspect of light Saturation: The chromatic strength of a hue Brightness: The distance from black in color space

Question 34

CMYK color Space

Answer 34

Cyan, Magenta, Yellow and Black. | Used by printers.

Question 35

Hue

Answer 35

The chromatic (color) aspect of light

Question 36

Saturation

Answer 36

The chromatic strength of a hue How much hue in light. White = zero saturation.

Question 37

Brightness

Answer 37

The distance from black in color space physical intensity of light

Question 38

Whole shape represents all visible colors.

Answer 38

Triangle represents what a computer monitor can reproduce.

Question 39

response curves from 2 cones (M and L)…

Answer 39

GREEN: 80 units of response from M cone, 40 units of response from L cone. Add red: RED: 80 units of response from L cone, 40 units of response from M cone. Total activity in L=120, in M=120 YELLOW LIGHT: M=120 L=120.

Question 40

Metamers

Answer 40

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

Question 41

Additive color mixture

Answer 41

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.

Question 42

Subtractive color mixture

Answer 42

A mixture of pigments. If pigments 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.

Question 43

2 warnings:

Answer 43

Mixing wavelengths does not change the physical wavelengths! In order for a mixture of a red light and a green light to look perfectly yellow, you have to add just the right amount of red and just the right amount of green.

Question 44

Mixing wavelengths does not change the physical wavelengths!

Answer 44

ADDING a wavelength of 500 to one of 600 does not create a wavelength of 550!! Nor the sum 1100. It produces a change in our PSYCHOPHYSICAL REALITY, not in the PHYSICS of the light.

Question 45

The visual system begins by picking up light from the environment

Answer 45

- visible light has a wavelength in the hundreds of nanometers - respond to only a narrow range of light wavelengths - when light reaches an object, part of the light is reflected while part is absorbed - our perception of brightness is based on the intensity of the reflected light that hits our eye - ex. Completely white objects reflect all light while black absorb all light - color of light is called hue - we are attuned to 3 primary colors of light: red, green, and blue - the mixing of these 3 colors through additive color mixing can produce any color - differs from subtractive color mixing such as that with paint or ink

Question 46

additive

Answer 46

Lights | LCDs

Question 47

subtractive

Answer 47

Surfaces Filters Pigments

Question 48

Filters are subtractive because they absorb light

Answer 48

Different filters absorb different wavelengths

Question 49

“Blue” light + “Green” light + “Red” light =

Answer 49

“White” light

Question 50

If we shine “blue” and “yellow” lights on the same patch of paper....

Answer 50

the wavelengths will add, producing an additive color mixture

Question 51

When a pattern of non-overlapping blue & yellow pigments is blurred....

Answer 51

...the resultant mixture is additive (gray) as opposed to subtractive (green).

Question 52

Pointillism

Answer 52

Additive Color Mixing Style of painting developed by the neo-impressionist Georges Seurat, in which additive color mixtures are achieved by visually by placing dots of different colors in close proximity to each other, rather than the subtractive mixtures that are obtained when pigments are mixed together in the same location.

Question 53

George Seurat

Answer 53

A Sunday Afternoon on the Island of La Grande La Parade, 1889

Question 54

Pop culture

Answer 54

Roy Lichtenstein Pointillism

Question 55

Opponent color theory

Answer 55

The theory that perception of color is based on the output of 3 mechanisms, each of them based on an opponency between 2 colors: Red–green, blue–yellow, and black–white Some LGN cells are excited by L-cone onset in center, inhibited by M-cone onsets in their surround (and vice-versa) Red versus green Other cells are excited by S-cone onset in center, inhibited by (L + M)-cone onsets in their surround (and vice-versa) Blue versus yellow

Question 56

Ewald Hering

Answer 56

noticed that some color combinations are legal while others are illegal We can have bluish green, reddish yellow (orange), or bluish red (purple) We cannot have reddish green or bluish yellow

Question 57

Hue cancellation experiments

Answer 57

Start with a color, such as yellowish green The goal is to end up with pure green Shine some blue light to cancel out the yellow light Adjust the intensity of the blue light until there is no sign of either yellow or blue in the green patch

Question 58

We can use the hue cancellation paradigm to determine the wavelengths of unique hues

Answer 58

Unique hue: Any of four colors that can be described with only a single color term: Red, yellow, green, blue For instance: unique blue is a blue that has no red or green tint

Question 59

The 3 steps of color perception, revisited

Answer 59

Step 1: Detection. S, M, and L cones detect light Step 2: Discrimination. Cone opponent mechanisms discriminate wavelengths [L – M] and [M – L] compute red vs. green [L + M] – S and S – [L + M] compute blue vs. yellow Step 3: Appearance. Further recombination of the signals creates final color-opponent appearance

Question 60

Color in the Visual Cortex

Answer 60

Some cells in LGN are cone-opponent cells. These respond to RED-center/GREEN-surround and vice-versa. In primary visual cortex, double-opponent color cells are found for the first time. These are more complicated, combining the properties of 2 color opponent cells from LGN.

Question 61

double-opponent color

Answer 61

In primary visual cortex, double-opponent color cells are found for the first time. These are more complicated, combining the properties of 2 color opponent cells from LGN

Question 62

Yellow

Answer 62

L+M responsecontrasted against S response

Question 63

Afterimages

Answer 63

A visual image seen after a stimulus has been removed

Question 64

Negative afterimage

Answer 64

An afterimage whose polarity is the opposite of the original stimulus Light stimuli produce dark negative afterimages Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice-versa) This is a way to see opponent colors in action There is no way of explaining these after-images with Trichromacy ALONE.

Question 65

Does everyone see colors the same way? — Mostly Yes

Answer 65

General agreement on colors Some variation due to age (lens turns yellow)

Question 66

Does everyone see colors the same way? — No

Answer 66

About 8% of male population, 0.5% of female population has some form of color vision deficiency: Color blindness

Question 67

Dichromats (M 2.4%, F 0.03%)

Answer 67

Protanope: No L-cones (M 1.3%, F 0.02%) Deuteranope: No M-cones (M 1.2%, F 0.01%) Tritanope: No S-cones (M 0.001%, F 0.03%)

Question 68

Anomalous Trichromats (M 6.3%, F 0.37%)

Answer 68

Protonomalous - L-cone (M 1.3%, F 0.02%) Deuteranomalous - M-cone (M 5.0%, F 0.35%) Tritanomalous - S-cone defect (M/F 0.0001%)

Question 69

Anomalous Trichromats (M 6.3%, F 0.37%)

Answer 69

Protonomalous - L-cone (M 1.3%, F 0.02%) Deuteranomalous - M-cone (M 5.0%, F 0.35%) Tritanomalous - S-cone defect (M/F 0.0001%)

Question 70

Color Blindness is sex linked

Answer 70

The genes that produce photopigments are carried on the X chromosome if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome

Question 71

Achromatopsia

Answer 71

An inability to perceive colors that is due to damage to the central nervous system

Question 72

Tetrachromacy

Answer 72

the condition of possessing 4 different types of cone cells. Some Human Females have a normal cone gene on one X chromosome and a mutated cone gene on the other X chromosome. One study suggested that 2–3% of the world's women might have the kind of 4th cone that lies between the standard red and green cones, giving them a significant increase in color differentiation. This finding is still debated.