SCI 200 Physical Science Lecture 9 Color & Color Vision

SCI 200 Physical Science Lecture 9

Color & Color Vision

Rob DaniellJuly 21, 2011

NEiA SCI 200 Lecture 9 2revised 20 Jul 2011

Physical vs. Psychological Color

Psychological ColorSubjectiveIndirectly

measurableBased on the

response of cones and subsequent processingGanglionsBrain

• Physical color• Objective• Directly

measurable• Based on

wavelength• Any “color” can be

defined by the relative intensity of light at each wavelength


Physical Color

Electromagnetic SpectrumColor vs. wavelength


Physical Color

Electromagnetic Spectrum Intensity vs. wavelength


Physical Color

Spectroscope Light source Entrance slit Dispersive element (prism or grating) Screen or detector


Physical Color

Simplified Grating Spectroscope Project STAR Spectrometer Transmission grating Adjustable scale Do not point directly at sun


Physical Color

Diffraction Grating


Physical Color

Simplified Grating Spectroscope Project STAR Spectrometer Transmission grating Adjustable scale Do not point directly at sun


Physical Color

Wavelength spectra of various light sources Intensity units are relative Gilbert and Haeberli [2007] Am. J. Phys., 75, 313-319.


Physical Color

Wavelength spectra of fluorescent light bulbs As seen through Project STAR Spectrometer


Physical Color

Discrete spectrumHelium


Physical Color

Discrete spectrum: more examplesHydrogen, Sodium, Helium, Neon, Mercury


Physical Color

Continuous spectrumWhite light, sunlight, etc.


Monochromatic vs.Non-monochromatic Colors

Monochromatic colors:Consist of a single wavelengthSometimes called “spectral colors”

Non-monochromatic colors:1. A discrete spectrum

several discrete wavelengths 2. A continuous spectrum

Most colors in nature are non-monochromaticExample: sunlight


Psychological Color• Physical color as perceived by the

human eye and brain• Color perception is mediated by the

cones in the retina• There are (usually) three kinds of cones

operating– Each cone type responds differently to a specific physical

color– The signals from the cones are processed in a non-intuitive

way to produce the sensation of color


Psychological Color• Color specification systems:

– HSV: Hue, Saturation, Value • Also:

– HSL (hue, saturation, lightness)– HSB (hue, saturation, brightness)

• Corresponds most closely to human color perception• Preferred by many artists

– RGB: (Red, Green, Blue)• Used in additive color systems• Used in many digital graphics applications

– Displays– Software

– CMYK: (Cyan-Magenta-Yellow-blacK)• Used in subtractive color systems• Used for printing inks, etc.

– “Four Color Printing”


Color Vision• HSV: cylinder

Hue:perceived

color0°-240°240°-360°

(“purples”)Saturation:

Purity of color0-1

Value:Light intensity0-1 or black to

white (brightest)


Color Vision

Another representation of HSL


Trichromacy

HistoryThomas Young (1773-1829)

Observed that it only takes three quantities (Hue, Saturation, Value) to specify a color

Three output quantities require three input quantities

Postulated three kinds of photoreceptors


Trichromacy History (continued)

Hermann von Helmholtz (1821-1894)Suggested that Young’s

three photoreceptors were Short wavelength Intermediate wavelength Long wavelength

Must overlap Monochromatic light of

different wavelengths have different colors


TrichromacySuppose there were

no overlap:Monochromatic light

would appear to consist of exactly three colors

400 500 700

Response

600

For example, (above) any monochromatic light source between 400 and 500 nm would appear blue. Yet we know that 450 nm light is a very different shade of blue

than 475 nm light

S I L


A. Overlap of Response Curves

Example: six monochromatic emission lines from atomic HeliumEach a different colorConclusion: There

must be at least two overlapping cones at each wavelength in the visible region

Line spectrum of helium (He) Blue-violet: 447.1 nm Blue: 471.3 nm Green: 501.5 nm Orange: 587.5

nm Red-orange: 706.5 nm Dark red: 728.1

nm


Trichromacy Where do the curves

cross?This requires exploring

the properties of psychological color


Trichromacy

• Complementary colors:• R + C W• G + M W• B + Y W

•White can be produced by• Broadband light (e.g.,

sunlight)• Pairs of complementary

colors• Stimulate the three kinds of

photoreceptors “equally”• An infinite variety of other

combinationsC = Cyan, M = Magenta, Y = Yellow

W = White


Color Perception Mechanisms

If Helmholtz is right, how can we determine the actual response curves?A. Overlap of response curvesB. Spectral complementariesC. Hue discriminationD. Microspectrophotometry



cross?Consider a

monochromatic color at about 430 nm.Stimulates S with a little I Another monochromatic

color near 610 nm could stimulate some I and more L to produce white.

