Question From Last Class

Transcription

1 Question From Last Class What is it about matter that determines its color? e.g., what's the difference between a surface that reflects only long wavelengths (reds) and a surfaces the reflects only medium wavelengths (greens)? 1. Question from Last Class 1 1

2 Answer: It depends on the physical and chemical characteristics of the matter. There are 15 different ways that light can interact with matter to produce colors. They are called the 15 causes of color. 2. Question from Last Class 2 2

3 All of the causes involve changes in the excitation of electrons in the atoms of the matter. The kind of excitation depends on the physical structure and chemical composition of the matter. 3. Question from Last Class 3 3

4 If you are interested... Nassau, K. (1983). The physics and chemistry of color: the fifteen causes of color. New York: John Wiley & Sons. McLauren, K. (1986). The color science of dyes and pigments. Bristol, U.K.: J.W. Arrowsmith Ltd. 4. Question from Last Class 4 4

5 2. The relationship between light and the perception of color i. Color dimensions: hue, brightness and saturation ii. Color mixing iii. The color spindle 5. The relationship between light and the perception of color (review) 5

6 iii. The Color Spindle 6. Color Spindle 1 -well all of these things I have talked about so far can actually be represented in a relatively simple geometric form -there have been several versions of this throughout history, the earliest being Newton s color wheel -but this one is the most modern -it s called the color spindle, and it shows how colors vary with hue, brightness, saturation, and mixing 6

7 The Color Spindle 7. Color Spindle 2 -so each of the three basic dimensions we talked about at the beginning are represented here -hue is represented as wrapping around the color spindle -brightness is represented as the distance from the bottom -and saturation is the distance from the center 7

8 The Color Circle: One Slice of the Color Spindle 8. Color Circle 1 -often times people just talk about 1 slice out of the color spindle -the center slice is sometimes called the color circle -so, brightness is not represented in the color circle -only hue and saturation -let s look at some of the properties of the color circle and how they relate to how we perceive colors 8

9 Single Wavelengths (monochromatic lights) Single wavelengths are fully saturated, and fall on the perimeter of the circle (except for purple). Monochromatic: composed of a single wavelength. 9. Color Circle 2 -in the color circle, the perimeter corresponds to single wavelengths -these are usually called monochromatic lights -however, there is one exception, purple,which we will get to in a moment -notice that all of the monochromatic lights are as far from the center as they can be -this means they are fully saturated -this agrees with our experience--monochromatic lights, such as laser pointers, look very saturated 9

10 Mixtures Additive mixtures are desaturated. They fall on the inside of the color circle. 10. Color Circle 3 -what about mixtures? -mixtures fall inside of the color circle -this means that mixtures will look less saturated that monochromatic lights -and this also agrees with our experience -think about a white light: it is a mixture of all lights, and looks completely desaturated -and look, it is in the center of the color circle, exactly where it should be -so one way to think about saturation is the amount of white light a mixture has in it -well you can figure out what a mixture will look like by using the color circle -for example, suppose I mix a green and red light -first, I just draw a line between the two wavelengths that I mixed -then I place a point on the line that corresponds to how much of each light I put in -for example, if I put in a lot of green then the point will be close to green -and then I just look at where this point falls in the color circle -well we know that it will fall inside the circle -and for equal amounts of red and green, the point will fall in the middle of the line -now if we look at the hue that corresponds to this mixture, it will be a yellow, and it will be desaturated 10

11 Non-Spectral Hues Purple is a nonspetral hue. It can t be matched by any single wavelength. It is obtained by mixing very long and very short wavelengths. Spectral hues: hues that can be matched by a single wavelength of light. 11. Color Circle 4 -now one weird thing that I have been ignoring is purple -it has this big section up at the top, with no wavelength along the perimeter -purple is often called a non-spectral hue -spectral hues are those that you can match with a single wavelength -and any of the mixtures that fall outside of the purple range can be matched in hue with a monochromatic light -however, that is not true with purple -purple requires a mixture of at least two wavelengths to match it -and as the circle shows, you can make purple by mixing red and blue light -again, we will talk about why purple is so weird later on -but the point is that the color circle accurately represents it 11

