W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

Wavelength 1 produces a response of size XWavelength 1 produces a response of size X

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

Wavelength 2Wavelength 2 produces a response of size X produces a response of size X

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

T w o w a v e l e n g t h s ,

o n e r e s p o n s e .

The problem of “univariance”The problem of “univariance”

So, we have a problem.So, we have a problem.

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

Here is a solution….add another cone type. Here is a solution….add another cone type.

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

Two cones can give you color visionTwo cones can give you color vision

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

X

Y

Z

X/Y = X/Y = redred, X/Z = , X/Z = greengreen

COMPARISONS ARE CRITICALCOMPARISONS ARE CRITICAL

Three cones give you Three cones give you TrichromacyTrichromacy

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

LM

S

Three cones give you Three cones give you TrichromacyTrichromacy

AnyAny light = aL + bM + cS light = aL + bM + cS

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

L

M

S

Let’s add some patches togetherLet’s add some patches together

W a v e l e n g t h ( n m )

Let’s take GREENLet’s take GREEN

W a v e l e n g t h ( n m )

M1

L1

And add REDAnd add RED

Red + GreenRed + Green = (M1+M2)/(L1+L2) = 1 = (M1+M2)/(L1+L2) = 1

W a v e l e n g t h ( n m )

M1

L1 M2

L2

Compare that to YELLOWCompare that to YELLOW

YellowYellow = M3/L3 = 1 = M3/L3 = 1

W a v e l e n g t h ( n m )

M3 L3

It follows thatIt follows that

REDRED

It follows thatIt follows that

REDRED

plusplus

GREENGREEN

Yields Yields YellowYellow

R+G and Y areR+G and Y are

METAMERSMETAMERS

This is ADDITIVE color mixtureThis is ADDITIVE color mixture

But what about color paint in kindergarten?But what about color paint in kindergarten?

x x x x x x x x x

Blue paintBlue paint

x x x x x x x x x

Yellow paintYellow paint

Mixing paint is SUBTRACTIVEMixing paint is SUBTRACTIVE

The The intersection intersection ofof Blue paint and Yellow Blue paint and Yellow paint looks paint looks GreenGreen

x x x x x x x x x

MWC

LWC

Not all comparisons are used

LWC - MWC gives you a red-green axis.

MWC

LWC

SWC

SWC - (LWC+MWC)gives you a blue-yellow axis.

LWC

SWC

MWC

MWC

LWC

GrayEverything is balanced

.

MWC

LWC

“unique” yellow

“unique” green

.

Purple… A “non-spectral” hue

Recall…Three cones give you Recall…Three cones give you TrichromacyTrichromacy

Suppose: Suppose: if S=M=L, then WHITEif S=M=L, then WHITE

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

LM

S

Suppose that L gets Suppose that L gets tired?tired?

What does S=M>L look like?What does S=M>L look like?

W a v e l e n g t h ( n m )

4 0 0 5 0 0 6 0 0 7 0 0

LM

S

BoringBoring

Stare at the centerStare at the center

Boring?Boring?

Pretty boring……Pretty boring……

Try thisTry this

Negative afterimageNegative afterimage