Table of Contents


Part I - Introduction

Part II - Spectral Power Distribution

Part III - Metamerism and Color Matching

Part IV - The Optical System of the Human Eye

Part V - Chromaticity Coordinates and the Color Triangle

Part VI - Color Spaces and the CIE-XYZ System

Works Consulted

The Optical System of the Human Eye

The eye is a detector of light whose sensitivity to the quality or wavelength of the light has developed into color perception. Color is only one of several qualities that go to make up our field of vision. The physical features of the light, the physiological processes in retina and brain, and finally the psychological interpretation of the physiological response determine the color sensation that we experience.

Due to the limited scope of this project, we will focus only on those physical features of the eye that relate to the perception of color.

The retina, located in the inner surface of the eye, contains light receptors (specialized photoreceptor cells), called rods and cones. Since rods are unable to distinguish color (being used only under low illumination), we will only concern ourselves with cones.

Relation Between Spectral Composition and Visual Sensation

There are three types of cones, each containing a different pigment with different absorption spectra (that is, the three types of cones have different spectral sensitivity). Hence responses of the three types of cones depend on the spectral distribution of the light reaching them.

There are several techniques for determining the spectral sensitivity of the cones from experimental results. The goal is to find the energy required at each wavelength in the visible spectrum to produce a response (from the cones) of constant magnitude. This can be done using a threshold technique, a direct luminosity match between the monochromatic light and a fixed comparison field, and several others. The spectral sensitivity curve can then be graphed by plotting the inverse of the energy against the wavelength.

The cones have their maximum sensitivity at wavelength of 555nm. All three are sensitive to all wavelengths of visible spectrum but each type has a value at which it is most sensitive. One type responds best to red light, another to green, and the third to blue. Figure 4.1 shows a typical graph of the relative sensitivity of each kind of cone to all the wavelengths of the visible spectrum. When light of some mixture of wavelengths hits the retina, all three would be stimulated to some degree. The relative amount of stimulation produced in the three kinds of receptors would then determine the perceived color.

Figure 4.1
Typical set of color sensitivity curves