> > > >

1954-55 Theatre Catalog, 12th Edition, Page 305 (269)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 305
Page 305

1954-55 Theatre Catalog, 12th Edition, Page 305



FIGURE 3 is a diagram which is an illustration of the fields of observation of a plane mirror.

dent on the surface from without the field, This effect is most beneficial when the screen must be located in positions where it is impossible to eliminate completely extraneous light. Thus, for example, the classroom and the outdoor theatre use of this screen will yield vastly superior results than will a diffusing type surface, because of the preservation of better contrast values over the entire screen surface.



The design of a reflecting type Magniglow screen assumes that specular surfaces are involved and that consequently the theories which pertain to the surfaces may be applied to this problem. Let us, for a moment, consider what happens when we look into a plane mirror. First of all, it is noticed that an image is formed seemingly as far in back of the mirror as we stand in front. Now, as we move to the right, our image correspondingly moves to the right and this motion may continue until our sight is impeded by the presence of the mirror boundary. The observeris eye then perceives the image disappearing past this boundary until the line of sight can not longer pick up the reflected light.

Another observation which can be made is that the distance of the eye from the mirror has a bearing on the mangitude of the lateral displacement which can be allowed before the image disappears at the boundary. If the viewers eye is moved closer to the plane mirror, the lateral displacement will be greater and still retain the image within the mirror boundary.

From a study of the properties of reflecting surfaces, several rules have been formulated whereby it is possible to predict the behavior of these sur faces under different conditions of observations. It has been stated that the edge of the mirror imposes restrictions on the field of view and this, .then, is the field stop of the system. Likewise, it has been stated that the proximity of the eye to the plane mirror determines in part the field of observation. The angle that the pupil of the observeris eye makes with this mirror boundary is called the tiexit pupils and determines the volume of space within which the image must lie in order to be seen by the observer. The entrance pupil of such a redecting system is the virtual image of the observers eye and the volume of space encompassed by drawing rays from the entrance pupil point to the field stop determine the boundaries within which the object must reside in order to be seen by the observer. Perhaps the pictorial presentation of a plane mirror described in terms of the field stop, entrance and exit pupils, as shown in Figure 3, will facilitate the understanding of these concepts.

There is one additional concept which must be covered before the detailed theory of Astrolite refiection may be presented. This factor involves curved mirrors, either concave or convex in nature, and the location of their focal points for imaging purposes. If light consisting of parallel rays or bundles are allowed to impinge on a concave mirror, it is noticed that upon reflection all these rays meet at the focal point of the system. Again, as with the plane mirror, if we move in a lateral direction, say to the right, the image formed by the mirror appears to move to the left. Similarly, as we analyze the relative motion of a virtual image formed by a convex mirror, the image appears to move in the same direction as the observer. Illustrations of these two cases are shown in Figure 4. As a final observation, which can be made in a qualitative fashion, it is recognized that with a curved mirror, the image appears to move at a slower rate than does a corresponding image viewed on a plane mirror,

Illustration of the Field Concept

for o Concave Mirror

Consider a concave mirror which has a radius of curvature of one inch. Then, since it is assumed that the incident

FIGURE 4 helps to show the relative motion of images formed by a convex and concave mirror.

FIGURE 5 illustrates methods of obtaining specific fields of view with concave mirrors.

light is composed of a parallel beam, the image will be formed at the focal point which in this case is one-half the radius of curvature. If it is desired to obtain a viewing angle of 90 degrees as is shown in Figure 5, the element diameter must be approximately 0.84 inch. To obtain a vertical angle of 45



\. \ \



1954-55 Theatre Catalog, 12th Edition, Page 305