Additive. Subtractive

Transcription

1 Physics 106

2 Additive Subtractive

3 Subtractive Mixing Rules: Mixing Cyan + Magenta, one gets Blue Mixing Cyan + Yellow, one gets Green Mixing Magenta + Yellow, one gets Red Mixing any two of the Blue, Red, Green one gets Black.

4 Subtractive mixing can be tricky

5 T,P,S Fluorescent bulbs don t usually cover the entire spectrum. If I had a bulb which put out just two colors: narrow bands of cyan and red in a ratio of 2:1 so that it looked white, and I look at a yellow (in daylight) object in only this light, what color would it appear?

6 Dependence of subtractive color on the light source The color of the light reflected from an object also depends on the light source. To find the color of an object under a nonwhite light source, we need multiply the intensity distribution of the light source times the reflectance spectrum of the surface.

7 Things are not always as they appear Even if two illuminating lights look the same, an object may still appear different colors in them. Under a white light consisting of two narrow bands of cyan and red, the yellow object can only appear as black. X = BLACK

8 Things are not always as they appear Even if two illuminating lights look the same, an object may still appear different colors in them. Under a white light consisting of two narrow bands of cyan and red, the yellow object can only appear as black. However, the same yellow object appears yellow under sun light. X = YELLOW

9 If we just had rods How would we tell a small amount of light at l 1 from a large amount of light at l 2? We would see in monochrome - as we do under scotopic conditions

10 1801-Thomas Young (same Young as wave nature of light) We can distinguish colors - therefore there must be more than one type of color receptor We can describe colors with 3 parameters: hue, saturation and intensity There must be 3 types of color receptors trichromacy

11 1852 Helmholtz Von Helmholtz postulated three response curves for the three types of cones: S-cones: has the best response to short wavelength of light L-cones:. to long wave length of light I-cones:.. to the intermediate wavelength of light. Different colors correspond to different patterns of responses in these cones. No two (different) colors produce the same response from the three cones.

12 A Guess response 400 nm 700 nm wavelength A problem : how can we tell different hues of reds from each other?

13 If one chooses a point on the edge of the chromaticity diagram, then draws a straight line passing through white you will hit the complement on the other edge

14 Key Feature: Any color has at least two types of cones that respond: takes care of the problem of distinguishing different hues of red

15 S I L Helmholtz hypothetical spectral response curves of human photoreceptors.

16 Determine the response curves For white light, all cones respond equally. All wavelengths contribute equally to broad-band white light. An additive mixture of two complementary lights can also yield white. From the region of spectral colors without spectral complementarity (greens between 490 and 565 nm), we determine where L & I and S & I responses cross. It s because of the overlap that you don t get perfect addition!

17 S I L Region without monochromatic complement - determines where the curves cross

18 S I L I > S and L then complement must have S & L > I (because green + purple = white I green + I purple = S green +S purple =L green +L purple ) Can t be done with a single wavelength

19 Region without monochromatic complement - determines where the curves cross The region is in the greens between 490 and 550 nm and the complements lie on the purple line of the CIE chromaticity diagram.

20 S I L Measured spectral response curves of human photoreceptors.

21 The ability for hue discrimination depends on the wavelength. From this, one can find the steep rise and falling segments of the response curves. Bad! Good!

22 Nowadays, we can look at the cones themselves Microspectrophotometry

23 Understanding adding colors Spectral cyan: we needed blue (460), green (530) and negative red (650) Cyan excites I and S equally, L ~ half as much If we make cyan from RGB, we need a bit more green than blue, but then we get too much response from the L cones: so we need to subtract some red this leads to our idea of negative red.

24 The information contained in the chromaticity diagram is consistent with that of the (human) cone response curves (i.e., that s how the diagram got created). A bee would have a different chromaticity diagram.

25 Adding green and red yields yellow There is no such color as reddish-green or yellowish-blue?

26 Four Psychological Primaries When we additively mix red and green, we don t see reddish green; we see yellow. When we subtractively mix cyan and yellow, we don t see yellowish cyan, we see green. Thus to describe what colors look like, we need four primaries: blue, green, yellow and red. Any hue can be verbally described as a combination of them.

27 Color Opponents Using the psychological primaries, we can name all spectral colors. Red and green are opposite colors; they never appear at the same wavelength. So are yellow and blue. We call them opponents. The opponent color theory as put forward by Ewald Hering, was in competition with trichromacy.

28 Afterimage + +

29 Negative afterimage When you are adapted to yellow, you will see blue in the white. When you see too much yellow, the L and I cones excited and tired. So when you see the white, the S cones respond more than L and I cones. This gives you a feeling of blue!

30 Color Cancellation If a color is too bluish, it can be made less bluish by adding yellow. The amount of yellow that one adds to cancel the blue entirely determines the blueness of the original color. The yellowness and blueness, and redness and greenness can be used to describe a color quantitatively.

31 Opponents: Green: red and Blue:yellow

32 Which is correct : trichromacy or opponent processing? T,P,S Biology : trichromacy Psychology: opponent processing Answer: 3 cone types are wired in the retina to give opponent processing!

33 Opponent Processing The responses of the L,I,S cones are processed by three opponent channels: yellow-blue, red-green, and white-black. It is the latter information which is passed to the brain. S I L type of cones Y-B contribution to signal R-G W-K + + +

34 Opponent Processing S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel - to brain -

35 S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel to brain - +1

36 S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel to brain - 0

37 S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel to brain - -1

38 S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel to brain - +1

39 S I L Y-Blue W-Black R-G Chromatic channel Luminance channel Chromatic channel to brain - +1

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41 Simultaneous Color Contrast Imbed a gray region among yellow and green. The yellow region makes the gray look bluish and the green region makes the gray reddish. Thus a region of the retina contains spatial opponency---lateral inhibition of color. Used widely in impressionists paintings.

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45 Color Constancy Objects tend to retain the same perceived color even though the coloration of the overall illumination may change. A biological necessity. Comes from lateral inhibition. The color constancy is not perfect. It depends on the state of adaptation.

46 Chevreul French Chemist - Director of Dyes Theory of Contrasting Colors Gospel of the Impressionists In his 1839 work demonstrates that a colour will lend its adjacent colour a complementary tinge (of colour hue). As a result, opposing complementary colours will brighten, and noncomplementary colours will appear "contaminated", for example a yellow next to a green receives a violet tinge.

47 Monet

48 Van Gogh

49 Van Gogh

50 Color Deficiency Monochromats -- very rare Dichromats Trichromats - normal vision

51 Various deficiencies

52 Usually sex-linked ~5-10% of males 0.5% of females different varieties Most common - dichromat

53 Color Test

54 What a dichromat sees

55 Do insects have color vision? Look at flowering plants

56 Insect Color Vision UV sensitive Bees butterflies Some dragonflies - quintchromat 340,410,490,540,620

57 Canine Color Vision Similar to human dichromat