12 Metamers Any mixtures that fall on the same point will be metameric. That is, they will be perceptually indiscriminable. Metamers: two or more things that produce the same percept. 12. Color Circle 5 -well another neat thing that this color circle predicts is that any mixture that falls on the same point will look the same, even though they are made of different wavelengths -things that are perceptually equivalent--that is, perceptually indiscriminable--are called Metamers -so here, the color circle says that if we take our red and green mixture and compare it to a blue and orange mixture that has a lot of orange in it, they should look exactly the same -and they do! -this is another good demonstration of how color perception is a psychological process -the mixtures have completely different wavelength content, yet produce the same percept -we will talk about why this happens a little later on 12

13 A special case of metamers: complementary color pairs Complementary colors are mixtures of 2 wavelengths that, when additively mixed, are metameric with white light. 13. Color Circle 6 -now special cases of metamers are complementary colors -complementary colors are just pairs of wavelengths that, when mixed in the same amount, produce the percept of white -you can use the color circle to figure out what these pairs should be -again, we will talk about why this happens a little later on 13

14 Opponent Hues Have you ever seen a reddish-green? How about a yellowish-blue? 14. Color Opponency 1 -have you ever seen a reddish-green? Or a yellowish-blue? -probably not -it is impossible to conceive of these mixtures 14

15 Opponent Hues Red/Green and Yellow/Blue are opponent hues. It is impossible to imagine a reddish-green or a yellowish-blue. 15. Color Opponency 2 -well the color circle captures this aspect of color perception as well -notice that there are no greenish-reds or yellowish-blues on the color circle -and remember what happened when we mixed red and green light- -we got something very different: yellow! -and again why this happens will become clear soon 15

16 ?So Far The physical nature of light 2. The relationship between light and the perception of color 3. The mechanisms of color vision 4. What s different about people with color blindness? 16. So Far 1 -well that s an overview of some of the basic phenomena associated with color perception -now any mechanisms that we propose will have to be able to account for all of those things -and that s what we will talk about next -the mechanisms that mediate color vision 16

17 Color Matching People can match any monochromatic light by mixing and adjusting the relative intensities of 3 different monochromatic lights. They can t do this with less than Color Matching -well remember when we talked about additive color mixing and your TV screen -I told you that you can produce any color if you just put the right amounts of red, green, and blue light into it -well this is actually true of lots of sets of 3 lights, not just red, green, and blue -but what does this tell us about the way the visual system is encoding color? 17

18 What would happen if you only had 1 kind of color mechanism? Monochromatic System You would only need 1 wavelength to match any other wavelength color mechanism system -well lets step back and consider some different color sensing devices -lets say I have a system that only has 1 kind of light sensitive mechanism -and lets also say that it has some wavelength that it responds best to; and it will respond less and less as wavelengths get higher or lower than this best wavelength -well if I show this system some wavelength, there would actually be no way that it would be able to tell what wavelength it was -because look: I could just take some other wavelength, and adjust it s intensity so that the mechanism responded exactly the same amount -so here, the two wavelengths shown produce the same exact response from this mechanism -and if you think about it, any mixture would also be indisciminable from it -so what this means is that all information about wavelength would be lost -so a system like this could not see any colors 18

19 What would happen if you had 2 kinds color mechanisms, with different but overlapping spectral sensitivities? Dichromatic System You would need at least 2 different wavelengths to match all other wavelengths color mechanism system -well what would happen if we had two mechanisms instead of 1? -well now lets say they had different best wavelengths, but that they overlapped --they were both sensitive to some of the same wavelengths -now the system would have 2 responses for each wavelength, one from each mechanism -well now we could produce the same ratio of responses in the two mechanisms by showing two wavelengths instead of 1 -so in this kind of a system (a dichromatic system), any wavelength can be matched by adjusting two other wavelengths 19

20 What would happen if you had 3 kinds of color mechanisms, with different but overlapping spectral sensitivities? You would need at least 3 different wavelengths to match all other wavelength (trust me). Trichromatic System color mechanism system -well what about a 3-mechanism system? -a similar reasoning applies, only now the system has 3 responses to compare! -it s hard to make an example because it is so complicated with 3 mechanisms, but you can show that you need at least 3 wavelengths to match all other wavelengths in such a trichromatic system -well, what does this suggest about out visual system? -we are able to match any light with 3 monochromatic lights -and we need at least 3 lights to match all of the different colors 20

