Announcements 1. This week's topic will be COLOR VISION. DEPTH PERCEPTION will be covered next week. 2. All slides (and my notes for each slide) will be posted on the class web page at the end of the week. 3. Don't be afraid to contact me via e-mail at: jasong@psych.utoronto.ca 1. Announcements 1
Color Vision 2. Color Vision 2
The World to a Person With Red-Green Color Blindness Normal Observer Red-Green Color Blind Observer 3. Red-green color blindness -this is a guess at what the world might look like to a person who is red-green color blind -they have difficulty discriminating between reds and greens -the picture on the left shows what a person with normal color vision sees -the picture on the right has been modified so as to look like what a person with red-green color blindness might see -notice that the fruit just looks yellow and sort of bluish, and the places where there are red and green in the first picture just look the same color in the second one 3
What number is shown here? 4. Red-green Ishihara 1 -can anybody see what is in this circle of blobs? -probably not -this image is meant to simulate what the original version of it would look like to a person with red-green colorblindness 4
5. Red-green Ishihara 2 -and this is the original version -these are called Ishihara plates -they are designed to test for color blindness 5
The World to a Person With Yellow-Blue Color Blindness Normal Observer Yellow-Blue Color Blind Observer 6. Yellow-blue color blindness -this is a guess at what the world might look like to a person who is yellow-blue color blind -they have difficulty discriminating between yellows and blues -the picture on the left shows what a person with normal color vision sees -the picture on the right has been modified so as to look like what a person with yellow-blue color blindness might see -notice that the fruit just looks red and sort of greenish, and the places where there are yellow and blue in the first picture just look the same color in the second one 6
What number is shown here? 7. Yellow-Blue Ishihara 1 -what number is shown here? -this is meant to simulate what the original image might look like to a person with Yellow-Blue color blindness -this one was not made very well -but it shows the effect pretty well 7
8. Yellow-Blue Ishihara 2 -and this is the original image 8
The World to a Person With Achromatopsia (complete color blindness) Normal Observer Observer with Achromatopsia 9. Achromatopsia 1 -there are also people who cannot see any colors whatsoever -the world is literally colorless -again, the image on the left is what a normal person would see -and the image on the right is what somebody with achromatopsia might see 9
What number is shown here? 10. Achromatic Ishihara 2 -this is what either of those ishihara plates might look like to a person with achromatopsia -it is impossible to see the number 10
11. Achromatic Ishihara 1 -but with color the 5 jumps right out 11
Pictures by Color Blind Artists Original Picture Patient with Red-Green Color Blindness Patient with Achromatopsia 12. Pictures -these are some drawings by artists that suffer from color blindness -they were asked to draw the image on the far left -the person in the middle suffers from Red-Green color blindness -notice that many of the colors are misused, but most of them are reds and greens -the image on the right was made by an artist that suffers from achromatopsia -notice that no color was used at all 12
Why are some people color blind?? 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? 13. Why are some people color blind? -well why have I told you all of this -well, it turns out that, in order to understand why these patients have all these different symptoms, we have to know something about how the visual system represents color -so what I thought we would do is spend the first part of the class looking at how normal people perceive color -and then we will be able to understand what these disorders are all about -so first, we will discuss the physical characteristics of light--the thing that gives rise to our percepts of color -then, we will talk about the relationship between light and color -I think one of the key insights in the study of color vision is that color is a purely psychological phenomenon--it is a code that you visual system uses to organize information transmitted by light -hopefully in not too long I will have convinced you of this -next, we will talk about neural mechanisms involved in color perception -one we understand all of that, we will see why some people are unable to see colors like the rest of us 13
1. The physical nature of light Light has four basic properties. i. Light is composed of tiny particles, called photons*. ~300,000,000 meters/second *Light is both a particle and a wave, depending upon the conditions. 14. Physical properties of light 1 -there are four basic properties of light that we need to know i. light is composed of tiny particles, called photons -photons are extremely tiny bundles of energy that are emitted by light sources, such as a light bulb or the sun -these photons travel at a speed of 300,000,000 meters/second in a vacuum -technically, light acts like both a particle and a wave, depending upon the conditions -but we will consider it only as a particle 14
ii. Every photon has a wavelength. Photons constantly vibrate, and the distance they travel during 1 complete vibration is their wavelength. wavelength (λ) space *The lambda ( ) symbol is usually used for wavelength. 15. Physical properties of light 2 ii. Every photon has a wavelength -photons are constantly vibrating, and they vibrate at a particular frequency or rate. -they are traveling across space at the same time, and the distance that they travel during 1 full vibration or cycle is their wavelength -the symbol lambda is usually used to signify wavelength -now some of these wavelengths are very short, some very long -so there is a very wide range of wavelengths -but it turns out that only a small portion of that range is what we see as light 15
