Chapter 5 Flashcards
3 steps to color perception:
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.
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.
Color
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
COLOR of a surface depends on….
the mix of wavelengths that reach the eye from that surface.
In the electromagnetic spectrum, we perceive light of a wavelength of 700 nm as ___ .
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
Mostly red light emitted:
Heatlamp & Candle
(only red)
Halogen, Maglight, & Incandescents
(sloped up towards red end)
Basically all wavelengths (colors) of light are emitted in ….
Daylight
Mostly green light (intermediate wavelengths) emitted:
Standard Fluorescent
(green with some dark blue)
Lab Fluorescent, LCD
(green with some orange)
Only violet light
Blacklight Fluorescent
Strong Orange, with a lot of green and blue
Cathode Ray Tube TV
The light coming from an object is composed of ….
…a distribution of different wavelengths
Reflectance Curve:
Proportion of light at different wavelengths that is reflected from a pigment.
Scotopic
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
All rods have same sensitivity to wavelength, making it impossible to….
discriminate light
Scotopic vision
with rods only:
The moonlit world
moonlit world appearing to be drained of color
Photopic
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.
Cone photoreceptors: Three varieties
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.
blue cones
S-cones
(420 nm):
Cones that are preferentially sensitive to short wavelengths
green cones
M-cones (535 nm):
Cones that are preferentially sensitive to middle wavelengths.
red cones
L-cones (565 nm):
Cones that are preferentially sensitive to long wavelengths.
Never put dark blue text on a dark background.
…
Problem of univariance:
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.
Show single photoreceptor’s response to lights of different wavelength.
M-cone example.
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.
Lights of 2 different wavelengths can produce same response from photoreceptor.
PLUS, it is actually an INFINITY of possible stimulations giving rise to the same response!!!
Trichromacy
Trichromatic Color Theory
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.
two wavelengths that produce the same response from one type of cone produce different types of responses across three types of cones
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.
Color Discrimination….
With 3 cone types, we can tell the difference between lights of different wavelengths!
Color space
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!
Color space:
A 3-dimensional space that describes all colors.
There are several possible color spaces:
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.
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
Saturation: The chromatic strength of a hue
Brightness: The distance from black in color space
CMYK color Space
Cyan, Magenta, Yellow and Black.
Used by printers.
Hue
The chromatic (color) aspect of light
Saturation
The chromatic strength of a hue
How much hue in light.
White = zero saturation.
Brightness
The distance from black in color space
physical intensity of light
Whole shape represents all visible colors.
Triangle represents what a computer monitor can reproduce.
response curves from 2 cones (M and L)…
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.
Metamers
Different mixtures of wavelengths that look identical.
More generally, any pair of stimuli that are perceived as identical in spite of physical differences.
Additive color mixture
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.
Subtractive color mixture
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.
2 warnings:
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.
Mixing wavelengths does not change the physical wavelengths!
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.
The visual system begins by picking up light from the environment
- 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
additive
Lights
LCDs
subtractive
Surfaces
Filters
Pigments
Filters are subtractive because they absorb light
Different filters absorb different wavelengths
“Blue” light + “Green” light + “Red” light =
“White” light
If we shine “blue” and “yellow” lights on the same patch of paper….
the wavelengths will add, producing an additive color mixture
When a pattern of non-overlapping blue & yellow pigments is blurred….
…the resultant mixture is additive (gray) as opposed to subtractive (green).
Pointillism
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.
George Seurat
A Sunday Afternoon on the Island of La Grande
La Parade, 1889
Pop culture
Roy Lichtenstein
Pointillism
Opponent color theory
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
Ewald Hering
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
Hue cancellation experiments
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
We can use the hue cancellation paradigm 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
For instance: unique blue is a blue that has no red or green tint
The 3 steps of color perception, revisited
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
Color in the Visual Cortex
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.
double-opponent color
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
Yellow
L+M responsecontrasted against S response
Afterimages
A visual image seen after a stimulus has been removed
Negative afterimage
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.
Does everyone see colors the same way? — Mostly Yes
General agreement on colors
Some variation due to age (lens turns yellow)
Does everyone see colors the same way? — No
About 8% of male population, 0.5% of female population has some form of color vision deficiency: Color blindness
Dichromats (M 2.4%, F 0.03%)
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%)
Anomalous Trichromats (M 6.3%, F 0.37%)
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%)
Anomalous Trichromats (M 6.3%, F 0.37%)
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%)
Color Blindness is sex linked
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
Achromatopsia
An inability to perceive colors that is due to damage to the central nervous system
Tetrachromacy
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.
