British Kinematography (1950)

November, 1950 ROSS : NEW STILL PROCESS PROJECTOR 163 The holder for the objective lenses is provided with adjustments in three directions — focus, up and down, and cross movements — which may be operated manually at the projector or by remote control from a desk at the camera position. No noticeable distortion or degradation of the picture occurs on using these controls. The amount of vertical adjustment is ±18 per cent, of the picture height, and the horizontal adjustment is ±16 per cent, of the picture width. These movements are found to be of particular value for accurately positioning the picture on the screen in relation to the foreground set. Method of Measuring Light Output. Perhaps the most important quality in the practical performance of a projector is its light output. It is, however, inadequate to state merely the total light output ; for it is necessary to have fairly uniform distribution of light over the screen, and, what is even more important, the brightness at the edges and corners of the screen must be known. Due to the action of a back-projection screen, the edges and corners are the least bright parts of the picture, and the exposure at the camera is primarily determined by these areas. It is concluded, therefore, that the light output can be best defined by stating the distribution and also the total output. It is not particularly easy to measure the light from a projector and obtain a figure which may be relied upon. To assess the distribution, it is found best to use a photo-electric photometer and make repeated measurements comparing rapidly the brightness of the required point on the screen with the centre. When averaged out, the result should be independent of the small fluctuations which arise, for example, from rotation of the positive carbon, variation in the carbons, or changes in the line voltage. To measure the total light output, a white surface of known reflectivity has been held at the centre of the screen, and the brightness of this measured with an accurately calibrated visual photometer (an S.E.I, photometer was used, after special calibration at the G.E.C. Research Laboratories, London). The average of a number of readings was taken. This brightness, in foot-lamberts, corrected for the reflectivity of the surface, was multiplied by the area illuminated on the screen (in square feet), and a correction added for the non-uniformity of illumination, TABLE 3 — Light Output Lens Light Output Distribution ins. Lumens 6.4 m .... 52,000 Edges 8% down on centre. Corners 15% down on centre. 8 m 55,000 Edges equal to centre. Corners 5% down on centre. 10 7/2 ... 55,000 Edges 10% up on centre. Corners equal to centre. 12.5 774.5 ... 49,000 Uniform. 16 7/4.5 ... 50,000 Edges 5% up on centre. Corners 5% down on centre. 22 fl 5.6 ... 32,000 Edges 8% down on centre. Corners 15% down on centre.