A Technique for Measuring the Saturation of Phosphors at High Current Densities

Abstract

A technique is described which allows for the measurement of the saturation of luminous intensity of phosphors up to high current densities. The actual light intensity profile of the spot is measured and is related to the current density profile across the spot, the latter being obtained from measurements on a nonsaturating phosphor. This technique thus uses a point-bypoint measurement of relative efficiency vs. beam current density, and therefore no assumptions concerning the beam current profile have to be made, and no averaging of beam current over spot diameter is required. Typical data are presented from several classes of materials. For the samples reported, La202S:Tb showed the least degree of saturation, (Zn, Cd)S:Ag the greatest degree, and Zn2SiO4:Mn was intermediate to these two.Most phosphors, when excited by an electron beam, show a less than linear increase in brightness with increasing beam current density. This phenomenon is known as saturation and is indicated schematically in Fig. 1 where both intensity and efficiency are displayed as a function of beam current density. This effect can be particularly troublesome in multicolored cathode ray tubes using high current densities, e.g., post deflection focusing (PDF) type color television tubes. * Electrochemical Society Active Member. ]Key words: luminescence, phosphors, saturation. CURRENT DENSITYFig. 1. Effect of current density on: a) brightness or b) efficiency of a nonlinear phosphor.Two disadvantages will accrue if the phosphors chosen for this type of tube exhibit saturation in the current density range used by the tube: (i) the anticipated brightness increase will not be achieved, and (ii) the color balance and white field produced by color mixings will vary with beam current density. For these reasons, the intensity vs. beam current density data is an important factor in choosing the appropriate phosphors for these types of applications.Measurement of the saturation of various phosphors has been reported by a number of authors (1-6), with the work of Meyer and Palilla (6) being most directly concerned with television tube application. In these previous works, some assumption was made about the current density profile across the electron beam and a calculated average current density was used in the saturation plots. The technique reported in this article determines the actual current density profile of the spot, and so provides a point by point measurement of intensity vs. beam current density. ApparatusPhosphors used in this experiment were either obtained commercially (so designated in the appropriate figures) or prepared in the laboratory. Samples were settled onto 2.5 • 7.5 cm microscope slides using a potassium silicate-barium acetate settling method. After lacquering, the slides were aluminized in a vacuum evaporator. Aluminum deposition was regulated by a quartz crystal thickness monitor such that all samples measured had an aluminum backing of 1000 _ 50A. Saturation measurements were made in a demountable electron beam gun system sho...

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