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2. Chapter 9: How we characterize colors: Hue, Saturation, and Brightness (HSB) What they mean in terms of intensity distribution curves? Hue is specified by the dominant wavelength color in the intensity-distribution curve Saturation is the purity of a color (absence of other wavelengths). The pure spectral colors are the most saturated Brightness refers to the sensation of overall intensity of a color

3. The same color sensation can often be produced by 2 or more different intensity distribution curves Here is an intensity distribution curve which gives us the sensation of yellow Here is a different intensity distribution curve which also gives us the same sensation of yellow The two colors described by the two different intenstiy curves are called metamers

4. Hue, Saturation and Brightness (HSB): One way to use 3 numbers to specify a color instead of using an intensity-distribution curve Color tree (e.g. Fig. 9.5 in book) Moving up the tree increases the lightness of a color Moving around a circle of given radius changes the hue of a color Moving along a radius of a circle changes the saturation (vividness) of a color These three coordinates can be described in terms of three numbers Photoshop: uses H, S and B

6. Red, green and blue (RGB): RGB is another way to use 3 numbers to specify a color instead of using an intensity-distribution curve or HSB In addition to using Hue, Saturation and Brightness (HSB); Many (but not all) colors can be described in terms of the relative intensities of a light mixture of a certain wavelength red, wavelength green and wavelength blue lights 650-nm red 530-nm green 460-nm blue These are called the additive primaries The mixing of the additive primaries is called additive mixing Additive mixing is usually done by mixing primary color lights with different intensities but there are other ways to be discussed later Demonstrate with Physics 2000

7. Complementary additive colors Definition of complementary color (for additive mixtures): The complement of a color is a second color. When the second color is additively mixed to the first, the result is white. Blue & yellow are complementary B + Y = W. Green & magenta are complementary G + M = W Cyan and red are complementary C + R = W Magenta is not a wavelength color� it is not in the rainbow There is at most one wavelength complementary color for each wavelength color (Fig 9.9)

10. Chromaticity diagrams: Yet another way to represent colors by (3) numbers The chromaticity diagram is in many ways similar to a color tree A chromaticity diagram has a fixed brightness or lightness for all colors Wavelength colors are on the horseshoe rim but non-wavelength colors like magenta are on the flat part of the rim Inside are the less saturated colors, including white at the interior

11. Using the chromaticity diagram to identify colors The numbers that we use to identify a color are its x-value and y-value inside the diagram and a z-value to indicate its brightness or lightness x and y specify the chromaticity of a color Example: Apple pickers are told around the country that certain apples are best picked when they are a certaim red (see black dot) Since the chromaticity diagram is a world standard the company can tell its employees to pick when the apples have chromaticity x = 0.57 y = 0.28 The "purest" white is at x = 0.33 and y = 0.33 Chromaticity diagram can be related to colors in Photoshop

12. Using the chromaticity diagram to understand the result of additive mixing of colors An additive mixture of two wavelength colors lies along the line joining them Example: The colors seen by mixing 700 nm red and 500 nm green lie along the line shown Where along the line is the color of the mixture? Answer depends on the relative intensities of the 700 nm red and the 500 nm green. Here is what you get when the green is much more intense than the red (a green) Here is what you get when the red is much more intense than the green (a red) Here is what you get when the red is slightly more intense than the green (a yellow)

13. Using the chromaticity diagram to understand complementary colors The complement to any wavelength color on the edge of the chromaticity diagram is obtained by drawing a straight line from that color through white to the other edge of the diagram Example: The complement to 700 nm red is 490 nm cyan Example: The complement to green is magenta - a non-wavelength color

14. Using the chromaticity diagram to find the dominant hue of a color in the interior of the diagram To find the dominant hue of the color indicated by the black dot Draw st. line from white through the point to get dominant wavelength, and hence, hue (547 nm green) Works because additive mixture of white with a fully-saturated (wavelength) color gives the desaturated color of the original point

15. Partitive mixing is another kind of additive color mixing but not achieved by superimposing colored lights! Instead, it works by putting small patches of colors next to each other. From a distance these colors mix just as though they were colored lights superimposed on each other Examples: Seurat pointillism Color TV and computer screens (Physics 2000) Photoshop example What is partitive mixing?