Also, vice versa



cross?Note that in the region

where the Intermediate photoreceptors dominate, no single complementary spectral (monochromatic) color existsNo one spectral color can

stimulate both the S and the L photoreceptors equally.

Empirically, this is the region from 490 nm to 565 nm


Trichromacy

So 490 nm and 565 nm represent the crossover points between S and I and between I and L, respectively

Between these wavelengths, it takes two additional monochromatic sources to combine with a “green” source to produce white

A “blue” source and a “red” source - hence “purple” (or magenta) This has consequences for color mixing (Lecture 10)


Trichromacy Hue discrimination:

The difference in wavelength (Δλ, pronounced “delta lambda”) at which two monochromatic sources are barely distinguishable

Varies with wavelength Where Δλ is small, the

photoreceptor response must be changing rapidly

Further Details of the spectral response curves required microspectrophotometry

The physical measurement of the amount of light of each wavelength absorbed by each kind of cone

Although many cones have been measured this way only three basic types have been found


Trichromacy Cone Mosaic:

Simulation based on measured cone densities

No “blue” cones in the central fovea!

Visual acuity in blue light is less than in green and red light

Over the entire retina There are about 100 “red” and

“green” cones for every “blue” cone There are about 150 “red” cones

for every 100 “green” cones However: Much variation among

individuals


Trichromacy

Spectral sensitivity of the three types of cones in the human eye Intensity of each wavelength

is the same There is considerable

overlap among the three cone types

Type II & Type III cones have the same sensitivity at about 560 nm

Figure 6.4 from text


Trichromacy

Spectral sensitivity of Type II (green) cones Two different wavelengths

can produce the same response

Figure 6.5 from text

Using all three types of cones, the four colors can be distinguished. Figure 6.6 from text


Trichromacy• Color vision:

– : 3 kinds of cones• Type I: Short (S), beta (β), or blue (B)• Type II: Intermediate (I), gamma (γ), or green (G)• Type III: Long (L), rho (ρ), or red (R)

Note that the three kinds of cones do not actually correspond to blue, green, and red. The RGB model is merely a convenient means of representing color.


Color Vision

• RGB color system:• Based (loosely) on the three cones of the human eye• Z ~ blue, Y ~ green, X ~ red (even though it peaks shortward of red)


Color Vision

• Additive color rules:• R + G + B = W• R + G = Y• G + B = C• R + B = M

• Complementary colors:• R + C = W• G + M = W• B + Y = W

• Can any 3 colors be combined to produce any other color?• Can R, G, & B be combined to

produce any other color?C = Cyan, M = Magenta, Y = Yellow

W = White


Color Vision

• Red, Green, & Blue can be combined to produce most colors, but some saturated (or nearly saturated) colors cannot be reproduced.• Will be considered in more

detail in Lecture 10

C = Cyan, M = Magenta, Y = Yellow

W = White


Color Vision

• Subtractive color combination:• Filters that absorb or block light

of certain colors• Ink or pigments that reflect only

certain colors and absorb the others

• Primary Subtractive Colors:• Cyan, Magneta, Yellow• Supplemented by Black in “four

color printing”• Will be considered in more

detail in Lecture 10 C = Cyan, M = Magenta, Y = Yellow

K = Black


Trichromacy• Where does “yellow” come from?


Trichromacy or Opponent Colors?

Statements:Magenta looks like a mixture

of Red & BlueCyan looks like a mixture of

Green & BlueYellow looks nothing like a

mixture of Red & Green


Trichromacy or Opponent Colors?

Based on the trichromacy theoryWe should expect an additive mixture of red and green to give a

reddish green (or a greenish red).Instead it gives yellow

In fact, it takes four psychological primaries to verbally describe any color

Blue, green, yellow, and redOrange looks yellowish redCyan looks bluish greenPurple looks reddish blueEtc.


