Understanding Color Vision: Trichromacy and Opponent Process Theory, Study Guides, Projects, Research of Computer Vision

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TV’s with three phosphors work because almost any color canbe generated by adding different amounts of the three primarycolors.

TV’s with three colors (phosphors) work because almost anycolor can be generated by adding different amounts of the threeprimary colors.

Why? Because we have three types of photoreceptors.

Wavelength (nm)

Absorption (%)

Trichromatic theory of color vision

Young-Helmholtz Theory (1802,1852).Our ability to distinguish between different wavelengths depends on theoperation of

three different kinds of cone receptors

, each with a unique

spectral sensitivity.Each wavelength of light produces a

unique pattern of activation

in the

three cone mechanisms.Perceived color is based on the

relative amount of activity—the pattern

of activity—

in the three cone mechanisms.

The

Principal of Univariance

For example, the M cones will respond equally to a dim green light as a brightred light. As far as the M cones are concerned, these lights look the same.

the absorption of a photon of light by a cone produces the same effect no matter

what the wavelength.

A given cone system will respond the same to a dim light near peakwavelength as a bright light away from the peak.

We can add lights to predict L, M and S responses to different amounts of 3 primaries

S M L

S M L

S M L

We can also predict L, M and S responses to different levels of saturation 400

S M L

S M L

S M L

S M L Because of the principle of univariance, two different spectra that produce the same L, M and S cone responses will look exactly the same. These pairs are called ‘ metamers ’. They make a ‘metameric match ’ 400

S M L

So the theory of trichromacy explains why we only need three primaries to produce a variety of colors. But what does an arbitrary sum of primaries look like? In 1878, Hering argued that trichromacy wasn’t enough. He asked:Why don’t we ever see yellowish blues? Or reddish greens?

Why are red and blue opposites? Why are yellow and green opposites?

blue

green

red

yellow

Color aftereffects

Hering proposed the

Opponent Process Theory

Color vision is based on the activity of two opponent-process

mechanisms:

A

RED

/GREEN

opponent mechanism.

A

BLUE

/YELLOW

opponent mechanism

S

M

L

The Blue-Yellow opponent system subtracts S from L+M cone responses

Blue-Yellow

Now with RG = L-M and BY = (L+M) – S, we can predict the appearance of any arbitrary spectrum of light. Yellow: 560 nm Blue: 450 nm Red: 650 nm Green: 520 nm 400

S M L R-G B-Y

S M L R-G B-Y

S M L R-G B-Y

S M L R-G B-Y

Our experience of color is shaped by physiological mechanisms, both in the receptors and in opponent neurons.

But, color perception depends on more than wavelength

•Light/dark adaptive state (Purkinje shift)•Adaptation aftereffects•Simultaneous contrast effects•Color constancy

Examples: