The Cine Technician (1935-1937)

102 The Journal of the Association of Cine-Technicians ^t-c., iq36-Jan., 1937 normal and presbyopic observers. For a brightness range of 0-4 to 80 foot-Iamberts, they secured increases of 112 and 160 per cent, respectively, for the two classes of observers. Their results also disclose the fact, which has been largely overlooked, that increases in acuity are not pronounced for young eyes beyond 8 foot-lamberts, but with advancing age that eyes still show an increase at 80 foot-lamberts. These data clearly demonstrate that age has a very important influence on the effect of the intensity of light upon clear vision. Since patterns of low contrast necessarily involve brightness sensibility, it is entirely to be expected that an increase in intensity beyond that level governed by the laws of brightness sensibility vs. intensity is wasted. Hartridge*! and others claim that acuity and the laws controlling it ultimately resolve themselves into a matter of brightness discrimination. Bloom and Garten,*^ and Broca*^ also have studied the effect of adaptation upon acuity and found that dark adaptation does not compensate for low illumination, and consequently does not provide for an acuity equal to that of the light-adapted eye. Pupillary size and time of exposure all have an effect, although it is slight. The influence of colour on acuity has been carefully investigated and the conclusion reached that although there is a difference in favour of the region of the spectrum possessing maximum visibility, the difference is practically negligible so far as the common illuminants are concerned. The effect of glare, which is a component of contrast and which depends upon the distribution of light, in general, reduces acuity due to the production of local desensitisation of the retina. Nutting's^ ^ analysis of glare on the basis of pain or unpleasantness resulting from a light stimulus of high intensity showed that the glare point so defined is proportional to the cube root of the brightness to which the eye is adapted. Cobb and Moss^* find that discomfort due to a glare source is a function of its intrinsic brightness as well as the intensity of illumination incident at the eye. They also find that reduction in visibility by a glare source is greater, as the source approached the line of vision, affecting large objects of low contrast more than small ones in which the contrast is high. Speed of vision or the ability of the eye to see quickly is another of the perceptual functions with which we are concerned. Visual efficiency as based on reaction time and absolute brightness was placed by Johnson, ''^ following a review of the available data, at field brightnesses of between 1 and 2 foot-lamberts, provided the task was above threshold dimensions. In 1834, Talbot^*^ formulated the law that intermittent illumination of the retina yields a luminous impression which is determined by the average energy incident upon it. A large amovmt of data indicates that the speed with which any degree of visual excitation is approached will increase as the inten.sity of the stimulus is increased. How important this principle is in determining the illumination level for practical work depends upon an analysis of the BERT WAITE 105 BEDFORD ROAD, CLAPHAM, S.W.4 GERRARD 2366 CAMERAMAN 15 vears' American and British experience. actual operations involved ; for instance, whether the eye is required to move quickly from one dark area to another in which certain details are brighter. The persistence of visual sensation after removal of the stimulus has been recognised since early times. In fact, Aristotle made use of the fact in an attempted explanation of dreams. D'Arcy^^ made probably the first serious attempt to measure the persistence of the image of an object, by determining the rate at which a burning stick must be whirled around in order that the impression would be that of a continuous circle of light. Modern technic, however, has refined this method by determining the critical frequency of flicker. Critical frequency is studied by measuring the speed with which a sectored disc must be rotated between a source of light and the eye in order that the sensation becomes that of a continuous light. That the degree of flicker and rate of change of stimulus at which flicker disappears is a function of the absolute intensity has been definitely established from a large body of data. Ferry,** Porter,*" and Ives^" have independently established that the rate of alternation at which flicker disappears is proportional to the logarithm of the intensity. Lythgoe and Tansley^^ have measured the critical frequency of flicker for both the foveal and peripheral retina during the course of dark adaptation following adaptation to light, and also during the course of light adaptation following dark adaptation. Their conclusions were that the results of the critical frequency of flicker method are in good agreement with the performance of the eye when applied to other visual tasks. In order that an approach to the ordinary conditions of vision might be secured, Conn^ introduced the use of confusion patterns in the test field and demonstrated that persistence of vision breaks down under certain conditions so that increased intensities offer no gain in sensibility. His conclusions were that, for the conditions existing in his experiments, visual efficiency is improved up to brightnesses of the order of 32 foot-lamberts. Luckeish^^ studied the speed of reading and found an average increase of 15 per cent, for illuminations between 4 and 16 foot-candles, when the test was, as is the usual case, for black type on white paper. With black on grey paper having a reflection factor of 22 per cent., the result was an increase in speed of 50 per cent, for the same brightness range. Feree and Rand^* have found quite large increases in speed of vision with increasing illumination and with a proportionately greater gain, as the angular size of the object diminishes. The perception of motion, a close ally of speed of visioh, involves a combination of space discrimination and changes in the intensity of stimulation at given points on the retina. Basler^^ found a progressive increase in motion acuity up to 8-8 foot-lamberts. All available information shows that the voluntary functions of the eyes, such as accommodation, convergence and fixation, do not require high intensities. As evidence of this, Israel^^ foimd that the average error in accommodation was 1/23 of the total distance for a brightness of ()•()( )936 foot-lamberts, and that, for both convergence and accommodation, it was about 1/58 of the distance. Although accommodation breaks down at very low intensities, both convergence and fixation continue to the absolute threshold, which will be remembered to be V-S-t-lO'^" candle 1 foot from tiie eye. The threshold for the in\-oluntary contraction or expansion of t!ie pupil was found by Engelking^' to be about 00024 foot-lambert. Reeves'^* measurements show