The maximum possible efficiency of a phosphor excited by an electron beam depends primarily on the mean excitation energy required to create, by collision, a free electron‐hole pair or a free carrier‐ionized activator pair or an excited but nonionized activator. This energy is expressed as a multiple (factor α) of the band gap energy, E g . Values of α measured for various phosphors scatter over a wide range depending on host material and activator and, in many cases, strongly deviating from α ≈ 3 observed in free‐carrier‐collection experiments in semiconductors.
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W. Lehmann (1971) studied this question.