Quote:
Originally Posted by old_tv_nut
Actually, no -the minimum beam current is a certain fraction of the maximum, and that ratio sets the contrast ratio. Darkening the faceplate darkens both highlights and lowlights and keeps the ratio the same in this case. A dark faceplate does imrove poor contrast due to reflection of room light, since it darkens the reflected light twice (light going in and reflection coming back out), while darkening the highlights only once (light coming out).
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Where the issue is here is the Indextron needs a minimum amount of beam current to excite the index phosphor, to provide the feedback to the color switching circuit, keeping the timing in lock. This made the image appear as if the brightness was too high since it also illuminated the other phosphors as well.
In the very few industrial aviation indextron displays I saw, they placed OLCI glass in the front of the tube face to darken the background, since the high black level would be a nuisance to the pilots in the aircraft. This had the net effect of as you say cutting the maximum brightness, but the OLCI coating on the glass prevented the glare from sunlight from washing out the picture so it was a favorable tradeoff.
These little sets were really pricey when they were new. I remember it was a toss up between one of these, a 8" or a 19" at >$500.