1 kilometer The range of wavelengths that photons can have is called the electromagnetic spectrum. 1 trillionth of a meter Radio Infrared Ultraviolet Gamma Rays Microwave Visible X-Rays The Electromagnetic Spectrum 16. Physical properties of light 3 -the entire range of wavelengths that a photon can have is called the electromagnetic spectrum -the wavelengths of photons range from radio waves, which can be as much as 1 kilometer in length, to gamma rays (or cosmic rays), which can be less than 1 billionth of a meter -light is just a small portion of this range, falling somewhere in the middle 16
The part of the electromagnetic spectrum that we can see --the visible light spectrum -- ranges between about 400 and 700 nanometers (nm) in wavelength. 400 500 600 700 short wavelength light wavelength (nm) long wavelength light 1 nm = 1 billionth of a meter *Remember ROY G. BIV : Red, Orange, Yellow, Green, Blue, Indigo, Violet. 17. Physical properties of light 4 -this section of the electromagnetic spectrum is called the visible light spectrum -the wavelengths are usually measured in nanometers, which is 1 billionth of a meter -we will talk more the relationship between wavelength and color in a few minutes, but basically the wavelengths in the visible light spectrum correspond to different colors -the shorter wavelengths correspond to violets and blues, the medium to greens and yellows, and the long to oranges and reds -a good pneumonic for remember the order is : ROY G. BIV -any wavelengths outside this range is invisible to us -there are creatures that are able to see outside this range; -certain snakes and lizards are able to see into the infrared range -and some insects, such as bees, are able to see into the ultraviolet range -and we will talk about why we cannot see in this range a little later on 17
iii. Light can vary in intensity. Intensity = # of photons Area x Duration less intense: same size & duration, fewer photons more intense: same duration and # of photons, smaller area 18. Physical properties of light 5 iii. Light can vary in intensity -light can also appear be brighter and dimmer -and as we saw in the introduction, this is sometimes due to psychological processes, not the light itself -but most of the time it is due to changes in light, and this change is called a change in the light s intensity -intensity of the light is equal to the number of photons, divide by the area over which you are measuring and the time over which you are measuring -so, for example, if we have a light bulb and measure it s intensity, and then reduce the number of photons it gives off but keep it s size and time on the same, it will be less intense -this is exactly what you do when you turn a 3-way bulb to a lower setting -likewise, if we had a light bulb that was half the size but emitted the same number of photons, it would be twice as intense 18
iv. Photons travel in straight lines, unless reflected, absorbed, or refracted by matter. Reflection & Absorption reflected Surfaces reflect certain wavelengths, absorb others. light source absorbed mirror absorbed reflected green paper 19. Physical properties of light 6 iv. Photons travel in straight lines, unless reflected, absorbed, or refracted -even though photons are constantly vibrating, they nonetheless end up going in only 1 direction; they just vibrate on the way there -and the photons will always travel in a straight line unless they reach some obstacles -two of things that these obstacles can do is are Reflect and Absorbed the light -different surfaces cause photons of various wavelengths to bounce off of them, and there are very simple geometric rules for reflection which we won t go into -surfaces also absorb certain wavelengths of photons, and they are translated into things like heat or electricity -and so what we see when week look at a surface are the collection of wavelengths that it reflects -so a mirror reflects all wavelengths and absorbs none, and it reflects the wavelengths in a coherent way --it doesn t just scatter them all over the place -a surface like a green piece of paper reflects only medium wavelengths, around 500 nm, and absorbs the rest -non mirror-like surfaces reflect wavelengths in an incoherent fashion--they scatter them across space 19
Refraction When light passes from one medium to another, it changes direction. This change is greater for shorter wavelengths. Example: In a prism, light is refracted as it goes from air to glass to air again. The result is a spectrum, because shorter wavelengths bend more than the longer ones. 20. Physical properties of light 7 -the last thing that an obstacle can do to light is called refraction -with reflection and absorption, light either bounces off an obstacle or is stopped by the obstacle -however, light also passes through various media, such air, water, and glass -and when the light passes from 1 medium to another, it will change direction slightly; it will be bent -this change in direction depends upon the difference in density between the two media; the greater the difference, the more the light is bent -it also depends on the wavelength; short wavelengths will be refracted more that long wavelengths -this is how a prism works -light travels from air to glass to air again, and is thus refracted along the way -what come out on the other side is a spectrum, because the different wavelengths are refracted by different amounts 20
