Color TV Perception

additive color mixing - tri-stimulus theory - eye/brain logic

What it shows:
The full spectrum of colors (including white) in a television picture is produced by the additive mixing of only three colors: red, green, and blue.

How it works:
In a color television tube, three separate electron beams are focused so as to strike the appropriate phosphor dot on the screen. By looking at the television screen under considerable magnification, one can clearly see that there are only three phosphors which are stimulated by the electron beam(s). The apparatus is diagrammed below.

To provide for a pattern of colors, a color bar generator 1   is used as the signal source to produce a standard pattern on a small studio monitor. 2   A high-quality achromatic lens, 3   positioned directly in front of the monitor, serves as a magnifying glass. A video camera, 4   aimed at the monitor, shows the color-bar pattern on a large 35" studio monitor. 5   The wide-angle view shows the entire monitor with lens in front. By zooming in through the magnifying lens, one can see the individual phosphors that make up the particular color bar.

We suggest the following procedure: Start with a wide-angle shot and position the small monitor so that the color-bar of interest is behind the magnifying lens (the monitor sits on a pair of sliding tracks so that it can be easily moved from side to side to facilitate rapid repositioning). After pointing out the particular color to the audience, zoom in to reveal its composition. Zoom back out to a wide-angle shot, move on to the next color, and then zoom in again for another close look, etc. Move from right to left, ending with the more dramatic results - the yellow and white color bars.

Setting it up:
The entire setup fits on one of our large lecture carts. The Mitsubishi monitor sits on its own cart and should be positioned next to the apparatus. It's best to have everything off to the side so as not to block the blackboard. A few important details:

(1) White balance the Ikegami with the Pattern Generator on raster.

(2) Use progressive scan (not interlace) on the Generator.

(3) Position the Nikon lens as close as possible to the monitor screen. The sliding lens mount on the optics rail allows for course focusing - because the monitor screen is curved, the demonstrator will need to refocus every time the monitor is moved to another color bar. Fine focus with the Ikegami and/or Nikon lens as needed.

(4) Use manual aperture on the Ikegami and stop the lens down on the wide-angle shot to obtain truer colors; open the lens to wide aperture on the close-ups.

Comments:
Aside from the fact that the setup uses all our best (and most expensive) video equipment, the demonstration itself is quite nice. It never ceases to amaze people that red, green, and blue is all there is. The yellow and white bars are particularly striking. How does a tricolor phosphor-dot screen produce non-spectral colors such as gold and silver? This can lead into a whole discussion of the psychology of color perception. Rating ****


1 Leader model LCG-400 NTSC Pattern Generator
2 12" Sony monitor (Trinitron model KX-1201A); the front cover-glass must be removed to get close enough to the screen.
3 50 mm Nikon camera lens
4 Ikegami model ITC-730A with an f/1:1.7, 9-108 mm zoom lens
5 35" Mitsubishi model AM-3501R color monitor; our rear-view projection TV system will not faithfully reproduce the colors as well as the Mitsubishi