Pigmentation of Crt Phosphors

To improve the contrast of a TV screen, not only the phosphor brightness, but also the daylight reflectivity of a screen has to be considered. The relation of contrast on the one hand and luminance, as well as reflectivity on the other hand, can be quantified by the Luminance-Contrast-Performance (LCP): [ LCP = frac {Luminance}{sqrt Diffuse reflectance} ]

In order to reduce the reflectivity of the white phosphor powders (e.g. ZnS:Ag,Al, Y₂O₂S:Eu), each individual phosphor particle is covered with a pigment. The pigment colour should correspond to the emission of the respective type of phosphor, for instance, CoAl₂O₄ for blue emitting ZnS:Ag,Al or Fe₂O₃ for for red emitting Y₂O₂S:Eu.

The pigment particles (e.g. CoAl₂O₄, Fe₂O₃) have to be quite small in size (80 to 120 nm) to guarantee a sufficient adhesion on the phosphor particles. In addition, other ingredients like inorganic oxides or organic polymers can be required to achieve a good adhesion and to establish a homogeneous thin layer of pigment particles on the phosphor surface.