21 Trichromacy There are 3 cone types, each with different peak spectral sensitivities. These are short (S), medium (M) and long (L) wavelength sensitive cones. S Rods M L 21. Trichromacy -well what it suggests is that we have 3 different kinds of light sensitive receptors that have different spectral sensitivities -remember when professor sekuler talked about rods and cones, she showed their spectral sensitivity functions -well, there are actually 3 different kinds of cones, and each has a different spectral sensitivity function -one is very sensitive to short wavelengths --the S cones -one is very sensitive to medium-long wavelengths --the M cones -and the other is very sensitive to long wavelengths --the L cones -what this suggests is that the visual system looks at the relative activity across L,M, and S cones to figure out wavelength -so notice what this means -remember, cones are only active under photopic conditions -and there is only 1 kind of rod -so what this means is that we cannot discriminate colors at low light levels-- we are all color blind at night 21

22 Purple (non-spectral hue) Recall purple results from the combination of very short and very long wavelengths. This produces activity in only S and L cones. No single wavelength does this. S Rods M L 22. Purple -what about purple? -remember, unlike all other hues, purple requires 2 wavelengths to match it -and remember it can be made by mixing red and blue -well notice that this is the only case where only the L and S cones are active -and there is no single wavelength that does this 22

23 Metamers / Complementary Colors Complementary colors produce the same amount of relative activity across S, M, & L cones as white light when mixed in equal amounts. S Rods M L 23. Metamers / Complementary colors -what about complementary colors? -well, remember they are just a special case of metamers -in this case, they produce the same activity across L, M, and S cones that white light does -white light produces the same activity across cones, because it has equal intensity at all wavelengths -here is an example 23

24 What about Afterimages & Opponent Hues? Trichromacy alone cannot account for these phenomenon. 24. Afterimages & Opponent hues -but what about afterimages and opponent hues? -it turns out that they cannot be accounted for by just looking at the relative activity across the cones -there must be something else going on 24

25 Color Opponent Cells in the Retina & LGN Respond well to either blue or yellow. B+Y- Respond well to either red or green. G+R- Y+B- R+G- 25. Color sensitive cells in Retina & LGN -well the responses from the cones feed into retinal ganglion cells, which in turn feed into LGN cells -and it turns out that in the retina and LGN, there are not only cells that respond in an opponent nature to black and white, but cells that respond in an opponent nature to either yellow and blue or green and red 25

26 Spatial and Chromatic Double-Opponent Cells in Primary Visual Cortex Respond well blue and yellow gratings. G+R- Y+B- B+Y- Respond well red and green gratings. G+R- Y+B- B+Y- R+G- R+G- 26. Center-Surround Color Receptive Fields in the cortex -and these LGN cells feed into a more complex opponent cell that is spatially opponent as well as color opponent -these are usually called double-opponent cells, because they are opponent for color and spatial location -some of them are red-green opponent cells -they will respond well to alternating green and red bars -and some of them are yellow-blue opponent cells -they will respond well to alternating blue and yellow bars 26

27 Achromatic & Chromatic Channels Achromatic Channel L M White/Black Red-Green Channel L M S Red/Green Yellow-Blue Channel L M S Yellow/Blue 27. Color Channels -and so the result of all of this is that we have are three different color channels -an achromatic channel that just encodes intensity -and two chromatic channels that encode color, in an opponent fashion -the red part of the red-green channel receives input from the L and S cones -why the S cones? If you ask people to just judge the color of monochromatic lights, often times they report the very short wavelengths as reds -this may be why s cones feed into the red part of the channel -also notice that this is represented in the color circle: red wraps around to the short wavelengths (through purple) -the green part of the red-green channel receives input from the M-cones -the yellow part of the yellow-blue channel receives input from the L and M-cones - and the blue part of the yellow-blue channel receives input from the S-cones -notice also that the S-cones don t connect to the achromatic channel -most models of color vision suggest that there is little or no role for S-cones in this channel, so it is just left out 27

28 Adaptation Adaptation fatigues photoreceptors, which can produce changes in their relative contribution to each of the channels. Adapting to yellow will produce a blue afterimage. Adapting to green will produce a red afterimage. Adapting to black will produce a white afterimage. 28. Adaptation -we can also account for adaptation after-effects by looking at the activity of the channels -consider the weird American flag stimulus -it has black stars and stripes, a yellow background for the stars, and green stripes -based on the activity of our channels, what would we predict the flag should look like after adaptation? -adapting to yellow will produce a blue afterimage -adapting to green will produce a red afterimage -adapting to black will produce a white afterimage -so it should look like a normal American flag after adaptation (if you look a white screen) 28

