Light Output and Secondary Emission Characteristics of Luminescent Materials

Light output and screen potential data are given for zinc sulphide, calcium tungstate, willemite and several other silicate phosphors sprayed (from suspension) on Nonex glass and bombarded by electrons. The bombarding voltage range from 0.5 to 10 kilovolts is covered. The limiting potentials of willemite and other zinc silicate screens fall in the range from 6 to more than 10 kilovolts depending on the conditions of exhaust and preparation. The limiting potentials of two zinc sulphides fall in the range from 6 to 9 kilovolts while calcium tungstate has a limiting potential usually less than 5 kilovolts. Light output as a function of screen voltage over the range from 0.5 to 10 kilovolts is shown to follow a law of the form: $${\mathrm{L=Af(i)V}}^n$$ , where L is intensity of light, i is current density, V is electron energy at the screen in kilovolts, and A is a constant. The data show that n<2 for willemite or other zinc silicates, n=2 for sulphides, and n>2 for calcium tungstate and a fused layer of willemite. Analysis and comparison of the light output current density characteristic and the persistence characteristic shows that a definite relation exists between them in the case of silicate phosphors. The function of current density may be represented by $${\text{f(i)=Ai(1−(a/(a+b))e}}^{\mathrm{-\alpha /Bi}}{\mathrm{-(b/(a+b))e}}^{\mathrm{-\gamma /Bi}})$$ . The constant α and γ are those appearing in the persistence characteristics of the material as: $${\mathrm{L=(a/(a+b))e}}^{\mathrm{-\alpha t}}{\mathrm{+(b/(a+b))e}}^{\mathrm{-\gamma t}}$$ , a formula which expresses well the persistence of silicate screens.