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The requirements of low-energy excitation combined with practical constraints of commercial supply and other issues, mandate the use of readily available commercial CRT phosphors, such as ZnS and Y 2 O 3 -based P22, for first-generation field-emission flat-panel displays. The use of these phosphors at low ͑e.g., р2-4 kV͒ excitation energies places considerable problems with brightness, efficacy, spectral response, long-term reliability, screen manufacture and materials synthesis, surface conditioning and outgassing protection, and low-cost manufacturing. The tradeoffs imposed by using phosphors designed for optimum performance in the 15-30 kV range at the low voltages employed by field-emission displays are presented and discussed.

The efficiency of major types of cathodoluminescent phosphors has been investigated at high-brightness ͑up to 30 000 cd/m 2 ͒ operational conditions using thermionic electron source. The accelerating voltage was varied from 4 to 14 kV and electron beam current density was independently varied from 2 to 50 A/cm 2. Under those conditions both thermal quenching and current saturation of the phosphors were observed. Due to combined influence of those factors, it was found that the best way to characterize the phosphor is plotting the efficiency ͑in lm/W͒ versus the specific power density ͑in mW/cm 2 ͒ graph. The resulting plots show the integrated influence of current loading and elevated temperature on the phosphor efficiency. The best efficiencies at 200 mW/cm 2 and 10 kV were measured for a green color phosphor Ϫ55 lm/W, for a blue one-10 lm/W, and for a red-13 lm/W.

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The major issues and tradeoffs surrounding phosphor selection for field-emission flat panel displays are identified. The two main classes of commercially available phosphors applicable to flat panel displays are contrasted, and the major physical, electrical, chemical and optical factors effecting phosphor selection are discussed. The implications of screen layering designs and cathode materials are described as they relate to phosphor characteristics. Resolution requirements for displays severely limits the maximum anode voltage, which in turn forces specific phosphor choices. Possible solutions to these limitations are explored.