On the Limits of the Filter Concept for Color TV Screens

Abstract

The image contrast of color television tubes is reduced by the diffuse reflection of ambient light at the tube face. A recent method used to compensate this effect makes use of color filters, which are transparent to the phosphor light and absorb the rest of the visible spectrum. The subject of this paper is an investigation of the fundamental optical capability of the color filter concept. The main emphasis is placed on the sandwich system, involving a filter layer between the phosphor layer and the screen glass. Experiments were performed on test samples, using a spectrophotometer for the reflection measurements and a scanning electron microscope for measuring the phosphor light output. They yield a gain in brightness contrast performance (BCP) of 15–28% for combinations of red or blue phosphors with commercially available inorganic filter pigments. The corresponding increase of brightness can be twice as high if the contrast is held constant. Model calculations, assuming isotropic light scattering within the powder layers and linear superposition of multiple reflections between adjacent layers, show good agreement with the experimental results. Ideal square filters yield a BCP gain of 100–120%, whereas a shift of real phosphor spectra or filter spectra along the wavelength axis only yields a gain of a few percent. The conclusion drawn is that the inorganic filter pigments used set a practical limit to the gain in BCP that can be achieved with the color filter concept. Some results on mixtures of phosphors and filters are included. The experimentally found BCP gain amounts to about 7–10%, i.e., only about one‐half of the sandwich gain. A simplified model leads to approximately the same values. The different optical behavior of a color pigment in sandwich and mixture arrangements is apparently due to differences in the effective optical path length for ambient light and phosphor light.