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COLOR TELEVISION

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.raFor the Reading Challenged
By the Writing Challenged

NTSC Spectrum

Suggested Texts:
Television Engineering Handbook, Benson Rev Ed. Mc Graw Hill ISBN: 0-07-004788-X
Video Demystified, K. Jack (Brooktree's Guru) Hightext, Brooktree ISBN: 1-878707-09-4
Basic Television and Video Systems, Grob, Bernard Mc Graw Hill ISBN: 0-07-024933-4

The light that lights up our world and allows us to see that world is solar energy in what is known as the visible region of the Spectrum. This visible region is a very narrow segment of this spectrum extending from ~ 440nm in the extreme blue (near ultra violet) to ~ 690 nm in the red region--with green in the middle @ ~ 555 nm.

Human vision is such that what appears as white light is really composed of weighted amounts of a continuum of so-called Black Body energy. Tungsten lamps have a similar spectral distribution.

Sodium, Mercury vapor, fluorescent (a variant of Mercury), etc., on the other hand, do not have this continuum of spectral energy, but are composed of several discrete wavelengths in proportions that "fool" the eye. There is mounting evidence that this is an unhealthy environment. [1]

Color cameras are designed to "see" three (overlapping) segments of this spectral continuum by the action of red, green and blue optical bandpass filters. The encoded color signal from the camera does not convey any real wavelength information relative to the original hue. For example, if a predominantly orange color is imaged the red sensor will describe the light as some intensity of Red only. However, the green sensor will also image some part of this orange light and convey some intensity of what is essentially green light. This only works because the optical color filters are bandpass in nature and posses finite selectivity. If they were discrete monochromatic filters the color imaging system would fail. This points out the ratiometric nature of this imaging system, i.e., the overlapping gradual gradation of the color filters--all three filter have a weighted proportion of the visible spectrum.

An ideal transfer function in this overlapping selectivity would be such that as one filter's selectivity increases the adjacent filter's selectivity decreases having a total gain of unity throughout this continuum.

On the display side of this arrangement is a display device capable of producing only three narrow nearly discreet wavelengths of Red, Green, and Blue light. This is a result of electron bombardment of certain selected phosphors inside the CRT, each releasing a quanta of photons which are essentially "Monochromatic. "The wavelength of which is a function of each's atomic structure.

This all works because human vision can be easily fooled when it comes to absolute color discrimination. Within reason, the actual color or hue of each of these three colors is not critical.

Each phosphor is formulated as a compromise between its quantum efficiency and desired hue or color. An example of this is the fact that red phosphor requires more energy to cause it to "appear" equally bright to the human observer. Evidence of this can be seen when a CRT is over driven, the first color to bloom, is red.

By now it may be obvious that an imaging system for people is different than one for machines. Machine vision is not weighted to complement a second vision system--the human eye.

This all works because human vision can be easily fooled when it comes to absolute color discrimination. Within reason, the actual color or hue of each of these three colors is not critical.

For example, in order to produce "White" light to the human observer there needs to be 11 % blue, 30 % red and 59% green (=100%). However, if you shifted, say the red light source to a longer wavelength, the white light would appear more toward cyan. White balance could be restored by changing the three color's weights, i.e. other than the original 11, 30, 59 percent ratios.

One point should be made: the human observer is very discriminating when it comes to flesh or skin tones.

By now it may be obvious that an imaging system for people is different than one for machines. Machine vision is not weighted to complement a second vision system--the human eye.

The addition of colors in the correct proportion creates white; unlike paint which darkens, e.g., black is the addition of Yellow, Cyan and Magenta pigments.


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