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Josef Koudelka

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• Even if two illuminating lights look thesame, an object may still appeardifferent colors in them.– Under a white light consisting of two

narrow bands of cyan and red, the yellowobject can only appear as black.

Things are not always as theyappear…

X =BLACK

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• Even if two illuminating lights look thesame, an object may still appeardifferent colors in them.– Under a white light consisting of two

narrow bands of cyan and red, the yellowobject can only appear as black.

– However, the same yellow object appearsyellow under sun light.

Things are not always as theyappear…

X = YELLOW

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Reading

Seeing the LightChapter 10

Color Perception Mechanisms

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Questions to think about…If we add two colors (frequencies) of light, I perceivea single color - green + red = yellow

But if I hear two pitches (frequencies) of sound, Iperceive both of them at the same time.

What does this imply about our auditory vs. visualsystems?

What about the nature of sound vs. light might makethis so?

Try to imagine if your hearing worked like yourvision…

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If we just had rods…

How would we tell a small amount of light at λ1 from alarge amount of light at λ2?

We would see in monochrome

- as we do under scotopic conditions

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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 8

A Guess

resp

onse

wavelength400 nm 700 nm

A problem : how can we tell different huesof reds from each other?

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1852 Helmholtz• Von Helmholtz postulated three

response curves for the three types ofcones:– 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 colors produce the same response from the three cones.

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Helmholtz hypothetical spectral response curves of humanphotoreceptors.

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Key Feature:

Any color has at least two types ofcones that respond: takes care of

the problem of distinguishingdifferent hues of red…

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Determine the responsecurves

• 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 complementary (greens between 490and 565 nm), we determine where L & I andS & I responses cross.

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I > S and L then complement must have S & L > I(because green + purple = whiteIgreen + Ipurple = Sgreen+Spurple=Lgreen+Lpurple)

Can’t be done with a single wavelength14

Region without monochromatic complement -determines where the curves cross

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• The ability for hue discriminationdepends on the wavelength. From this,one can find the steep rise and fallingsegments of the response curves.

λ

λ + Δλ

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Look at the cones themselvesMicrospectrophotometry

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Spectral absorptions by 3-cone types

0.00E+00

2.00E-01

4.00E-01

6.00E-01

8.00E-01

1.00E+00

1.20E+00

400 450 500 550 600 650 700

Series1

Series2

Series3

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Understanding adding colors

• recall - to make spectral cyan weneeded blue, green and “negative red”

Cyan excites I and S equally, L ~ half as muchIf we make cyan from RGB, we need a bit more green thanblue, but then we get too much from the L cones : would need tosubtract some red - “negative red”.

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• The information contained in thechromaticity diagram is consistentwith that of the cone responsecurves.

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Adding green and red yieldsyellow

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There is no such color as reddish-green or yellowish-blue

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Four Psychological Primaries• When we additively mix red and green,

we don’t see reddish green; we seeyellow.

• When we subtractively mix cyan andyellow, we don’t see yellowish cyan, wesee green.

• Thus to describe what colors look like,we need four primaries: blue, green,yellow and red.Any hue can be verbally described as acombination of them. 22

Color Opponents

• Using the psychological primaries, wecan 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 was put

forward by Ewald Hering, was incompetition with trichromacy.

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Afterimage

+ +

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Color Cancellation

• If a color is too bluish, it can be madeless bluish by adding yellow. Theamount of yellow that one adds tocancel the blue entirely determines theblueness of the original color.

• The yellowness and blueness, andredness and greenness can be used todescribe a color quantitatively.

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Opponents:Green: red and Blue:yellow

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Which is correct :trichromacy or opponent

processing?

Biology : trichromacy

Psychology: opponent processing

Answer: 3 cone types are wired in the retina togive opponent processing!

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Opponent Processing• The responses of the L,I,S cones are

processed by three opponent channels:yellow-blue, red-green, and white-black. Itis the latter information which is passed tothe brain.

S I L type of conesY-B - + + contribution to signalR-G + - +W-Bl + + +

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Opponent ProcessingS I L

W-BlackY-Blue R-G

- -+ + + + + ++

- to brain -

Luminance channelChromatic channel Chromatic channel

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Opponent Process Curves