White light (e.g., sunlight) is composed of all visible wavelengths (of approximately equal energy). 21. Physical properties of light 8 -and this actually points out the fact that most light sources transmit various combinations of wavelengths of light, not just a few -one good example of this is the sun -sunlight typically has equal intensity at all wavelengths -this kind of light is usually referred to as white light -there are some light sources that transit just one or a few wavelengths of light -a good example of this is a laser pointer -but these are relatively rare, and do not occur that often in the natural environment 21
summary... Four basic properties of light i. Light is composed of tiny particles, called photons. ii. Every photon has a wavelength. iii. Light can vary in intensity. iv. Photons travel in straight lines, unless reflected, absorbed, or refracted by matter. 22. Physical properties of light 9 Summary: Four basic properties of light i. Light is composed of tiny particles, called photons ii. Every photon has a wavelength iii. Light can vary in intensity iv. Photons travel in straight lines, unless they are reflected, absorbed, or refracted 22
?So Far... 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? 23. So Far 1 -well that s an overview of the basic properties of light -let s next talk about some of the basic phenomena associated with color vision, in the context of light wavelength and how it relates to perceived color 23
2. The relationship between light and the perception of color i. Color dimensions: hue, brightness and saturation ii. Color mixing iii. The color spindle 24. The relationship between light and the perception of color -well now that we know something about the physics of light, we can consider how light is related to various aspects of color perception -the first thing we will consider is the ways colors can vary, and how they are related to physical properties of light -it turns out that colors vary along 3 very closely related dimensions: hue, brightness and saturation -and these dimensions have a close but not direct mapping on to 3 physical dimensions of light -next, we will consider what happens when you mix colors together -it turns out there are 2 very different ways to do this -finally, we will talk about the color spindle -- a geometric tool that has been developed to organize color percepts in terms of the physical dimensions of light -so lets first talk about the three dimensions of light: hue, brightness and saturation 24
i. Color dimensions Hue: Psychological Correlate of Wavelength 25. Hue: Psychological Correlate of Wavelength -well, recall that I said that what we normally think of as color seems to change with wavelength -this quality of light--how it changes with wavelength--is associated with a color s HUE -so HUE is what we usually think of when we think of color -it s what we mean when we say red or green or blue -however, it is not the same as color 25
However, there is not a direct mapping between hue and wavelength. 26. Spot 1 -and one of the arguments for this is that the same exact hue can produce very different percepts -and this will be my first bit of evidence that color is a psychological phenomenon -so what does this slide look like to you? -most people would say white, right? 26
27. Spot 2 -stare at the white spot for about 40 seconds 27
28. Spot 3 -now what do you see? -most people see a greenish background with a red dot in the center -this is actually the same white uniform field you saw a moment ago -what you are seeing is called a color afterimage -we will explore why this happens a little later on -but notice that something that has the exact same wavelengths in it at one time can look white and, depending upon what you have just seen, can look green and red another time -this should convince you that there is not a direct mapping between wavelength and hue 28
29. Flag 1 -here is another example -stare at the fixation point for about 40 seconds 29
30. Flag 2 -what do you see? -most people see a normal American flag: white stars with blue background, and red and white stripes -but again, this is the same white field you saw before 30
Brightness: Psychological Correlate of Intensity 31. Brightness: Psychological Correlate of Intensity -well a color can also vary in brightness -and, as you might expect, brightness is strongly associated with the physical intensity of light 31
However, there is not a direct mapping between brightness and intensity. 32. Brightness 2 -however, just like with hue, there is not a direct mapping between brightness and intensity -the light check in the shadow has exactly the same luminance as the dark checks outside of the shadow -this example illustrates again that there is not a direct mapping between the physical world and our perception of it 32
Saturation: Psychological Correlate of Vividness 33. Saturation: Psychological Correlate of Vividness -well the last way color can vary is in terms of saturation -saturation basically refers to how washed out something looks -as something becomes more saturated, it tends to look more vivid -notice that as things become de-saturated (less vivid), they tend to look more white 33
However, there is not a direct mapping between vividness and saturation. 34. Saturation 1 -you could have guessed this, but there is not a direct mapping between saturation and vividness -and I will prove this to you -stare at the fixation point for about 40- seconds 34
35. Saturation 2 -what do you see? -most people see a highly saturated yellow on the right and a much less saturated yellow on the left -colors tend to look less saturated when you stare at them for long periods of time (adaptation) -however, this is a uniform field, with the same saturation throughout -again, this shows quite nicely that there is not a direct mapping between saturation and vividness 35
