Patent GB-1272782-A (GB)
Title: Improved dual persistence phosphor screens for electron tubes and tubes including such screens
Inventor: BISHOP ALFRED EDWARD
IPC: H01J29/32
Filed: 1969-06-23
Abstract: 1,272,782. Luminescent screens. RCA CORPORATION. 23 June, 1969 [15 July, 1968], No. 31495/69. Heading H1D. [Also in Division C4] A luminescent screen for an electron tube, wherein a reference pattern may be produced by the scanning electron beam in air traffic control, military identification, stock market processing and computer data processing for instance comprises two different phosphor materials (e.g. blue emitting ZnSiAg and yellowgreen emitting ZnCdSiCu respectively) of persistencies p 1  and p 2  separated from the electron source by a third phosphor coating (e.g. red emitting manganese activated zinc magnesium cadmium silicate) of persistence p 3 , wherein p 2 >p 3 >p 1 . The first phosphor is excited by high energy electrons penetrating the third phosphor coating, the second phosphor is excited by light from the first phosphor and also by the high energy electrons and the third phosphor emits on incidence by low energy electrons from the source which may be a single beam operated alternately at high and low potentials or two separate guns each operated at different energies. In the latter case, the beams may scan separately or simultaneously. Mesh 34 connected to coating 26 prevents raster distortion, and the tube is then operated according to post deflection acceleration principles. The first and second phosphor materials (50, 54) (Figs. 3 and 4, not shown) may comprise separate particles coated with the third phosphor material (52, 56) and intermixed in a single layer of thickness substantially greater than the size of the particles (e.g. of 5 to 40 microns). The layer is substantially free of perforations and voids and a light reflective layer (38) of Al may be included. The first core of each coated particle may comprise a light-transparent, non-luminescent particle or bead and the persistencies of the phosphors (50, 52, 54) respectively may be 0À01 to 1À0 ms. (e.g. 0À035 ms.), 1À0 to 100À0 ms. (e.g. 35 ms.), and 100À0 to 1000À0 ms. or more (e.g. 400À0 ms.). Alternatively, the first and second phosphor may be admixed in a layer beneath a second layer of the first phosphor. Phosphor particles 54 may be excited by high voltage electrons, for instance as from a low frame signal in a PPI radar system, to produce a persistent image, blue emission from phosphor 50 indicating the instantaneous presence of a signal and a stationary or moving reference mark or pattern may be in the third colour. In the preparation, the particles may be bathed with a particle absorbing mixture, e.g. gelatine, in a water solution, settled, excess poured off, washed, bathed in a submicron particle suspension of phosphor 52, to form the coating and separated and washed. These steps may be repeated to produce the desired thickness. Adherence may be improved by washing with a solution of formaldehyde, chrome alum or potassium silicate. The particles are applied as an aqueous suspension by slurry coating or settling. A nitrocellulose or acrylic polymer film may be laid over the particle layer beneath the Al film, the screen being heated subsequently to remove all volatile material.