What is the theory behind low voltage focus CRTs?

[old_tv_nut]

Originally Posted by Electronic M

Actually that spot size 1.5x the phosphor dot size is a post 1970 thing. In 1970 Zenith introduced the Chromacolor black matrix CRT. In black matrix CRTs you can have dot size larger than the phosphor because there's a black guard band where the screen is unable to light...in the older CRTs you had to underscan the phosphor dots to keep from illuminating the space between the dots or adjacent dots since that space could have a random mix of phosphor from imperfections in the phosphor deposition.

You are confusing two different things. The use of black matrix has nothing to do with line pitch/spot size.

Black matrix, also called "negative guard band" means that the visible phosphor dot area is smaller than the shadow mask hole. This makes white field purity more consistent since the whole visible phosphor dot is always fully illuminated even if the electrons coming through the hole are slightly off center. It also has the major effect of decreasing screen reflectance.

The older positive guardband tubes and the newer negative guardband tubes had illuminated dot sizes and triad spacings approximately equal. In the early tubes, the illuminated area was limited by the shadow mask hole, and in the later tubes it was limited by the black matrix opening.

The moire issue is one of line pitch vs. triad spacing. The older positive guardband tubes and the newer negative guardband tubes had illuminated dot sizes approximately equal. The ratio of line pitch to triad spacing should be about 1.5:1 so that the moire pattern is fine and therefore less visible. The strength of the moire (rather than its fineness) is dependent on the sharpness of the spot. The moire spatial frequency is a beat note between line spacing and triad spacing. If the line spacing and triad spacing are approximately the same, the moire spatial frequency is close to zero, that is, it makes a very coarse pattern. 1.5:1 makes the pattern much finer. Going to smaller triad spacing would make the tube much more expensive and more difficult for purity adjustment. 1.5:1 also is adequate for the triad spacing not to seriously limit video resolution.