British Kinematography (1951)

I August, 1951 STABLEFORD : TELEKINEMA SCREEN 53 being produced by the B.T.H. Co.6 under similar working factors as the Telekinema. The throw is on the short side, just over 50 ft., and the spacing between the two projectors was the usual five feet. There was thus a substantial angle subtended between the two projectors and the screen. Since the two projectors were producing respectively a right and left eye image this difference in angle could produce a noticeable difference in light value between the right and left eye images as viewed from the extreme sides of the Telekinema, if a very high efficiency screen with an adequate but, nevertheless, pronounced directional effect was used. As a result of this, the reflection of the screen was kept down to a factor of 2 and this lower factor enabled the distribution of the polar curve to be maintained, with very little falloff over the whole of the viewing arc. Again, this brought us into dangerous country, as, to over-simplify, polarised light is depolarised as a function of the degree of diffusion in the reflector. A matt white surface being a nearly perfect diffuser depolarises completely. The specular metallic mirror does not depolarise at all. If there was any substantial degree of depolarisation introduced into the screen, it would intrude the left eye image into the right and vice versa. It was a nice point to produce the right degree of levelling the polar curve without introducing any noticeable depolarising effect. Large Screen Television Dealing with some of the other requirements for the screen, large screen television was the next requirement to be considered. In the earlier experimental work which has gone on with this type of screen, the aim has been to produce a screen with the highest possible reflecting factor, consistent with adequate coverage. As stated, factors up to three have been obtained consistently. Such a screen can give a remarkable picture in a normal theatre and it was with such a screen that Cinema-Television, Ltd., have given their 20 ft. wide picture demonstrations during the past year or two.2 Incidentally, that screen is also acting as the normal screen at the theatre for everyday projection of films. To be fair to Cinema-Television, Ltd., it must be stated that the picture they are producing at the Telekinema is not as good as could be produced in an ordinary theatre with ordinary viewing requirements, as the peculiar requirements of the Telekinema from stereoscopy, shortness of throw, have acted against using the most efficient screen. However, very few kinema theatres have a throw as short as 50 ft., so that, even if they were showing stereoscopy in the future, a higher factor of screen reflectance could be used. Colour Pictures With regard to the projection of normal pictures in colour, it is known that the normal matt white screen is strongly reflective at the red end of the spectrum and somewhat lacking at the blue end. When the screen discolours with age, staining by tobacco tar, the shift to the red becomes much more pronounced and the response in the blue is largely filtered out. This chromatic distortion is carried right through the visual octave. Of all the better known reflective surfaces, there is one which is distinctly better than most others from a chromatic point of view, and that is aluminium. Its reflectance goes very far into the ultra-violet field, whereas silver, the normal by which reflecting standards are judged, tails off badly by the early violet. Since aluminium is so freely available it is the obvious choice of surface and with its inherent resistance to staining and long stabilised surface life, it would enable the colour film producers, for the first time, to work to exact data of colour. Blended Colour Surround The blended colour surround went through many phases of development, and from practical experiments early on . it was soon discovered that any surface which diverted from the plane of the screen by however slight an amount produced from different parts of the surround very tricky conditions of light and shade. It was the obvious thing to put the surround in the plane of the screen by a continuation of the latter beyond the picture line. It is conceivable that the surround could be curved gradually into the