Black layer

A black layer is a coat on the filament of the indirectly heated hot cathode in a cathode ray tube, intended to optimise the output of electrons from the cathode, thus minimising the energy consumption and heat generated while maximising the picture brightness.

It has been shown that applying a layer of black color to the tungsten cathode improves the electron output. The layer has to be deposited evenly on the metal, has to be rigid and must be able to withstand elevated temperatures.

The coating process of the cathode is realised using a chemisorbed monolayer of an aliphatic monocarboxylic acid such as stearic acid. The coating is applied using a tungsten-aluminum suspension, chemically stabilised, as the heavy tungsten particles tend to sediment, thus disturbing the coating process. A patent describing this stabilisation process is US Patent 4983420 (Wolfrum, Stefan M.; Ponjee, Johannes J.; Van de Poel, Angela C.L.), filed Jan 8th, 1991.