Announcements - psych.utoronto.capsych.utoronto.ca/users/vislab/CH6_1.pdfPsy 280 Fall 2000: Color...

Psy 280 Fall 2000: Color Vision (Part 1) Oct 23, 2000

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Announcements

1. This week's topic will be COLOR VISION.DEPTH PERCEPTION will be covered nextweek.

2. All slides (and my notes for each slide) willbe posted on the class web page at the end ofthe week.

3. Don't be afraid to contact me via e-mail at:

[email protected]

1. Announcements


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Color Vision

2. Color Vision


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The World to a Person WithRed-Green Color Blindness

Normal ObserverRed-Green ColorBlind Observer

3. Red-green color blindness

-this is a guess at what the world might look like to a person who isred-green color blind

-they have difficulty discriminating between reds and greens

-the picture on the left shows what a person with normal colorvision sees

-the picture on the right has been modified so as to look like what aperson with red-green color blindness might see

-notice that the fruit just looks yellow and sort of bluish, and theplaces where there are red and green in the first picture just look thesame color in the second one


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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 itwould look like to a person with red-green colorblindness


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5. Red-green Ishihara 2

-and this is the original version

-these are called “Ishihara” plates

-they are designed to test for color blindness


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The World to a Person WithYellow-Blue Color Blindness

Normal ObserverYellow-Blue Color

Blind Observer

6. Yellow-blue color blindness

-this is a guess at what the world might look like to a person who isyellow-blue color blind

-they have difficulty discriminating between yellows and blues

-the picture on the left shows what a person with normal colorvision sees

-the picture on the right has been modified so as to look like what aperson with yellow-blue color blindness might see

-notice that the fruit just looks red and sort of greenish, and theplaces where there are yellow and blue in the first picture just lookthe same color in the second one


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7. Yellow-Blue Ishihara 1

-what number is shown here?

-this is meant to simulate what the original image might look like toa person with Yellow-Blue color blindness

-this one was not made very well

-but it shows the effect pretty well


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8. Yellow-Blue Ishihara 2

-and this is the original image


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The World to a Person WithAchromatopsia (complete color blindness)

Normal ObserverObserver withAchromatopsia

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 achromatopsiamight see


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10. Achromatic Ishihara 2

-this is what either of those ishihara plates might look like to aperson with achromatopsia

-it is impossible to see the number


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11. Achromatic Ishihara 1

-but with color the 5 jumps right out


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Pictures by Color Blind Artists

Original PicturePatient withRed-Green

Color Blindness

Patient withAchromatopsia

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 arereds and greens

-the image on the right was made by an artist that suffers fromachromatopsia

-notice that no color was used at all


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?Why are some people color blind?

1. The physical nature of light2. The relationship between light and the perception of color3. The mechanisms of color vision4. 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 thesedifferent symptoms, we have to know something about how the visual systemrepresents color

-so what I thought we would do is spend the first part of the class looking athow “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 thatgives 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 apurely psychological phenomenon--it is a code that you visual system uses toorganize 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 seecolors like the rest of us


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Light has four basic properties.

i. Light is composed of tinyparticles, called photons*.

*Light is both a particle and a wave,depending upon the conditions.

~300,000,000 meters/second

1. The physical nature of light

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 bylight sources, such as a light bulb or the sun

-these photons travel at a speed of 300,000,000 meters/second in avacuum

-technically, light acts like both a particle and a wave, dependingupon the conditions

-but we will consider it only as a particle


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ii. Every photon has a wavelength.

Photons constantly vibrate, and the distance they travelduring 1 complete vibration is their “wavelength”.

*The lambda ( ) symbol is usuallyused for “wavelength”.

space

wavelength (λ)


ii. Every photon has a wavelength

-photons are constantly vibrating, and they vibrate at a particularfrequency or rate.

-they are traveling across space at the same time, and the distancethat they travel during 1 full vibration or “cycle” is theirwavelength

-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 wesee as “light”


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RadioMicrowave

InfraredVisible

UltravioletX-Rays

Gamma Rays

The Electromagnetic Spectrum

The range of wavelengths that photons can have iscalled the electromagnetic spectrum.

1 trillionth of a meter1 kilometer


-the entire range of wavelengths that a photon can have is called theelectromagnetic spectrum

-the wavelengths of photons range from radio waves, which can beas 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 themiddle


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*Remember “ROY G. BIV”:Red, Orange, Yellow, Green,Blue, Indigo, Violet.

