1. Physics 1230: Light and Color Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Phone:
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
Class # 22
Chapter #9 and 10

2. Announcements Good job on the 2nd Midterm Exam; Average ~80
Exam Results Posted;
Enjoyed the science Lab Tour?
Finish Chapter 10 ;
New exciting developments in science of Light & Color
Move to Chapters 11 and 13.

3. What final grade I can get given my Exam #1 and Exam #2 grades?
One exam is dropped;
Example: you can get A if you had 90 points and more on at least one of the first two exams;
Questions?

4. When should we have the 3rd exam?
(A) Week of final exams, as scheduled;
(B) Regular class time on Thursday during the week before final exams;
(C) Other time during the week before finals;
(D) Other time during the week of finals.

6. A colored filter subtracts colors by absorption.=
Cyan
filter subtracts
red
Yellow
filter subtracts
blue
Incident white light
Only green
gets
through

9. Demonstration
Compare to LC display

10. First: Non-spectral colors (mixtures)
Brown = red + green
Cyan (turquoise) = blue + green
Silver (shiny gray)
Magenta (red + blue)
Pink = blue + red + white
Gold
Plum
Tan
White (red + blue + green) ?
Many purples are non-spectral

11. We can distinguish a million of different colors!!!

12. A transmittance curve
Just as a reflectance curve shows what percent of each of the wavelengths in white light reflect from this magenta surface
A transmittance curve shows what percent of each of the wavelengths are transmitted through this magenta filter
Reflectance
Transmittance

13. What color does this spectral curve look like?
Answer: White
400 nm nm nm nm
100%

14. What color does this spectral curve look like?
Answer: Gray
400 nm nm nm 700 nm
100%

15. What color does this spectral curve look like?
gray + red = pink
400 nm nm 600 nm 700 nm
100%

16. Cyan and magenta are most important nonspectral colors
Cyan = blue + green
Magenta = red + blue

17. Spectral curves for nonspectral colors
Magenta = Red + Blue
400 nm nm nm nm
100%

18. We have three different kinds of cones — whose responses are mainly at short, intermediate and long wavelengths
s-cones absorb short wavelength light best, with peak response at 450 nm (blue)
L-cones absorb long wavelength light best, with peak response at 580 nm (red)
i-cones absorb intermediate wavelengths best, with peak response at 540 nm (green)
i-cones
L-cones
s-cones
Spectral response of cones in typical human eye
relative response

19. 100%
400 nm nm nm nm
Which cone(s) are most stimulated by light that appears magenta (450 nm nm) S M L
S and M
L and S

20. Complementary colors
The complement is the color you would add to get white.
Since R + G + B = White, then
cyan
Complement of Red = G + B = cyan
Complement of Green = ?
A) MAGENTA B) RED C) BLUE

21. White = Red + Blue + Green
Color “math”
Now we have all the tools to add color
Magenta = Red + Blue
M = R + B
Cyan = blue + green
C = B + G
Yellow = Red + Green
Y = R + G
White = Red + Blue + Green
W = R + B + G
Complement Corollaries:
W = M + G W = C + R W = Y + B

22. Color “math” – you try it
Use worksheet and these definitions:
M = R + B
C = B + G
Y = R + G
W = R + B + G

23. Concept Question:
White is an equal mixture of red, green and blue. What is another metamer for white light? A. Red and cyan; B. Cyan, magenta and yellow; C. Blue and yellow; D. A,B, and C

24. Why these “complements”?
Blue excites S. Complement excites M and L = Yellow.
Red excites L. Complement excites S and M = Cyan.
Green excites “M”. Complement excites S and L = Magenta.
Cyan = overlap of S and M. Stimulates both S and M. So Cyan + Red = white.
Yellow = cross of M and L. Stimulate both, add S (blue), get white
Cyan
Yellow

25. What happens if a person is missing one type of cone?
Spectral response of cones in protanopic eye
relative response
Missing one type of cone results in one type of color-blindness
Dichromats

26. What happens if a person is missing 2 (or all 3) types of cones?
Missing 2 or all 3 type of cones results in a different (rare) type of color-blindness called monochromacy
Cone monochromats have only one type of cone (s, i or L).
Rod monochromats have no cones and have difficulty seeing with their rods under bright light (photopic) conditions
Monochromats
Truly color-blind because they cannot distinguish any wavelength color from any other
They see in blacks & whites

27. Concept question
Can rod monochromats distinguish red color from green color?
A. Yes
B. No
C. Only during a bright day;
D. Only during the night;

28. The four psychological primaries
In addition to the additive primaries (RGB) and the subtractive primaries (CMY) there is another set of (4) primary colors, called the psychological primaries
Blue
Green
Yellow
Red (closer to magenta)
These hues can be used to describe all other hues.
All hues can be verbally described as combinations of these colors. For example,
Yellowish red
Greenish yellow
Bluish green
Bluish red
BUT we don't recognize hues such as
Reddish green
Yellowish blue
Red and green are opponent hues
Yellow and blue are opponent hues
Note, nonwavelength hues along the bottom line connecting the horseshoe ends in the chromaticity diagram can also be described by these hues. Not shown in Fig

29. We can verify color naming of hues in terms of the psychological primaries on the chromaticity diagram
Opponent nature of
red vs green (r-g) perception
yellow vs blue (y-b) perception
Greenness &
yellowness
Greenness &
blueness
Redness &
yellowness
Redness &
blueness
Note that pink, for example has the same hue as red or red-blue. However it is desaturated.

30. How can this "wiring" work to produce the chromatic channels?
The neural cell for the y-b chromatic channel has its signal
inhibited when (bluE) light excites the s-cone INTERPRETED AS BLUE
enhanced when light excites the i & L cones INTERPRETED AS YELLOW
The neural cell for the r-g chromatic channel has its signal
inhibited when (green) light falls on the i-cone INTERPRETED AS GREEN
enhanced when light excites the s and L cone INTERPRETED AS MAGENTA (Psychological red)
The neural cell for the achromatic channel has its signal enhanced when light excites any of the cones
s-cone
i-cone
L-cone  + +  + + +
neural cell for y-b chromatic channel
neural cell for r-g chromatic channel
neural cell for w-blk achromatic channel
Electrical signal to brain

31. We learned: how cone-neural cell "wiring" works to produce the chromatic channels
The neural cell for the y-b chromatic channel has its signal
inhibited when (bluE) light excites the s-cone INTERPRETED AS BLUE
enhanced when light excites the i & L cones INTERPRETED AS YELLOW
The neural cell for the r-g chromatic channel has its signal
inhibited when (green) light falls on the i-cone INTERPRETED AS GREEN
enhanced when light excites the s and L cone INTERPRETED AS MAGENTA (Psychological red)
The neural cell for the achromatic channel has its signal enhanced when light excites any of the cones
s-cone
i-cone
L-cone  + +  + + +
neural cell for y-b chromatic channel
neural cell for r-g chromatic channel
neural cell for w-blk achromatic channel
Electrical signal to brain

32. More systematic descriptions of color-blindedness (no need to memorize terminology)
Monochromacy (can match any colored light with any 1 spectral light by adjusting intensity)
Either has no cones (rod monochromat) or has only 1 of the 3 types of cones working (cone monochromat).
Sees ony whites, greys, blacks, no hues
Dichromacy (can match any colored light with 2 spectral lights of different intensities of (rather than the normal 3)
L-cone function lacking = protanopia
i-cone function lacking = deuteranopia
s-cone function lacking = tritanopia
no y-b channel but all 3 cones OK = tetartanopia
Anomalous trichromacy
Protanomaly
Shifted L-cone response curve
Deuteranomaly (most common)
Shifted i-cone response curve
Confusion between red and green.
Tritanomaly
Yellow-blue problems: probably defective s-cones
Neuteranomaly
ineffective r-g channel

33. Visualizing dichromacy: protanopia
No L-cone function
See yellows & blues instead of reds & greens
y-b
r-g

34. Visualizing dichromacy: deuteranopia
i-cone function lacking
Like protanopes, they see yellows & blues instead of reds & greens
y-b

35. Visualizing dichromacy: tritanopia
s-cone function lacking
They see reds and greens instead of blues and yellows
r-g

36. Take the color blindness test
The color blindness test consists of a set of five charts. Each chart shows a number in one color on a different backgound color.
People with normal color vision will have no problem seeing the numbers on the charts, but people with color blindness will see only random colored dots.