ii. Color Mixing Subtractive Color Mixing Additive Color Mixing 36. Color Mixing -well now you know the basic dimensions over which a color can vary: Hue, Brightness, and Saturation -well in the real world, we don t see too many monochromatic (single wavelength) lights -they are usually mixtures, and mixing colors can change all of these things (Hue, Brightness, and Saturation) -so let s talk about how color mixing works -now it turns out that there are two different kinds of color mixing -the one that we are all familiar with is subtractive color mixing -this is what you do when you mix paints -the other is additive color mixing, and it is much less intuitive -let s talk about subtractive color mixing first 36
37. Subtractive Mixing 1 -this picture shows what happens when you mix paints -each of the strips is line of paint squeezed from a bottle -at the bottom, the paint has been smeared together -and you get colors that we see when we mix paints -for example, mixing yellow and blue gives us green 37
How Paint Mixing Works Light Source The light that hits a surface (e.g., sunlight). Here, it has equal intensity at all wavelengths. Blue Paint Blue paint absorbs all longer wavelengths and some short wavelengths. We see the SHORT wavelengths that it reflects. Yellow Paint Yellow paint absorbs all shorter wavelengths and some long wavelengths. We see the LONG wavelengths that it reflects. 38. Subtractive Mixing 2 -well to understand how this works, we have to know about what the paints do to the light that falls on them -let s just assume that the light falling on the paint is something like sunlight--equal energy at all wavelengths -well the way surfaces work is that they reflect some wavelengths and absorb others -the ones they absorb, we don t see -the ones they reflect, we *do* see -so, for example, blue paint absorbs long wavelengths and it reflects short wavelengths -so if we were to shine a light on a surface that was covered by blue paint, the light that it reflected would look something like the second graph above -similarly, yellow paint absorbs short wavelengths and reflects medium-long wavelengths; and so if we were to shine a light on a surface that was covered by yellow paint, the light that it reflected would look something like the third graph above 38
Yellow Pigment Blue Pigment Subtractive Color Mixing Blue paint absorbs long wavelengths. Yellow paint absorbs short wavelengths. They both reflect MEDIUM wavelengths (green) 39. Subtractive Mixing 3 -so what happens if we mix them together? -well, the blue paint will absorb all of the long wavelengths -and the yellow paint will absorb all of the short wavelengths -and so the only wavelengths that will be reflected will be the ones they BOTH reflect -this is where the two curves overlap--right around green -so subtractive color mixing works by mixing two pigments together and taking away the wavelengths that they don t BOTH reflect 39
Additive Color Mixing White surfaces (reflect all wavelengths equally). Yellow light is composed of long wavelengths. Blue light is composed of short wavelengths. Together, they contains ALL wavelengths (white light) 40. Additive Mixing 1 -well there is another form of color mixing, and it is actually the one that is most relevant for color vision -it is called additive color mixing -unlike subtractive color mixing, additive mixing does not work by surfaces taking wavelengths away -instead, it works by adding wavelengths together -now think about what would happen if you had a surface that reflected all wavelengths and you put a blue light on it -it would look blue, right? -now what would happen if you put a yellow light on it at the same time? -well now it would reflect both the yellow and the blue light -and together, those two lights have both long, medium, and short wavelengths -and so they would add up to form something very similar to white light -this is additive mixing--adding lights of various wavelengths together to make a new light 40
An example of additive color mixing: Pointillism Georges Seurat, Un Dimanche apres-midi a Ile de la Grande-Jatte (1884) 41. Additive Mixing 2 -well it also works if you have a broad-band light (like sunlight) and you shine it on surfaces that are different colors but are really small and close together -they are so small that the light they reflect gets added together and you see them as an additive mixture -this is what is used in the technique pointillism -this is a famous painting by George Seurat that uses this technique -now from here, this painting look pretty normal 41
The light from small, nearby surfaces (points) combine to produce an additive mixture of light. 42. Additive Mixing 3 -but if you get really close, you can see that it is made up of a bunch of small, colored dots -well, when you get far enough away, the light from nearby dots becomes added together, and you see the additive mixture that is produced by the sum of all the the dots wavelengths 42
Another example of additive color mixing: TV s and computer monitors 43. Additive Mixing 4 -your TV and computer monitor also use this principle to make colored images -a TV screen is made up of a bunch of very small pixels, and each pixel is actually made up of three smaller units that are red, green, and blue -the relative intensities of these can be changed to produce any color -your visual system additive mixes the red, green, and blue parts of the pixels -now WHY you can make any color from red, green, and blue will become clear in a little bit -but this is another good example of additive color mixing 43
Again Subtractive color mixing works by removing wavelengths of light. Additive color mixing works by combining wavelengths of light. 44. Mixing Summary -summary -well we are concerned with additive color mixing because it is what takes place on the eye -subtractive mixing takes place on surfaces, before the light reaches the eye -so from here on out, when we talk about color mixing we will mean additive mixtures 44