Colour Vision Mechanisms 2

Synthesis of Trichromatic Theory and Opponent Processing

It seems that the trichromatic theory and opponent color theory are compatible. The three cone types (short, medium, and long wave) are incorporated into two parallel opponent processing channels (blue vs. yellow, and red vs. green). Strictly speaking, the comparison is between medium and long wave cones versus short wave and middle and long wave combined. For simplicity, the shorthand green, red, blue-yellow is used. However, this is an oversimplification.

Surface Spectral Reflectance (SSR) and Illuminating Light

The spectrum of light in an image affects the three cone classes, which then feed into parallel processing circuits (blue-yellow, red-green). The spectrum of illuminating light is not always spectrally flat (containing all wavelengths in equal proportion). The spectrum of light reflected from an object's surface is influenced by the wavelength composition of the illuminating light.

• The SSR of a surface is a fixed physical-chemical property indicating relative reflectance (light that isn't absorbed).

• The actual reflected light depends on the quality of the illuminating light.

Examples of Illuminating Light

• Daylight simulation: Light on a clear day in shadow has more power in the shortwave end of the spectrum, giving it a bluish quality.

• Tungsten lamps: Emit light across the entire spectrum, but with a steady increase towards the long wave, giving it a yellowish appearance.

Calculating Reflected Spectrum

To determine the actual reflected spectrum: for each wavelength, multiply the relative power of the illuminate by the corresponding relative reflectance (SSR). The resulting spectrum represents the light forming the image in the eye.

Color Constancy

Color constancy is the phenomenon where the perceived color of an object remains relatively constant despite variations in the spectrum of the illuminating light. Even with different spectra of light reaching the eye, the perceived color doesn't change.

Illustration

Consider a lime-green card under daylight and tungsten light:

• Daylight: The reflected spectrum has a peak in the middle (green) and some energy in the long wave regions (yellow).

• Tungsten light: The reflected spectrum is skewed, with less power in the shortwave and more in the long wave.

Despite these differences, the card appears lime green under both conditions.

Implications for Color Realism

Color realism suggests that SSR is the real-world property associated with color. The visual system extracts the real SSR from the image, discounting the illuminating conditions. If $C$ is the perceived color, $I$ is the spectrum of the illuminating light, and $S$ is the SSR, then $C$ is a function of both $I$ and $S$. However, the visual system needs to know the spectrum of the illuminate ($I$) to determine the real SSR ($S$) from the perceived color ($C$). If both $I$ and $S$ are unknown, the problem is not solvable.

$C = f(I, S)$

Possible Solutions

The spectrum of the illuminate may be reasonably predictable (e.g., daylight, indoor lighting), which could help the visual system achieve color constancy.

Broad Implications

Color appearance is more complex than spectral wavelength composition. The perception of an object's color doesn't directly equate to the wavelength composition of the light in the image.

Simultaneous Color Contrast

Simultaneous color contrast is the converse of color constancy, where the wavelength composition of light reaching the eye remains the same, but the color appearance changes due to the surrounding colors. The visual system takes into account the distribution of wavelength spectra across the visual scene, not just in one patch.

Examples

Two identical central squares surrounded by different colors appear to have different colors.

Implications

Color perception depends on the distribution of wavelength spectra across the visual scene, not just the wavelength composition of a single patch of an image.

Opponent Processing and Color Perception

The L vs. M channel in opponent processing signals changes in wavelength composition. The responses of opponent neurons vary with the actual wavelengths, not with subjective color sensation. Further cortical processing is needed to account for the subjective experience of color, incorporating phenomena like color constancy and simultaneous color contrast.

Next Steps

Next week's session will cover the cortical processing of color, as well as the evolution and genetics of color vision.