37. Receptive field of a double-opponent cell of the r-g type
2 different ways to INCREASE the signal the ganglion cell sends to brain
Red light falling on cones in center of receptive field attached to ganglion cell
Green light on surround
2 different ways to decrease the signal the ganglion cell sends to the brain
Red light on surround
Green light on center
Electrical signal to brain from ganglion cell is at ambient level when no light is on center or surround
When signal to brain is INCREASEDwe interpret that as red
When signal to brain is decreased we interpret that as green
signal to brain

38. We can summarize this by just showing the center & surround of the receptive field and indicating the effect of red (R) and green (G) on each
A double-opponent cell differs from a single opponent cell
In both of them R in the center increases the signal
In a single-opponent cell G in surround would inhibit signal, whereas in double-opponent cell G enhances
In a double-opponent cell
R in center enhances signal (ganglion cell signals red)
G in surround enhances signal (ganglion cell signals red)
R in surround inhibits signal (ganglion cell signals green)
G in center inhibits signal (ganglion cell signals green)
Fictional cell
real cell

39. Effect of red or green light falling in various combinations on the center or surround of a double-opponent r-g cell
Strongest signal (interpreted as red)
Weakest signal (interpreted as green)
No change in signal (color not noticed)
No change in signal (color not noticed)
Note, you would still "see" red if the center were grey!
Note, you would still "see" green if the center were grey!

40. y-b double-opponent receptive fields and cells work the same way
Strongest signal (interpreted as yellow)
Weakest signal (interpreted as blue)
No change in signal (color not noticed)
b+y-
Note, you would still "see" blue if the center were grey!
Note, you would still "see" yellow if the center were grey!
y+b-

41. Concept Question:
What is effect of red light falling on both the center AND surround of the r-g receptive field?
No color
Sensation of red
Sensation of green
Sensation of yellow

42. Concept Question:
What is effect of green light falling on surround only?
No color
Sensation of red
Sensation of green
Sensation of yellow

43. Concept Question:
What is the effect of green light falling on surround and red light falling on the center of the receptive field?
No color
Sensation of red
Sensation of green
Sensation of yellow

44. Concept Question:
What is the effect of blue light falling on surround of receptive field only?
No color
Sensation of blue
Sensation of green
Sensation of yellow
Sensation of red

45. Here is an optical illusion which can be explained by double-opponent retinal fields and cells
Look at the grey squares in your peripheral vision
Does the grey square surrounded by yellow appear to take on a tint?
What color is it?
Repeat for the grey squares surrounded by
Blue
Green
Red (pink)

46. Color constancy depends on double-opponent processing
Color constancy means we see the proper colors of a picture or scene or object relatively correctly even though the overall illumination may change its color
Color constancy developed in the evolution of mankind so that we could recognize colorful things in broad daylight, late afternoon, and early evening
No change in signal (color not noticed)

47. Illustration of how the three opponency channels work in your perception of the design below
Here are the enhanced edges resulting from your y-b chromatic channel
Here are the enhanced edges resulting from your r-g chromatic channel
Here are the enhanced edges resulting from your wt-blk achromatic channel

48. The artist Van Gogh knew how to use the opponency of yellow and blue to enhance each of them
Note also that we use yellow letters against a blue background in these notes for emphasis.
Red would be less effective than yellow because it is not an opponent to blue

49. Negative afterimages occur when you stare at an image for a long time without moving your eyes
1 Conditions for negative afterimages
o Prolonged stimulation by an image on the retina adapts or desensitizes part of retina.
That part of retina has a weaker response to subsequent stimulation.
Negative afterimages are a temporal version of lateral inhibition.
Try it in home;