sci 200 physical science lecture 9 color & color vision
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SCI 200 Physical Science Lecture 9 Color & Color Vision. Rob Daniell July 21, 2011. Psychological Color Subjective Indirectly measurable Based on the response of cones and subsequent processing Ganglions Brain. Physical color Objective Directly measurable Based on wavelength - PowerPoint PPT PresentationTRANSCRIPT
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
NEiA SCI 200 Lecture 9 3revised 20 Jul 2011
Physical Color
Electromagnetic SpectrumColor vs. wavelength
NEiA SCI 200 Lecture 9 4revised 20 Jul 2011
Physical Color
Electromagnetic Spectrum Intensity vs. wavelength
NEiA SCI 200 Lecture 9 5revised 20 Jul 2011
Physical Color
Spectroscope Light source Entrance slit Dispersive element (prism or grating) Screen or detector
NEiA SCI 200 Lecture 9 6revised 20 Jul 2011
Physical Color
Simplified Grating Spectroscope Project STAR Spectrometer Transmission grating Adjustable scale Do not point directly at sun
NEiA SCI 200 Lecture 9 7revised 20 Jul 2011
Physical Color
Diffraction Grating
NEiA SCI 200 Lecture 9 8revised 20 Jul 2011
Physical Color
Simplified Grating Spectroscope Project STAR Spectrometer Transmission grating Adjustable scale Do not point directly at sun
NEiA SCI 200 Lecture 9 9revised 20 Jul 2011
Physical Color
Wavelength spectra of various light sources Intensity units are relative Gilbert and Haeberli [2007] Am. J. Phys., 75, 313-319.
NEiA SCI 200 Lecture 9 10revised 20 Jul 2011
Physical Color
Wavelength spectra of fluorescent light bulbs As seen through Project STAR Spectrometer
NEiA SCI 200 Lecture 9 11revised 20 Jul 2011
Physical Color
Discrete spectrumHelium
NEiA SCI 200 Lecture 9 12revised 20 Jul 2011
Physical Color
Discrete spectrum: more examplesHydrogen, Sodium, Helium, Neon, Mercury
NEiA SCI 200 Lecture 9 13revised 20 Jul 2011
Physical Color
Continuous spectrumWhite light, sunlight, etc.
NEiA SCI 200 Lecture 9 14revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 15revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 16revised 20 Jul 2011
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”
NEiA SCI 200 Lecture 9 17revised 20 Jul 2011
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)
NEiA SCI 200 Lecture 9 18revised 20 Jul 2011
Color Vision
Another representation of HSL
NEiA SCI 200 Lecture 9 19revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 20revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 21revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 22revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 23revised 20 Jul 2011
Trichromacy Where do the curves
cross?This requires exploring
the properties of psychological color
NEiA SCI 200 Lecture 9 24revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 25revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 26revised 20 Jul 2011
Trichromacy Where do the curves
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
NEiA SCI 200 Lecture 9 27revised 20 Jul 2011
Trichromacy Where do the curves
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
NEiA SCI 200 Lecture 9 28revised 20 Jul 2011
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)
NEiA SCI 200 Lecture 9 29revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 30revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 31revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 32revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 33revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 34revised 20 Jul 2011
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)
NEiA SCI 200 Lecture 9 35revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 36revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 37revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 38revised 20 Jul 2011
Trichromacy• Where does “yellow” come from?
NEiA SCI 200 Lecture 9 39revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 40revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 41revised 20 Jul 2011
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”
NEiA SCI 200 Lecture 9 42revised 20 Jul 2011
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?
NEiA SCI 200 Lecture 9 43revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 44revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 45revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 46revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 48revised 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
NEiA SCI 200 Lecture 9 49revised 20 Jul 2011
Genetics of Color Vision Scientific American, April
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
NEiA SCI 200 Lecture 9 50revised 20 Jul 2011
Genetics of Color Vision Scientific American, April
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
NEiA SCI 200 Lecture 9 51revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 52revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 53revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 54revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 55revised 20 Jul 2011
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.”
NEiA SCI 200 Lecture 9 56revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 57revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 58revised 20 Jul 2011
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.
NEiA SCI 200 Lecture 9 59revised 20 Jul 2011
Genetics of Color Vision Genetics of and evolution of color vision
Subject of ongoing research Very complex system with lots of threads to unravel
NEiA SCI 200 Lecture 9 60revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 61revised 20 Jul 2011
Color Vision Problems
• 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)
NEiA SCI 200 Lecture 9 62revised 20 Jul 2011
Color Vision Problems
• 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
NEiA SCI 200 Lecture 9 63revised 20 Jul 2011
Color Vision Problems
• 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
NEiA SCI 200 Lecture 9 64revised 20 Jul 2011
Color Vision Problems
• 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.
NEiA SCI 200 Lecture 9 65revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 66revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 67revised 20 Jul 2011
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
NEiA SCI 200 Lecture 9 68revised 20 Jul 2011
Homework & Lab
Read Chapter 6 in textbookHomework Packet 9:
Due Thursday, July 28Next Lab: Lab 6: Water Prism
Thursday, July 21