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Phosphors perform the function of converting invisible high-energy particles to lower-energy visible light. The actual mechanism for the transfer of kinetic energy from the electron beam to the phosphor crystal is through the impact of the beam with the crystal. The penetration into the crystal’s outer shell of electrons ends with a collision that drives the valence electrons to a higher unstable energy state, i.e., a higher orbit. In a finite amount of time they fall back to their normal or an intermediate state, releasing a photon. The extent they fall back determines the wavelength and thus the emissive color. At 20,000 V of acceleration, the electron beam is traveling at 37% of the speed of light. The electrons’ negligible mass is offset by the benefits of the velocity squared, making anode potential a key performance modifier of phosphor luminance output.
Information in this chapter is limited to those phosphors used in medical displays. For further information on phosphors, the Electronic Industries Association (EIA) publication TEP-116C, “Optical Characteristics of Cathode-Ray Tube Screens,” should be consulted. Despite the proliferation of television and PC monitors, the amount of phosphors utilized in this way is a fraction of the total phosphor industry. Fluorescent lamps and other commercial devices consume more phosphor.
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