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January, 1952
(JUTH ! SURROUND BRIGHTNESS
area is in terms of the solid angle, Q, in steradians,* subtended by a surface which combines the actual projected area with the distance from the eye to the centre of the surface. Thus, the relative extent of a surface can be expressed as a percentage of the total solid angle subtended by the entire binocular visual field which is approxi
2W. A more significant comparison is the solid angle in steradians subtended by the screen and the per cent, of the visual field occupied at various viewing distances. Except for those who sit very closely to the front of the theatre, the screen occupies less than about 4% of the visual held, and for the average viewer less than 1%. Thus,
Fig. I
A diagrammatic representation of the monocular and binocular visual fields. The portions occupied by a motion picture screen when viewed at three distancis, corresponding to the screen width W, 3W and 5VV, are illustrated by rectangles A, B and C, respectively. Shaded areas (right and left) represent portions of the visual field seen only by the right and left eyes, respectively. Unshaded area represents that portion of the visual field seen by both eyes.
mately 5 steradians. The solid angle subtended by a motion picture screen is dependent upon its actual size and the distance from which it is viewed, and both of these may vary over a considerable range. However, when the viewing distance is expressed in terms of the screen width, W, it is possible to illustrate the ranges of apparent sizes of screens as in Table I. It is seen that as the viewing distance increases, the angular extent of the screen diminishes rapidly between W and 2\V ', and progressively more slowly for distances greater than
The solid angle, Q, in steradians is equal to the projected area of a surface divided by the square of the distance from the surface to the eye; i.e.,
0=-
it is obvious that the magnitude of the peripheral region of the visual held makes it extremely important to the viewer of projected pictures. Consequently, this area cannot be neglected when designing the lighting for comfortable seeing conditions.
When it is considered that the viewing of projected pictures involves a dynamic rather than a static visual situation, the area immediately surrounding the screen becomes even more important. In order to see all of the picture details, the eye may rove over the entire screen, the angular movement depending upon the viewing distance. Thus, at times, the line of vision may be directed toward the edge of the screen and then the screen surround is close to the line of vision. For the longer viewing distances, a rela