Opponent Processing

When asked to name the color of a spot of spectral (i.e., monochromatic) light, most people give responses similar to those at right

Note that there is no “reddish green” or “yellowish blue”


Opponent Processing

Yellow and blue seem to oppose each other

Red and green also seem to oppose each other

How can the three kinds of cones be wired together to produce this kind of color opposition?


Opponent Processing

S inhibits y-b and stimulates r-g & w-bkI inhibits r-g and stimulates y-b & w-bkL stimulates all three opponent systems


Opponent Processing

Net stimulation of y-b makes the light appear yellowish; net inhibition, bluish

Net stimulation of r-g makes the light appear reddish; net inhibition, greenish

The w-bk channel conveys brightness information


Opponent ProcessingThere are at least two rival theories for the details of

how the three kinds of cones get processed to produce the opponent activity.

One theory makes use of lateral inhibition in the form of center-surround antagonism among the various cones

Another assumes some kind of filter that narrows the wavelength range accessible to some cones but not others.


Genetics of Color Vision Review: basics of human

genetics Each cell in the human body

contains 23 pairs of chromosomes The chromosomes are numbered 1

through 22 plus the X and/or Y chromosome

In each pair, one comes from the mother, the other from the father.

The gender is (mostly) determined by the X and Y chromosomes

Females have 2 X chromosomes, one from each parent

Males have an X chromosome from their mothers and a Y chromosome from their fathers

Other primate species have differing numbers of chromosomes The Great Apes all have

24 pairsGender is generally

determined in the same was as for humans

The genes controlling color vision differ among primate species

NEiA SCI 200 Lecture 9 47

Genetics of Color Vision Color Vision

The number of cone types varies dramatically throughout the Animal Kingdom

Mammals Most mammals have only two types of cones – dichromats

Short vs. long wavelength: Yellow vs. Blue Red-Green color blind

Primates All new world primates are dichromats

But see next slide Many old world primates are trichromats

Especially monkeys, apes, and humans

revised 20 Jul 2011


Genetics of Color Vision Scientific American, April

2009, The Evolution of Primate Color Vision, pp. 56-63. Some Old World primates

(including humans) are trichromats

Gene for the short wavelength (“blue”) cone resides on chromosome 7

Genes for the medium wavelength (“green”) cone and the long wavelength (“red”) cone both reside on the X chromosome



2009, The Evolution of Primate Color Vision, pp. 56-63. New World primates are mostly

dichromats Gene for the short wavelength

(“blue”) cone resides on chromosome 7

Gene for one of the longer wavelength (“green”, “yellow”, or “red”) cones resides on the X chromosome



2009, The Evolution of Primate Color Vision, pp. 56-63. Some female New World

primates are trichromats One X chromosome has one of

the green, yellow, or red cones The other X chromosome has

a different “long wavelength” cone

These females can distinguish colors that their dichromat brothers and sisters cannot


Genetics of Color Vision It appears that “new world”

dichromacy is the ancestral condition: Among old world primates, a

recombination error resulted in both “green” and “red” genes appearing on every X chromosome.

Both males and females became trichromats

Strong selective advantage, so this system became the norm in Old World primates.


Genetics of Color Vision Every cone cell contains the genes for every cone

type Dichromats have two types Trichromats have three types

In any particular cone cell, only one gene is actually expressed Mechanism for selection of which gene to express is not

known For the genes on the X chromosome, it appears that the choice

is random Matrix of “red” and “green” cones is a random distribution

So an individual with both a red and a green cone gene on the X chromosome would have both red and green cones.


Genetics of Color Vision Having both red and green cones would give

the individual a strong survival advantage It would be much easier to distinguish ripe fruits

(yellow, orange, etc.) from unripe fruits (green) It would be much easier to distinguish some

predators (e.g., a leopard with a tawny coat) from the leaves or bushes (green) in which it was hiding.

The selection pressure was so strong that trichromats have completely displace dichromats among Old World monkeys and apes – and, of course, humans


Genetics of Color Vision Implications for “opponent processing”

If the ancestral color processing was dichromacy: It probably involved the opposition of blue cones and the

“yellow” (i.e., longer wavelength) cones The advent of trichromacy with the simultaneous

appearance of red and green cones on the X chromosome made a second color opposition possible

Red vs. green Thus, the psychological colors - blue, green, yellow, and red

- may have arisen naturally from the basic distinction between blue cones on the one hand and red, green, and yellow cones on the other.

