British Kinematography (1952)

Vol. 20, No. 1 SURROUND BRIGHTNESS: KEY FACTOR IN VIEWING PROJECTED PICTURES Sylvester K. Guth# Reprinted from the Journal of the Society of Motion Picture and Television Engineers, Vol. 57, No. 3, September, 1951. THE brightness characteristics of various portions of the visual field surrounding the central or task area are of overwhelming importance in providing a comfortable visual environment.1 These brightnesses, and their relationship> to the brightness of the task, contribute favourably or unfavourably to the seeing conditions. They may influence directly the visibility of the visual task, or their effects may be more subtle and result in decreased ease of seeing. Obviously, both effects may be and otten are produced simultaneously, especially when prolonged seeing is involved. The difficulty of obtaining adequate auditorium brightness in theatres has often resulted in minimizing the importance of the surround brightnesses for ability to see and comfort of viewing. The lack of reports of discomfort has been used as one of the principal arguments for considering that there is nothing wrong with the existing viewing conditions. Such ' lack of complaints should merely be taken as the audience acceptance of what it is used to, just as it has done in many other fields. Since the motion picture is a visual task, the consideration of light and lighting can and should include the same factors that apply to other visual situations. BASIC CONSIDERATIONS In order to understand the importance of the surrounding conditions in the central field, it may be well to consider briefly the relative magnitudes of the two areas. The angular extent of the entire binocular visual field varies with the individual physiognomy and averages about 200° horizontally and 130° vertically, and is approximately elliptical in shape. The limits of various portions of the visual field are illustrated in Fig. 1. The unshaded area indicates the portion of the visual field in which objects can be seen by both eyes. The two shaded areas on the right and left represent those portions of the visual field that can be seen only by the right and left eyes, respectively. The Task Area A visual task usually occupies a limited region in the central portion of the visual field and its apparent or visual size is a function of the distance from which it is viewed. A motion picture screen, for example, appears large or small depending upon whether it is viewed from the front or rear of a theatre. The three rectangles superimposed upon the visual field, illustrated in Fig. 1, represent a motion picture screen viewed from three different positions in an auditorium. In order to be applied generally to any size screen, the viewing distance is expressed in terms of the screen width, W* Thus, a screen viewed at a distance corresponding to the screen width, W , is represented by rectangle A, the angular extent of which is about 53° horizontally and 41° vertically. If the screen is viewed from the rear part of an auditorium, or a distance of 5W, it occupies a much smaller portion of the visual field and may be represented by rectangle C. When viewed at this distance, it extends approximately 11° horizontally and 8° vertically. The intermediate rectangle B corresponds to a viewing distance of about 3 times the screen width. It should be noted that in some theatres a screen may appear even smaller than the one indicated by C. It is seen that even when the screen is viewed from a short distance, it occupies a relatively small portion of the binocular visual field. The importance of the peripheral regions can be emphasized by considering the relative areas involved. A convenient and expressive unit of apparent ♦General Electric Co.