Color Monitors

The previous discussion considered monochromatic and gray scale monitors—monitors with a single electron gun that generates varying pixel intensities by bombarding single monochromatic phosphor with varying numbers of electrons. Modern color monitors use three electron guns that are offset slightly from one another. These three guns bombard red, green, and blue phosphors, respectively, from behind a perforated metal shadow mask, positioned near the monitor face, that isolates the three beams, preventing the beams from bombarding incorrect phosphor areas. Shadow masks vary in hole size, hole shape, and hole location and are also called dot masks and aperture masks.

Shadow masks are inserted in monitors early in the manufacturing process and, together with high intensity lamps and appropriate optics that simulate electron guns, allow red, green, and blue phosphors to be applied to the monitor face using photographic techniques. In operation, a shadow mask blocks up to 70 percent to 80 percent of the electrons produced by the electron guns and produces beams with reduced size on the monitor face. Shadow masks that pass more electrons produce higher intensity displays, all other things being equal. The electron gun offsets allow the remaining electrons to pass through the shadow mask at slightly different angles and strike the appropriate phosphor areas. Absorbing the electrons can generate heat which can cause shadow mask distortion, producing image chromatic distortions when electron beams strike inappropriate phosphors. To combat this, many popular masks use invar, an alloy composed of 64 percent iron and 36 percent nickel.

Color Image Challenges

High quality color monitors are more difficult to produce than high quality monochromatic and gray scale monitors. Each of the three guns may have different operational characteristics and require individual adjustment such as gamma correction. Moreover, each electron gun has its own video amplifier with its own individual slew rate.

Since the shadow mask absorbs up to 70 percent to 80 percent of all electrons, the electron guns must produce more electrons, generating an aggregate electron beam which is more difficult to focus. Though a display controller may be in a specific mode, the monitor video amplifier slew rates may not be able to support the mode. In addition, the monitor phosphor dot density and the display controller mode may together produce low quality images since an electron beam typically passes through more than one aperture mask hole at a time. Finally, dozens of choices exist for each of the red, green, and blue phosphors. Their chromaticities, persistence, and fading characteristics vary significantly, making the job of authentic color and intensity reproduction a never-ending task.

In the final analysis, making an image attractive on one type of monitor provides no guarantee that it will appear identically on another monitor, particularly with the existence of monitor adjustment controls that are easily accessible to users. However, the various sample programs do attempt to provide some compensation for varying electron gun operational differences.