Color CRT Types: Monochrome Cathode Ray Tubes were relatively straightforward devices. An electron beam accelerated to high speed by high voltage is painted across a screen by horizontal and vertical deflection plates. The electrons produces a color of varying brightness in response to the intensity of the beam. The phosphor persistence is selected such that the image will not fade too much while the beam is scanning elsewhere, but will fade rapidly enough so that "ghosts" of objects do not remain long if areas are not reilluminated on future scans. Early tubes sometimes used green or amber phosphors resulting in green or amber on black images. Modern usage is phosphors that are black when unilluminated and white when illuminated.

Color presents a problem because the color must come from the phosphors, and must be available in all parts of the screen. In theory, it would be possible to create a pattern of color dots and modulate the electron beam to illuminate specific dots with specific intensities. That would, however, require impractical precision in the focusing and aiming of the beam. Instead, separate electron guns are used for the three colors (red, blue, green) and physical barriers are used to prevent the electron beam from one electron gun from striking areas of the wrong color.

In shadow mask tubes, the colors are arranged in dots in a triangular pattern and the three offset color guns are aimed through a set of holes -- each centered on a triangle. The holes function as pinhole lenses that "focus" each color in the proper places. Either round or elliptical ("slot") holes may be used in the shadow mask, and the triangles may be either isosceles or equilateral. In extreme designs, the center slot in the triad may be brought into line with the other two slots forming a "slot mask" tube with three color slots in a line.

In Tension mask/Aperture Grill tubes, the colors are arranged in stripes and a row of vertical bars (the mask/grill) is used to align the color guns with the color stripes and preventing the wrong stripes from being illuminated. Tension mask tubes like the Sony Trintron block fewer electrons than shadow mask tubes and are brighter with fewer purity problems. On the other hand, one or more horizontal stabilizer bars are required to keep the tensioned wires from vibrating. They are visible (if you look closely) and some users find them extremely annoying.

Shadow mask tubes theoretically require a spherically curved screen. this can be fudged some to produce a flatter screen at the cost of slightly poorer resolution at the corners of the tube. Tension mask tubes perforce have only cylindrical curvature. Both types of tube block a considerable portion of the electron beam with the mask/bars. This causes the mask to heat and change geometry. That in turn can cause color purity problems.

One final problem is that electrons are negatively charged and thus repel each other. Electron beams, even if 'perfectly' focused, spread as they travel. This combines with other factors such as screen curvature compromises; focusing limits, mismatches between the dot/slot/aperture geometries and the software "pixel" grid; etc to ensure that one "pixel" will probably illuminate several color regions.

The following sites have some nice pictures of tube geometry:

And this one has a lot of detail:

Return To Index Copyright 1994-2008 by Donald Kenney.