Phosphor Screens for High-Current-Density Cathode-Ray Tubes

Abstract

Phosphors are evaluated for use in high-current-density cathode-ray tubes. Oxide-based phosphors exhibit a linear increase in brightness with current density, whereas conventional II-VI sulfide phosphors show a less-than-linear increase. At high current densities, the sulfides also exhibit spectral shifts and a decrease in decay times. Selected rare-earth-activated oxide phosphors incorporated in a postdeflection focusing tube demonstrated a substantially linear increase in brightness, improved contrast ratio, and good color balance.The intensity of the luminescence emitted by a phosphor screen excited by cathode rays increases as the number of electrons per unit area incident upon the phosphor is increased. The linearity of the increase in the intensity of the luminescence is, however, generally limited to low current densities with phosphors used in most color-TV display systems (1-5). At high current densities, the increase is less than linear. This nonlinear behavior is called the "saturation" effect. The most common type of color-TV tube in use today is the shadow-mask version in which a perforated electrode is interposed between the electron guns and the phosphor screen in a unipotential drift space. This electrode severely limits the number of electrons reaching the screen since 85% of the electrons emitted by the guns are not transmitted by the mask (6). Other types of color picture tubes are available which considerably increase the current density at the phosphor screen by providing for finer focusing of the electron beam. One such class of tubes, the post-deflection focusing (PDF) type, produces a focusing action at the screen by replacing the unipotential drift space by a retarding field region created by a grill in close proximity to the screen. The "Chromatron" or "Lawrence" tube (7) are examples of this tube type in which either one or three electron guns are used and in which the current densities on the phosphor screens are several orders of magnitude higher than the current densities in the shadow-mask tube. Recently, another type of tube has been announced (8) which utilizes current densities between those encountered in the shadow-mask and the PDF tubes. In this tube, electron guns with improved focus are used, and a grill system having a higher optical transmittance than that of the shadow mask is placed between the phosphor screen and the electron guns. Conceivably, the development of guns having improved focus could also be utilized in shadow-mask tubes to obtain higher current densities.The brightness advantage which would be expected from the PDF grill has, however, not been realized in practice. The PDF grill has an electron transmission about six times that of the aperture mask and, in addition, provides a focusing action which delivers a higher current density at the screen. However, the phosphor characteristics preclude full utilization of the higher current density because of the nonlinear response of the phosphors at higher current densities and the decrease in contrast ratio 1 caus...

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

“…This is consistent with the results of Ref. (6) which showed that the rare earth doped oxides showed the least degree of saturation of the phosphors reported. ZnS:Ag and (Zn,Cd)S:Ag showed the greatest loss in relative efficiencies.…”

“…These data all show a greater saturation effect for a given current density than has been reported previously (6). We attribute this difference to the use of an average current density for the previous data.…”

“…Measurement of the saturation of various phosphors has been reported by a number of authors (1)(2)(3)(4)(5)(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.…”

Investigation of Fluorescence Yield of Phosphors in Vacuum Ultraviolet Region

“…Saturation of emission in cathode ray tube ͑CRT͒ phosphors due to ground state depletion and other related processes has been extensively studied in the past. [1][2][3][4][5][6] This effect reveals itself as decreasing quantum efficiency of the phosphor with increasing input power in the form of increasing electron beam current density. The emitted photon flux first increases linearly with increasing beam current for low intensity and then the increase in fluorescence output becomes sublinear.…”

“…The emitted photon flux first increases linearly with increasing beam current for low intensity and then the increase in fluorescence output becomes sublinear. [1][2][3][4][5][6] However, similar studies in the context of excitation by ultraviolet radiation are few in number. 7 It is well known in the fluorescent lamp industry that many phosphors in VUV discharges exhibit quantum efficiencies less than those obtained from plaque measurements.…”