29 ?So Far The physical nature of light 2. The relationship between light and the perception of color 3. The mechanisms of color vision 4. What s different about people with color blindness? 29. So Far 2 -now we are in a good position to understand different kinds of color blindness 29

30 Red-Green Color Blindness Red-green color blindness almost always results from the absence of L or M cones. The Red-Green channel will only get 1 kind of input (except for a very small contribution from the S-cones in protanopia). So, the Red- Green channel will be essentially colorblind. Female Incidence: Protanopia = 1/5,000 Deuteranopia = 1/1,000 L M S M S L S Male Incidence : Protanopia = 1/100 Deuteranopia = 1/100 Normal Red/Green Missing L-Cones (protanopia) Red/Green Missing M-Cones (deuteranopia) Red/Green 30. Red-Green Color blindness - red-green color blindness almost always results from the absence of L or M cones. -missing L cones is called protanopia -missing M cones is called deuteranopia -the red-green channel will only get 1 kind of input (except for a very small contribution from the S-cones in protanopia) -so, the red-green channel will be essentially colorblind, because it will basically be a 1 cone system between about nm -color is still perceived, though, because there is still activity in the blue-yellow channel -it turns out that this is relatively common in men but not women -this is because it is due to a genetic defect on the X chromosome; males have 1 X chromosome, women 2, so it is likely that women will have at least 1 normal X chromosome Male Incidence : Protanopia = 1/100 Deuteranopia = 1/100 - Female Incidence: Protanopia = 1/5,000 Deuteranopia = 1/1,000 30

31 Yellow-Blue Color Blindness Yellow-Blue color blindness almost always results from the absence of S cones. The Yellow-Blue channel will only get 1 kind of input. So, the Yellow-Blue channel will be colorblind. L M S L M Normal Yellow/Blue Missing S-Cones (tritanopia) Yellow/Blue Female and Male Incidence: Tritanopia = 1/10, Yellow-Blue Color blindness - yellow-blue color blindness almost always results from the absence of S cones. -missing S cones is called tritanopia -the yellow-blue channel will only get 1 kind of input (it is pooled across the L and M cones) -so, the yellow-blue channel will be essentially colorblind between nm -it turns out that this is very rare, and is equally common in men and women, because it is not due to a defect in the X chromosome Male and Female Incidence: tritanopia = 1/10,000 31

32 Inherited Achromatopsia Inherited achromatopsia results from the absence of 2 or all three cone types. Male and Female (?) Incidence: Only L-Cones = 1/1,000,000 Only M-Cones = 1/1,000,000 Only S-Cones = 1/30,000 No Cones = 1/10,000,000 L M S Only L-Cones Red/Green Yellow/Blue Only M-Cones Red/Green Yellow/Blue Only S-Cones Red/Green Yellow/Blue 32. Inherited Achromatopsia -recall achromatopsia is the disorder where people are unable to see any colors at all -there are actually two kinds -the first one we will talk about is inherited achromatopsia -it is almost always a congenital disorder (people are born with it) -they are missing either 2 out of the three cone types or all three cone types -sometimes they are referred to as monochromats -and remember the hallmark of any system that has only 1 color mechanism is complete color blindness -I am note sure about the relative incidence in men and women -but they are all very rare Male and Female (?) Incidence: Only L-Cones = 1/1,000,000 Only M-Cones = 1/1,000,000 Only S-Cones = 1/30,000 No Cones = 1/10,000,000 32

33 Acquired Achromatopsia Damage to cortical areas that process color can result in acquired achromatopsia. Very little is known about this kind of dysfunction. L M S L M White/Black Red/Green Yellow/Blue White/Black 33. Acquired Achromatopsia -the other kind of achromatopsia is acquired achromatopsia -it is almost always due to damage to cortical areas that process color -specifically, it involves damage to cortical areas that involve both of the chromatic channels -it can also result from selective damage to two or all three cone types, but this is very very rare -very little is known about this kind of dysfunction, partly because it is rare but also because it involves brain areas that are not well understood 33

34 The End. 1. The physical nature of light 2. The relationship between light and the perception of color 3. The mechanisms of color vision 4. What s different about people with color blindness? 34 The end 34