21. What is the effect of combining (sandwiching) different colored filters together? Rules for combining the subtractive primaries, cyan, yellow and magenta: White light passed through a cyan filter plus a magenta filter appears blue White light passed through a yellow filter plus a magenta filter appears red White light passed through a yellow filter plus a cyan filter appears green Why?

24. When looking at a colored object in a colored light source what is the resulting color? � Rule: Multiply the intensity-distribution of the light source by the reflectance of the colored object to get the intensity distribution of the the illuminated object � Example: Look at a magenta shirt in reflected light from a Cool White fluorescent tube. � It appears grey (colorless) Confirm by multiplying the intensity distribution curve by the reflectance curve to get the new intensity distribution curve for the reflected light

25. Halftone Left: Halftone dots. Right: How the human eye would see this sort of arrangement from a sufficient distance or when they are small. Resolution: measured in lines per inch (lpi) or dots per inch (dpi); for example, Laser Printer (600dpi)

26. Color halftoning

28. Color Liquid Crystal Displays (LCDs)

29. Chapter 10: 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) Light at any wavelength in the visual spectrum from 400 to 700 nm will excite these 3 types of cones to a degree depending on the intensity at each wavelength. Our perception of which color we are seeing (color sensation) is determined by how much S, i and L resonse occurs to light of a particular intensity distribution. Rule: To get the overall response of each type of cone, multiply the intensity of the light at each wavelength by the response of the cone at that wavelength and then add together all of the products for all of the wavenumbers in the intensity distribution

30. Examples of two different ways we see white Our sensation of color depends on how much total s, i & L cone response occurs due to a light intensity-distribution Multiply the intensity distribution curve by each response curve to determine how much total S, i, and L response occurs We experience the sensation white when we have equal total s, i & L responses There are many ways this can occur!! E.g., when broadband light enters our eye Another way to experience white is by viewing a mixture of blue and yellow E.g., 460 nm blue of intensity 1 and 575 nm yellow of intensity 1.66 The blue excites mainly s-cones but also a bit of i-cones and a bit of L-cones The yellow excites i-cones and (slightly more) L-cones but no s-cones The result is an equal response of s-cones, i-cones and L-cones (details)

31. How does a normal person see yellow when only red and green lights are superimposed? Our sensation of yellow depends on a special s, i & L cone response We experience the sensation yellow when 575 nm light reaches our eyes What really gives us the sensation of yellow is the almost equal response of i and L cones together with no s-cones!! Another way to experience yellow is by seeing overlapping red & green lights E.g., 530 nm green of intensity 1 and 650 nm red of intensity 2.15 The green excites mainly i-cones but also L-cones, while the red excites mainly L-cones but also i-cones The total respone of s & i-cones due to the spectral green and red is the same as the total response due to spectral yellow In general need 3 wavelength lights to mix to any color

32. We can verify color naming of hues in terms of the psychological primaries on the chromaticity diagram All of the hues can be named qualitatively by how much green, red, blue or yellow is "in" them We don't need orange, purple or pink: orange can be thought of as yellow-red purple can be thought of as red-blue pink has the same hue as red but differs only in lightness We can break up the diagram into 4 different regions by drawing two lines whose endpoints are the psychological primary hues The endpoints of the yellow line are 580 nm "unique" yellow and 475 nm "unique" blue One endpoint of the red line is 500 nm "unique" green and the other is "red" (not unique or spectral - really more like magenta) Note that pink, for example has the same hue as red or red-blue. However it is desaturated.Note that pink, for example has the same hue as red or red-blue. However it is desaturated.

33. What is meant by the opponent nature of red vs green (r-g) perception and of yellow vs blue (y-b) perception. Viewing a progression of colors in the direction of the yellow line from 475 nm blue towards 580 nm yellow, we see more yellowness of each color and less blueness. We call this perception our y-b channel Yellow & blue are opponents Moving parallel to the red line from 500 nm green towards nonspectral red we see more redness in each color and less greenness. We call this perception our r-g channel Red and green are opponents The lines cross at white, where both y-b & r-g are neutralized In this case we are moving into and out of desaturated regions of the chromaticity diagram. We could also move around the perimeterIn this case we are moving into and out of desaturated regions of the chromaticity diagram. We could also move around the perimeter

