The Cine Technician (1935-1937)

Dec, 1936Jan., ig-: The Journal of the Association of CineTechnicians 101 lamberts. At the latter point, the departure from the lawis marked by a lack of further increase, although brightnesses went as high as 4320 foot-Iamberts. Between 8 and 43 foot-lamberts, he found an increase of 31 per cent, in acuity. Cobb and Geissler^^ also found a logarithmic relation between acuity and brightness for a range of from 0-0013 to 20 foot-lamberts. Cobb^^ and then Dunlop^^ made further studies with approximately the same results. Ferree and Rand^'^ found very little increase in acuity above 4 footlamberts, although there was some slight increase even at 20 foot-lamberts. Low intensities were found to be somewhat disadvantageous to eyes slightly defective, but at higher levels they appeared not very different from normal ones. Averaging the results of all observers, Troland^ arrives at a value of 5-2 foot-lamberts, beyond which further increases in brightness do not yield an appreciable gain in acuity. All of the results included in this average used test patterns which possessed maximum contrast. It has been pointed out several times, notably by Cobb and Moss,^^ that the angular specification of acuity has little significance unless the conditions under which the determinations are made are clearly specified. As an example, discrimination of form is very noticeably affected by the degree of contrast between the object and its background. It is further affected by the brightness level at which the test object is examined, and also by the time of exposure, although the latter is not quite so important as the first two, since exposure times are relatively quite long. Kolbe,^* Broca,^^ and others have investigated the effect of contrast in this connection, and Broca^^ found that, at an illumination of 4 foot-candles with a contrast of 15 per cent., the acuity was 90 per cent, higher than for a contrast of 0. Cobb and Moss^^ conclude from their results that, within the limits of 1-100 foot-lamberts, visual angle 0-8 to 16 minutes, and for exposure times of 0-075 to 0-300 seconds, the four variables, brightness level, contrast which includes glare, visual angle, and exposure time are mutually complementary. That is to say, a deficiency in one may be compensated for by an increase in one of the others. Luckiesh and Moss^^ have also found that for a given size of test object the contrast necessary for visibility becomes less as the level of brightness increases. Their data show that, for a visual angle of 2-5 minutes, a contrast of 5 per cent, is required for visibility with a brightness of 92-9 foot-lamberts. At a brightness of 0-93 foot-lambert, a contrast of 20 per cent, is needed, or if only 5 per cent, contrast is available, the angle must be 6 minutes. Conner and Gannoung^'' have examined the course of foveal and parafoveal acuity at low background luminosity and for a number of degrees of contrast between test object and background. Their results indicate that both foveal and parafoveal acuity bear a linear relation to the logarithm of the background luminosity and that contrast is a decided factor. The function increases continuously with increasing background luminosity for the brightness range covered, which was from 0-00013 to 1-0 lumens per square foot. With constant background luminosity, the acuity increases with contrast, but not quite so rapidly as in the former case. The linear relation between background luminosity and acuity has been substantiated by Lythgoe^* and by Kryswijk and Zwikker,-'^ and others. Ferree and Rand^° have also recently investigated the relation between acuitv and the intensity of light both for TECHNICIANS engaged in Film Production in STUDIO : NEWSREEL : LABORATORY SHOULD BE MEMBERS OF A.C.T. The only organisation which represents the professional interests of all Cine -Technicians. (Regisfered Trade Union No. 1995). OVER 1,200 ALREADY BELONG. — DO YOU? Membership Form, Annual Report, and full details from : — THE ASSOCIATION OF CINE-TECHNICIANS — 30 Piccadilly Mansions, : 17 Shaftesbury Avenue, : London, W. 1 'Phone : Gerrard 2366 Secretary : GEORGE H. ELVIN, A.C.I.S.