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1. Sensation (continued)

1.2 Some Neurological Facts of Color Perception
Input port for all visual information   --   and of course, color as well   --   is our eye. Compared to a modern camera, it doesn't look too complicated (fig. 1.2-a).

    fig.1.2-a: longitudinal section of human eye (192 kByte)

Together with the cornea's curvature, the lens forms an imaging system of about 34 dioptres (f = 29mm). This focal length is controlled by "accomodation" (focussing) of the lens's curvature.

The iris in front of the lens works like the f-stop ("iris diaphragm") of your camera. As such, it controls the image luminous incidance.

Instead of the film (or CCD-/CMOS-image sensor) of your camera, the eye contains a light-sensitive layer called "retina".

Numerous rods and cones are embedded within the retina. You might compare them to the silicon pixels of your electronic camera, since they are specialized nerve fiber endings for sensing light. Rods and cones perform different functions:

R o d s   are distributed over a great portion of the retina's area (see fig. 1.2-b, red). They react sensitively to faint illumination. All rods have the very same spectral characteristics of sensitivity; so they cannot be used for distinction between colors.

    fig.1.2-b: distribution of rods and cones in the retina (53 kByte)

C o n e s   are concentrated at a place in the retina which is called fovea (see fig. 1.2-a and fig. 1.2-b, blue). They are 40dB less sensitive to light exposure than rods are. But they exist in 3 different spectral types (fig. 1.2-c), enabling color vision.

    fig.1.2-c: three different cone types (10 kByte)

As shown in fig. 1.2-d, our field of view is neither of circular shape, nor does it have equal size for green, red, blue, and white light.

    fig.1.2-d: usual field of view (39 kByte)

Normally, people think that they have a circular field of view with full 2*(pi) steradians, exhibiting full spatial resolution and full color resolution all over this solid angle. This is not true.
Our very fine spatial-, grey level- and color-resolutions are only achieved in the very center of our attention. And this center is always the place to which we direct our fovea. While we are looking around, highly sophisticated image processing within our brain builds up this entire visual model of our surroundings, which our awareness then uses without thinking.

Given the 3 spectral types of cones from fig. 1.2-c, you might conclude that there must be exactly three principal colors. This ist true but it isn't the complete truth. Our brain links the red-green-blue color informations in such a way that it results in two color contrast pairs "red/green" and "yellow/blue". (And additionally, red+green+blue are summed up to give a "lightness" information.) Fig. 1.2-e shows a schematic of this psycho-physiological color processing.

    fig.1.2-e: how brain calculates contrasting color pairs from RGB (44 kByte)

Link List and Literature

Subject used in source
section ... human eye fig. 1.2-a Herder Standard Lexikon 1959 Band 1 Sp.239
rod & cone distribution fig. 1.2-b University of York (Canada)
3 different cone types fig. 1.2-c Uni Augsburg; Seminar Sinnesorgane; --> Sehen II: Sensorik
usual field of view fig. 1.2-d Pschyrembel: "Klinisches Woerterbuch",
256.Auflage, page 587
...color pairs from RGB fig. 1.2-e Marchesi 1993: "Handbuch der Fotografie",
Band 1 page 130

Continued: 1.3 Measuring Color Sensation   Contents of entire essay   Contents of entire web site

Last modified March 2nd, 2003; 23:47