34. How might the three types of cones be "wired" to neural cells to account for our perception of hues in terms of two opponent pairs of psychological primaries r-g and y-b? The 3 kinds of cones are related to r-g and y-b by the way they are connected to neural cells (such as ganglion cells) Cones of each kind are attached to 3 different neural cells which control the two chromatic channels, y-b and r-g, and the white vs black channel called the achromatic channel (lightness) "wiring" is the following: When light falls on the L-cones they tell all 3 neural cells to increase the electrical signal they send to the brain When light falls on the i-cones they tell the r-g channel cell to decrease (inhibit) its signal but tell the other cells to increase their signal When light falls on the s-cones they tell the y-b channel cell to decrease (inhibit) its signal but tell the other cells to increse their signal Achromatic cell is related to the z-coordinate not shown on the chromaticity diagram. Achromatic cell is related to the z-coordinate not shown on the chromaticity diagram.

35. 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 coneINTERPRETED AS MAGENTA (Psychological red) The neural cell for the achromatic channel has its signal enhanced when light excites any of the cones

36. Systematic description of color-blindness (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 (can match any colored light with 3 spectral lights of different intensities as in normal vision, but still have color perception problems) 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

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

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)

39. Here is an illustration of the effect of red or green light falling in various combinations on the center or surround of a double-opponent r-g cell

40. y-b double-opponent receptive fields and cells work the same way

41. 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)

42. 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 This is because our double-opponent receptiive fields compare neighboring colors and are not very sensitive to an overall change in color Color constancy developed in the evolution of mankind so that we could recognize colorful things in broad daylight, late afternoon, and early evening

43. 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 Note the edges that separate a yellowish from a bluish color are enhanced the most Here are the enhanced edges resulting from your r-g chromatic channel Note the edges that separate a reddish from a greenish color are enhanced the most Here are the enhanced edges resulting from your wt-blk achromatic channel Compare with the way a photocopy machine would see the design

44. Chapter 13: What can a light wave do when it encounters matter? Be TRANSMITTED laser aimed at water or glass Be REFLECTED specular reflection of light by a mirror diffuse reflection of the light in this room off all the other students reflection is re-radiation of light by the electrons in the reflecting material Be ABSORBED Cyan light shining on a red apple is absorbed by electrons in the apple A light wave shining on molecules in the air or plastic or other �transparent� materials can be SCATTERED Light ray moves over to the side in all directions rather than forward, backward or being absorbed. Intensity of the scattered light can depend on wavelength

45. What is Rayleigh scattering?(or why is the sky blue) The shorter the wavelength, the more light is scattered blue is scattered more than red. this is why the sky is blue and sunsets are red. (Fig. 13.1) Dust or smoke enhances red look of the sun by providing more scattering Larger particles scatter red as well as blue and hence look white. Clouds; Milk; Colloidal suspensions

46. What is polarized light? Light is polarized if the waveform and electric force field arrows remains in the same plane The (green) electric force arrows must always be perpendicular to the ray This is a light ray traveling in the z-direction and polarized in the y-direction Here is a light ray traveling in the same direction but polarized in the x-direction We will visualize the polarization in the x-y plane, looking at rays head-on The green force arrows point up and down or left and right, stacked up behind one-another. Here is the convention for visualizing vertical and horizontal polarization

47. What is unpolarized light? For unpolarized light the plane of polarization keeps jumping around But the electric force field arrows remain perpendicular to the ray (direction of travel of the wave) We visualize this in the x-y plane (looking into the ray) as shown at right The many crossed double sided arrows are the symbol for unpolarized light See Physics 2000

48. When unpolarized light reflects off a horizontal surface (such as water or beach) near a special angle, the reflected light is polarized in the horizontal direction The special angle of incidence is where the refracted ray and reflected ray are perpendicular to each other This is called Brewster's angle To understand, imagine the electric force arrows of the incident unpolarized light to be decomposed into two perpendicular polariza-tions the first polarization is horizontal (force arrows are parallel to the flat reflecting horizontal surface and perpendicular to the ray) in the 2nd (Fig. 13.5), the arrows are perpendicular to both the ray and the horizontal force arrows

49. Some material from Chapter #8

50. How do 3D movies use polaroid filters?