British Kinematography (1947)

38 statement that one of the solutions of avoiding light loss is to go back to the old solid screen. Of the i oo pe r cent incident ray, the screen is therefore giving back 93 per cent, less the reflection factor. The reflection factor is normally taken as 70 per cent in a brand new screen, but if manufactured with high quality materials, this can be as high as 80 per cent. We have therefore a loss varying between 20 per cent and 30 per cent for the reflection factor — say 25 per cent, making the net return from the screen 69.75 Per cent °f tne incident ray. This, of course, with a brand new screen or an efficiently resurfaced one. Standard of Reflectivity Let us see if we can do anything basically to improve the light from the screen. The physical standards of reflection of a diffusing white reflecting plane surface are based, in scientific circles, upon magnesium oxide. Under most elaborate laboratorv conditions, it is possible to obtain a reflection factor of 98 per cent of the incident ray. To obtain this, however, it is necessary to burn magnesium ribbon and allow the oxide smoke to collect upon a polished silver surface to a depth of 2mm. The slightest breath of air will scatter the prepared reflector in all directions and to make such a surface usuable as a projection screen, the particles will have to be bound by a medium. Xow, any medium, however waterclear, will degrade quite considerably the reflecting power of the pigment. Under conditions called for in a matt white screen, in the light of present knowledge, there is very little chance of avoiding a light loss through the binder of 10 to 15 per cent. Causes of Screen Contamination Many users have no sufficiently clear idea of the processes which bring about screen contamination. We will ignore the mechanical factors of dirt and dust and deal with the main problem, that is to say, discolouration from a smoky atmosphere. To make a practical test for yourselves, just hold a lighted cigarette in your fingers so that the smoke percolates upwards through them for two or three minutes. You will find that your fingers will be stained yellow. If you try to wash the stain off you will find that soap and water will have very little effect upon it. Now consider the concentration of smoke in a theatre. Your own living room might be 15 ft. X 18 ft. In this area, with the normal seat concentration of a theatre you would have 30 or 40 seats. Allow for a capacity density of 40 per cent from 12 noon to 11 o'clock at night ; also, realise that most of these people are smoking for a large proportion of the time, and I think you can appreciate the inevitable effect. Plastic Screens The Americans have done some experimental work in the newer types of plastics for screens and my own people have also been doing a lot of work in this direction. I say " newer " types of plastics because it does not seem to be appreciated generally that the existing matt white screen is made of nitro-cellulose, which is the mother of all plastics. The newer plastics have certain attractions from the point of view of standing rougher handling, but, once installed, this attraction, of course, ceases to exist. What we are interested in is the staining from tobacco smoke and it is an unfortunate fact that general experimental work to date indicates that the newer plastic material will absorb tobacco stain and hold it as tenaciously as the nitro-cellulose screen. Summing up the screen end of film projection, it is clear that, if in the present state of the art, a manufacturer provides the theatre with a screen, the surface of which has a reflection efficiency ranging between 70 and 80 per cent, the remainder of the story is largely up to the theatre. The Screen an Expendable Item The writer feels that it is insufficiently appreciated in this country that the screen