The part of the electromagnetic spectrum that we cansee --the “visible light spectrum” -- ranges betweenabout 400 and 700 nanometers (nm) in wavelength.

1 nm = 1billionth of a

meter

wavelength (nm)long wave-length light

short wave-length light

400 500 600 700


-this section of the electromagnetic spectrum is called the visiblelight spectrum

-the wavelengths are usually measured in nanometers, which is 1billionth of a meter

-we will talk more the relationship between wavelength and colorin a few minutes, but basically the wavelengths in the visible lightspectrum correspond to different colors

-the shorter wavelengths correspond to violets and blues, themedium 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 ultravioletrange

-and we will talk about why we cannot see in this range a little lateron


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iii. Light can vary in intensity.

# of photonsArea x Duration

Intensity =

less intense:same size & duration,

fewer photons

more intense:same duration and # ofphotons, smaller area


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 topsychological processes, not the light itself

-but most of the time it is due to changes in light, and this change iscalled a change in the light’s “intensity”

-intensity of the light is equal to the number of photons, divide bythe area over which you are measuring and the time over which youare 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 sizeand time on the same, it will be less intense

-this is exactly what you do when you turn a 3-way bulb to a lowersetting

-likewise, if we had a light bulb that was half the size but emittedthe same number of photons, it would be twice as intense


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iv. Photons travel in straight lines, unlessreflected, absorbed, or refracted by matter.

Reflection & Absorption

Surfaces reflectcertainwavelengths,absorb others.

greenpaper

mirror

lightsource

reflected

absorbed

absorbed

reflected


iv. Photons travel in straight lines, unless reflected, absorbed, or refracted

-even though photons are constantly vibrating, they nonetheless end up going in only 1direction; 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, andthere 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 thingslike heat or electricity

-and so what we see when week look at a surface are the collection of wavelengths thatit reflects

-so a mirror reflects all wavelengths and absorbs none, and it reflects the wavelengths ina “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 scatterthem across space


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RefractionWhen light passes fromone medium to another,it changes direction.This change is greaterfor shorter wavelengths.

Example: In a prism,light is refracted as it goesfrom air to glass to airagain. The result is aspectrum, because shorterwavelengths bend morethan the longer ones.


-the last thing that an obstacle can do to light is called refraction

-with reflection and absorption, light either bounces off an obstacle or isstopped 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 changedirection slightly; it will be “bent”

-this change in direction depends upon the difference in density between thetwo media; the greater the difference, the more the light is bent

-it also depends on the wavelength; short wavelengths will be refractedmore that long wavelengths

-this is how a prism works

-light travels from air to glass to air again, and is thus refracted along theway

-what come out on the other side is a spectrum, because the differentwavelengths are refracted by different amounts


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“White” light (e.g.,sunlight) is composed ofall visible wavelengths(of approximately equalenergy).


-and this actually points out the fact that most light sources transmitvarious 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 fewwavelengths of light

-a good example of this is a laser pointer

-but these are relatively rare, and do not occur that often in thenatural environment


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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.

summary...


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


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?So Far...

1. The physical nature of light2. The relationship between light and the perception of color3. The mechanisms of color vision4. 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 withcolor vision, in the context of light wavelength and how it relates toperceived color


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2. The relationship between light andthe 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, wecan consider how light is related to various aspects of colorperception

-the first thing we will consider is the ways colors can vary, andhow they are related to physical properties of light

-it turns out that colors vary along 3 very closely relateddimensions: hue, brightness and saturation

-and these dimensions have a close but not direct mapping on to 3physical 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 thathas been developed to organize color percepts in terms of thephysical dimensions of light

-so lets first talk about the three dimensions of light: hue, brightnessand saturation


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Hue: Psychological Correlateof Wavelength

i. Color dimensions

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--isassociated 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”


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However, there is not a direct mappingbetween hue and wavelength.

26. Spot 1

-and one of the arguments for this is that the same exact hue canproduce very different percepts

-and this will be my first bit of evidence that color is apsychological phenomenon

-so what does this slide look like to you?

-most people would say white, right?


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27. Spot 2

-stare at the white spot for about 40 seconds


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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 momentago

-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 itat one time can look white and, depending upon what you have justseen, can look green and red another time

-this should convince you that there is not a direct mappingbetween wavelength and hue


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29. Flag 1

-here is another example

-stare at the fixation point for about 40 seconds


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30. Flag 2

-what do you see?

