Journal of the Society of Motion Picture and Television Engineers (1950-1954)

(e) Screen saturation occurs only at a high brightness level. Silver-activated zinc sulfide has therefore been used both here and in America for television recording. With recording processes involving relative movement of the film and image, the decay time of the phosphor is more important than colour or efficiency, and zinc-oxide mixtures might be of advantage. The most suitable size of cathode-ray tube is governed by a number of factors : (a) The ratio of screen size to scanningspot size should be a maximum. This discriminates against the smaller cathoderay tubes, such as those of 5-in. diameter or under. (b) The brightness should be high. For a given beam current and accelerating potential, the screen brightness is inversely proportional to the area of the raster, which favours the use of a small cathoderay tube. (c) Blemishes and grain structure in the phosphor should be negligible. Since these tend to be of constant size, independent of screen diameter, it is desirable to have as large a screen as possible. (d) The curvature of the cathoderay-tube face should be slight. Bearing the above factors in mind a 9-in. cathode-ray tube with an optically flat screen would seem to be a good compromise. This size was ultimately chosen for the display units used in the Lime Grove recording suite. The raster size was 6.7 in. X 5.02 in. The effect of light reflections in the cathode-ray tube is worth consideration. Reflections may be divided into three types, as follows : (a) Reflections from glass surfaces other than those of the screen. (b) Reflections which cause a haze over the whole surface of the screen. (c) Reflections which cause a halo round a highlight in the picture. Reflections of type (a) can be eliminated by the use of aluminium backing for the phosphor. This is desirable in any case, since the light output is increased and the danger of ion burn reduced. The crystals of the phosphor emit light in all directions, and the aluminium coating tends to reflect back that portion of the light emitted in the direction away from the recording camera while considerably attenuating any light which may have penetrated the coating and been reflected from glass surfaces behind the screen. Reflections of types (b) and (c) are less easy to reduce as they are due to the vacuum/glass and air/glass surfaces of the screen itself. Figure 8 (Fig. 9 in the complete paper) shows an enlarged view of the cathode-ray-tube screen. From any point P, rays of light PR, PA and PC are emitted. The portion of the ray entering the glass will again be partially reflected at C and again at D, E and so on. Such rays near the critical angle give reflections of type (b). Reflections of type (c) arise from rays such as PA, which is partially reflected at A back on to the phosphor at B. The halo is intensified by light travelling from P in the direction of the arrows through the phosphor itself. A possible way of reducing type (b) and (c) reflections would be to make the glass attenuate the light. (6) TONE REPRODUCTION The television camera, and its associated apparatus, converts the light and shade of the original picture into electrical signals of particular amplitude. If these signals are used to modulate a cathode-ray tube, and to expose photo graphic film, then, when the developed and printed film is transmitted on a telecine machine, the electrical signals produced should exactly correspond in amplitude to the original signals. This would ensure that the tone reproduction W. D. Kemp: Television Recording — Abstract 381