Yellow being a synthesis of the red and green cones


Genetics of Color Vision The ability to distinguish between “blue” cones and the other

kinds of cones appears to be “hardwired” into the brain The ability to distinguish between “red” and “green” cones

appears to be “learned.” Female mice that have been genetically engineered to have a

“green” cone on one X chromosome and a “red” cone on the other can learn to distinguish hues that are indistinguishable to their dichromatic relatives

There is evidence that the neural circuitry for distinguishing “red” and “green” cones is the same as that used for spatial vision Detecting boundaries, etc. If so, then “trichromacy can be viewed as a “hobby” of the

preexisting spatial vision system.”


Genetics of Color Vision

• The trichromatic theory of color vision is based on the three types of cones

• However, it has recently been discovered that some people have a rare “yellow” cone– Similar (identical?) to the “yellow” cone in New

World Monkeys


Genetics of Color Vision

• For males, with only one X chromosome– Standard trichromat: red, green, blue– Non-standard trichromat (rare)

• Red, yellow, blue• Yellow, green, blue

• For females with two X chromosomes– Standard trichromat: red, green, blue– Tetrachromat: red, green, yellow, and blue

• Still rare, but different


Genetics of Color Vision Implications for “tetrachromacy” in some

women. If distinguishing between “red” and “green” cones

is learned, perhaps distinguishing among “red, yellow, and green” cones is also learned

Unfortunately, so far vision tests have not produced conclusive evidence for true tetrachromatic vision, but research is ongoing.


Genetics of Color Vision Genetics of and evolution of color vision

Subject of ongoing research Very complex system with lots of threads to unravel


Color Vision Problems

• There are various kinds of color vision “anomalies” or “deficiencies”• Some are sex linked, since they involve the red

and green (and yellow?) cones on the X-chromosome

• Some are more general genetic anomalies



• Monochromats: People who see only one color• Relatively rare• Two main types:

• Cone monochromats have cones, but only one type is actually functional• Can see under photopic conditions

• Rod monochromats lack all cone function• Have difficulty seeing in bright light• Poor visual acuity (no foveal rods)



• Dichromats: People who see only two colors (and their combinations)• Two subtypes:

• People with only two kinds of functional cones• Three classes, depending on which cone type is nonfunctional

• Protanopia: lacking L (red) cones• Deuteranopia: lacking I (green) cones• Tritanopia: lacking S (blue) cones

• People for whom one of the opponent color systems is not working• Two classes, depending on which of the two opponent color

systems is nonfunctional



• Trichromats: People who see all three colors (and their combinations)• Normal trichromats

• Slight variations in the cone pigments• Anomalous trichromats

• Large variations in cone pigments• Connections between one type of cone and the nerve cells is

defective• Protanomalous, deuteranomalous, tritanomalous variations

recognized• No sharp boundaries, however



• Ishihara “Test for Colour-Blindness” (2 sample plates)• On left: “Normals” see “26”; Protanopes and some protanomalous

observers see only the “6”. Deuteranopes and some deuteranomalous observers see only the “2”

• On right: Many color deficients can see a serpentine path between the two x’s; Normals cannot.


Summary Three variables or quantities are sufficient to

describe any color Trichromacy theory:

Developed during 19th centuryConfirmed and quantified in the 20th centuryThree kinds of cones:

Long, Intermediate, ShortRed, Green, and Blue

Not the whole story: Opponent Color processingSignals from the three kinds of cones are

processed to produce Yellow-Blue and Red-Green opponents


Summary Original Opponent Color processing seems to involve

Blue vs. Yellow (Short vs. “Long”) Still the dominant form of color vision in New World monkeys Also most mammals

In Old World Primates A recombination error enabled Red-Green opponent colors

Apparently “learned” behavior Also enabled three color processing and the rich colors

visible to humans There appear to be three kinds of “intermediate” and

“long” cones “yellow” cones are relatively rare Some women have four kinds of cones

May be able to sense an even richer range of colors


Summary Various kinds of color vision “anomalies” or

“deficiencies” Trichromats: normal color vision

Except some people have deficient cones of one or more colors

Dichromats: can see only two colors Monochromats: only a single color

Some have no cones, so have poor visual acuity and difficulty seeing in bright light


Homework & Lab

Read Chapter 6 in textbookHomework Packet 9:

Due Thursday, July 28Next Lab: Lab 6: Water Prism

Thursday, July 21

Documents

SCI 200 Physical Science Lecture 9 Color & Color Vision