Human Vision

CS200 Art TechnologySpring 2011

The Retina

• Contains two types of photoreceptors– Rods– Cones

Rods

• More numerous than cones, some 120 million• Concentrated at the outer edges of the retina• More sensitive than the cones; can function in

less intensive light than cones• Almost entirely responsible for night vision• Exposed to a high concentration of photons for

a prolonged period, they become desensitized (adapted) to the environment

• Not sensitive to color

Cones

• Provide the eye's color sensitivity• Function best in relatively bright light• Their response times to stimuli are faster than

those of rods• Are much more concentrated in the central

yellow spot known as the macula

Cones

• Humans usually have three kinds of cones with different photopsins

• The first responds most to light of long wavelengths, peaking at a greenish yellow color; this type is sometimes designated L for long

• The second type responds most to light of medium-wavelength, peaking at a green color, and is abbreviated M for medium

• The third type responds most to short-wavelength light, of a bluish color, and is designated S for short

Cones

• Yellow is perceived when the L cones are stimulated slightly more than the M cones

• Red is perceived when the L cones are stimulated significantly more than the M cones

• Blue and violet hues are perceived when the S receptor is stimulated more than the other two

Color Perception

• The perception of color depends on the firing of these three types of nerve cells

• Visible color can be mapped in terms of three numbers called tristimulus values

• Color perception has been successfully modeled in terms of tristimulus values and mapped on the CIE chromaticity diagram

CIE Color Space

• Created by the International Commission on Illumination (CIE) in 1931

• Derived from a series of experiments done in the late 1920s by W. David Wright and John Guild

• Experimental results were combined into the specification of the CIE RGB color space, from which the CIE XYZ color space was derived

CIE Color Space

• The tristimulus values are not the S, M, and L responses of the human eye, but rather a set of tristimulus values called X, Y, and Z, which are roughly red, green and blue, respectively

• Two light sources have the same apparent color to an observer when they have the same tristimulus values(no matter what spectral distributions of light were used to produce them)

CIE Color Space

• The tristimulus values depend on the observer's field of view

• To eliminate this variable, the CIE defined the standard (colorimetric) observer

CIE Color Space

• The standard observer is characterized by three color matching functions

CIE xyY Color Space

• A full plot of all visible colors is a three-dimensional figure

• However, the concept of color can be divided into two parts: brightness and chromaticity

CIE xyY Color Space

• Y is a measure of the brightness or luminance of a color

• The chromaticity of a color is specified by the two derived parameters x and y

CIE RGB Color Space

• Wright and Guild's experimental results are summarized by the standardized CIE RGB color matching functions r(λ), g(λ), and b(λ)