-most people see a “normal” American flag: white stars with bluebackground, and red and white stripes

-but again, this is the same white field you saw before


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Brightness: PsychologicalCorrelate 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 thephysical intensity of light


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However, there is not a direct mappingbetween brightness and intensity.

32. Brightness 2

-however, just like with hue, there is not a direct mapping betweenbrightness and intensity

-the light check in the shadow has exactly the same luminance asthe dark checks outside of the shadow

-this example illustrates again that there is not a direct mappingbetween the physical world and our perception of it


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Saturation: PsychologicalCorrelate 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 tolook more white


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However, there is not a direct mappingbetween vividness and saturation.

34. Saturation 1

-you could have guessed this, but there is not a direct mappingbetween saturation and vividness

-and I will prove this to you

-stare at the fixation point for about 40- seconds


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35. Saturation 2

-what do you see?

-most people see a highly saturated yellow on the right and a muchless saturated yellow on the left

-colors tend to look less saturated when you stare at them for longperiods of time (adaptation)

-however, this is a uniform field, with the same saturationthroughout

-again, this shows quite nicely that there is not a direct mappingbetween saturation and vividness


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ii. Color Mixing

SubtractiveColor Mixing

Additive Color Mixing

36. Color Mixing

-well now you know the basic dimensions over which a color canvary: 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 ofthese 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


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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


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How Paint Mixing Works

Blue Paint Yellow PaintLight SourceThe light that hits asurface (e.g., sunlight).Here, it has equalintensity at allwavelengths.

Blue paint absorbs alllonger wavelengthsand some shortwavelengths. We seethe SHORTwavelengths that itreflects.

Yellow paint absorbsall shorter wave-lengths and somelong wavelengths.We see the LONGwavelengths that itreflects.


-well to understand how this works, we have to know about what the paints do to thelight that falls on them

-let’s just assume that the light falling on the paint is something like sunlight--equalenergy 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 thatit reflected would look something like the second graph above

-similarly, yellow paint absorbs short wavelengths and reflects medium-longwavelengths; and so if we were to shine a light on a surface that was covered byyellow paint, the light that it reflected would look something like the third graph above


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Subtractive Color Mixing

BluePigment

YellowPigment

Blue paint absorbs longwavelengths. Yellow paintabsorbs short wavelengths.They both reflectMEDIUM wavelengths(green)


-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 onesthey BOTH reflect

-this is where the two curves overlap--right around green

-so subtractive color mixing works by mixing two pigmentstogether and taking away the wavelengths that they don’t BOTHreflect


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Additive Color Mixing Yellow light is composedof long wavelengths. Bluelight is composed of shortwavelengths. Together,they contains ALLwavelengths (white light)

White surfaces(reflect allwavelengthsequally).

40. Additive Mixing 1

-well there is another form of color mixing, and it is actually the one that ismost relevant for color vision

-it is called additive color mixing

-unlike subtractive color mixing, additive mixing does not work by surfacestaking wavelengths away

-instead, it works by adding wavelengths together

-now think about what would happen if you had a surface that reflected allwavelengths 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 shortwavelengths

-and so they would “add” up to form something very similar to white light

-this is additive mixing--adding lights of various wavelengths together tomake a new light


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An example of additive color mixing: PointillismGeorges Seurat, Un Dimanche apres-midi a Ile de la Grande-Jatte (1884)


-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 reallysmall and close together

-they are so small that the light they reflect gets added together andyou 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


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The light from small, nearbysurfaces (points) combine toproduce an additive mixture oflight.


-but if you get really close, you can see that it is made up of abunch of small, colored dots

-well, when you get far enough away, the light from nearby dotsbecomes added together, and you see the additive mixture that isproduced by the sum of all the the dots wavelengths


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Another example of additive colormixing: TV’s and computer monitors


-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 isactually 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 becomeclear in a little bit

-but this is another good example of additive color mixing


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Again…

Subtractive color mixing worksby removing wavelengths of light.

Additive color mixing works bycombining wavelengths of light.

44. Mixing Summary

-summary

-well we are concerned with additive color mixing because it iswhat takes place on the eye

-subtractive mixing takes place on surfaces, before the light reachesthe eye

-so from here on out, when we talk about color mixing we willmean additive mixtures

Announcements - psych.utoronto.capsych.utoronto.ca/users/vislab/CH6_1.pdfPsy 280 Fall 2000: Color...

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