British Kinematography (1951)

February, 1951 stableford: back projection :,'< the foveal vision is scanning at very high speed the dominating centre of interest The remaining visual impression is gathered by macular or peripheral vision, which is not sensitive at high brightness levels. However, it has about 16 times the sensitivity of foveal vision at low brightness levels, to enable us to deal with low levels of illumination. Objective consideration will suggest that only rarely does acute vision stray to tne edge of the screen. Matt and Beaded Screens By far the most generally used screen for front projection today is the matt white. This gives a first-class picture over a very wide angle, and is expected to give a reflection factor of about 07. It is rather lacking response in blue and this is accelerated as, with increasing age, it yellows and finally browns, due to the deposition of tobacco tar. This tar exists in astonishingly high concentration in our kinema theatres.2 The glass-beaded screen, the second in general use, also suffers from the latter defect, but its virtue is that it gives a much brighter picture over a narrow angle, refracting and reflecting the ray in the direction of the light source. Thus, it should be used only under appropriate conditions. At the peak of its reflectivity curve, it can give a factor of 2-0 or 3-0, taking 1-0 as the reflecting factor of a 100% perfect diffuser and reflector. This high response has fallen to 10 at about 12J° or 15° from the normal to the screen, that is a solid angle of 25° to 30°. The curve is not square-topped but has a sharp apex and falls steeply from the nodal point; it continues, but rather less, steeply, beyond the above angles. Centre-to-Side Ratio A matt white front projection screen gives a centre-to-side brightness ratio of about 1-5:1 with an average projection system, and it must be admitted that a centre-toside brightness ratio of 2 : 1 is not so rare nor so objectionable as it would appear. It is true that projection engineers as a rule strive, at great trouble and expense, to produce an illumination over a screen as even as possible, but there are many who consider that a completely even illumination over the whole of the screen is not desirable. In my experience, a completely even illumination, even when it is of high brightness, produces a flat and uninteresting picture, and side-by-side tests, with two projectors of the latest type, have shown that a picture with 1-5:1 or higher centre-to-centre brightness ratio has more sparkle and life than one with 1 : 1 ratio. Indeed, this is very much in accord with our normal sensation of seeing. Shortcomings of Kinema Projection There are several fundamentals in which motion picture presentation falls short of nature, i.e., Completely natural colour; Adequate contrast ratio; and Method of masking the picture. Stereoscopy is omitted for the reason that it is outside the orbit of this paper. Completely Natural Colour If in the studio colours are painted in with all the delicacy of nature it is to be feared that a high proportion of them will be reproduced in the kinema only as monotone grey. There are two reasons for this. Frequently the picture is inadequately lit4, consequently there is insufficient contrast ratio to generate colour in the lowest lights. The spectral response of even a new front projection screen is lacking in blue; as a screen gets dirty, there is a pronounced shift further to the red; it is equivalent to projecting colour through a pale yellow filter, then a dark yellow filter and finally a brown filter. Contrast Ratio Front projection screens as used today have been stabilised as to their contrast ratio. Their very efficiency as a reflecting agent limits their capacity for contrast. A picture is produced only by relative brightness; highlights must be produced at such intensity that the screen already illuminated by the house lights appears in the low-lights relatively black.5 An efficient back projection screen is a very inefficient front projection screen.