Patent GB-822017-A (GB)
Title: Improvements in and relating to cathode ray tube systems
IPC: H01J31/20
Filed: 1955-12-13
Abstract: 822,017. Cathode ray tubes. MORLE, C. W. (Philco Corporation). Dec. 13, 1955, No. 35742/55. Class 39(1). [Also in Group XL(b)] In a cathode ray tube picture display system a portion of the light from the screen is reflected by diffusive light reflector which compensates for the different path lengths from the screen to the photocell on to a photocell to generate a control signal which is fed to the video input circuit or to the scanning circuit to correct registration errors of the beam on the screen, or to derive signals identifying the beam position. The light to excite the photocell is preferably obtained from phosphor indexing strips which have a short persistence and fast decay, e.g. cerium activated calcium silicate having an exponential decay time of 1/20 of a microsecond, and the beam is keyed on at maximum intensity when the indexing strips are struck by the beam. The diffusive reflector is formed by a wall coating of aluminium or silver having a roughened surface or a non-conductive coating, e.g. MgO, deposited on the wall anode, or the diffuse reflecting coating may be deposited on a rough glass wall. The diffusive reflecting layer reflects light in different directions and does not obey the specular reflection law that the angle of incidence is equal to the angle of reflection ; this gives an improvement in the evenness of the spacing of the indexing pulses as the average transit times of the light paths are equal for all parts of the screen. The amplitude of the indexing pulses is also increased over a conventional tube with a light absorbing wall anode since only a small proportion of the light reaching the photocell is direct light from the screen. The signal to noise ratio of the indexing pulses is therefore improved, as indicated graphically in Fig. 3 (not shown). In one form, Fig. 1, phosphor indexing strips 16 are separated by aluminium film 18 from fluorescent layer 12 and light emitted from the indexing strips 16 when scanned by the electron beam is reflected by diffusive coating 20 having terminal 22 to window 24 near which a photocell (not shown) is placed to reduce noise in the photocell due to light conducted through the glass body of the tube cone a light absorbing coating 19 is provided at the screen periphery or a light absorbing glass envelope is used. In a colour receiver, Fig. 2 (not shown), the screen has colour phosphor triplets and indexing phosphor strips on opposite sides of an aluminium layer. The tube window outside which the photocell is arranged is preferably formed as a lens. The screen strips are separated by spaces of half the width of the strips. In another form, Fig. 4 (not shown), two photocells 80, 81 are arranged inside the tube envelope. A prior art device using a colloidial graphite wall anode which absorbs rather than reflects light and is associated with a photocell for generating indexing signals, is referred to. If the photocell is located inside the tube between the gun and screen, e.g. at the junction of neck and cone, the optical lens must be placed in front of the photocell to enable collection of light from any part of the screen area.