The Cine Technician (1935-1937)

98 The Journal of the Association of Cine-Technicians i^l-c, 1936-Jan., 1937 Screen Brightness and the Visual Functions E. M. LOWRY (Kodak Research Laboratories) T] fHE object of this paper is to present the known facts, as reported in the Hterature, regarding the visual functions in so far as tliey are influenced by the brightness of the projection screen and its surroundings in the motion picture theatre. Since the enjoyment of the entertainment offered depends for the most part upon the organs of vision, the importance of providing conditions conducive to maximal visual comfort will be unquestioned. In order that the motion picture engineer may provide such conditions, it is necessary that he have as complete knowledge as possible of those factors which govern the efficiency of the visual organs. The attention of the audience must necessarily remain fixed upon the projected picture for long periods and, therefore, it is of the utmost importance that the projection system of which the screen and its background are an integral part be so adjusted as to secure undivided appreciation of the picture presented. The projection .screen and its environment constitute the visual field and the perception of the detail in the j)rojected picture is the visual task set for the eyes to perform. Therefore, a complete understanding of all factors which influence the operation of the visual functions is the first requisite to the provision of a satisfactory arrangement of illumination conditions, and, consequently, of the screen brightness. We know that fundamentally radiant energy, possessing the appropriate wave lengths and intensities, is the governing factor in the operation of all visual functions. We do not, however, know completely the process by which radiant energy is transformed into visual sensation and, therefore, the functional responses of the visual process are beyond our control except by the indirect method of adjusting those external factors with which they are correlated. These factors are the quantity, quality, and distribution of light, together with the length of time that the eye is exposed to their action. It is through the study and regulation of these factors that we may secure maximum visual efficiency. Several years ago, in addressing a meeting of the Society of Illuminating Engineers, Dr. E. C. Crittenden remarked that "Man's eye and his sensations must remain the basis for the evaluation of light." It is on such a basis that the problem of securing satisfactory screen brightness must be attacked. Dr. Troland, in an exhaustive review of the Hterature, has summarised the majority of the data available up to 1925 and, in this paper, extensive use has been made of his work for which acknowledgment is here given. As already stated, we are j)riinarily concerned with innceplion, and, conse([uently, for our purpose, the most important of the visual functions are the perceptual ones, although the motor functions may not be entirely disregarded. It is fundamental to the problem that all of the functicjus of the eye owe their initiation to light but, as will appear later, the latitude of lighting conditions is a wide one. Because of this fact, it is natural that the first phenomenon to be considered is the ability of the eye to perceive light. The least amount of energy which the eye' can perceive is dependent upon the sensitivity of the retina and retinal sensitivity automatically adjusts itself for the brightness level to which it is exposed. That is to say, the threshold is influenced by the adaptation or intensity level by which the eye has been stimulated. As a consequence, the absolute threshold is reached only after complete dark adaptation. I'nder such conditions, the least perceptible quantity of energy is of the order of 4-2 + 10"^ erg per second. This value is stated by Troland^* to be a fair average based upon a large number of determinations by different observers. For our purposes, the above figure will ha\-e more significance if reduced to photometric terms. The transformation may be accompanied by employing what has been called the mechanical equivalent of light. As reported by Coblentz and Emerson, and by Hyde, Cady and Forsythe,^ the most probable value of the mechanical equivalent may be assumed to be 0-00156 lumens per watt for the wave length to which the eye is most sensitive, namely, about 556mu., or one watt should yield 641 lumens. Now, the energy value at the absolute threshold is 4-2 410"^® watt, and therefore its value in lumens is 2-7 + 10"^^. According to Troland, this value of the quantity of light striking the retina corresponds to 7-3 + 10"^ candle, one metre from the eye, assuming a natural pupil. To go a step farther, it is found that for foveal vision the threshold is dependent upon the total flux of energy entering the eye without regard to area, so that the brightness for threshold visibility, of an object of given size, may be computed. It has been calculated that, under conditions of dark adaptation, a square whose sides subtend an angle of two degrees at the eye must have a brightness of approximately two-millionths of a foot-lambert, if it is to be detected. The unit of brightness adopted throughout this paper is the foot-lambert, defined as the brightness of a perfect diffuser which emits or reflects one lumen per square foot. We may say that any surface whatever, when viewed in a definite direction, has a brightness of x foot-lamberts, meaning that the particular surface when so vdewed has a brightness equal to that of a perfect diffuser emitting or reflecting x lifmens per square foot. On such a basis, assuming more or less perfect diffusion, the product of the illumination is foot-candles, incident upon a surface, and its reflection factor is numerically equal to its brightness in foot-lamberts. Considerable confusion exists in the literature with respect to the unit of brightness, and for this reason an attempt has been made in this discussion to reduce all brightness values to the same unit, namely, the foot-lambert. To this end, a con\-ersion factor has been applied to results given originally in millilamberts, and in those cases where the illumination is si)ecified in footcandles, a reflection factor of 0-80 has been assumed. So far the results considered have been for the threshold of vision and dark adaptation. Let us examine the effect of adapting the eye to different brightness levels, because, due to the automatic adjustment of its sensitivity, the higher the intensity the lower the sensitivity becomes. Nutting, Blanchard, and Reeves'* carefully studied the * Numerals throuehout article refer to References at end.