Geometric Relationships

The shadows cast by an actor, or any foreground object, are largely obscured by the object as the projector is brought close to the camera. The shadows are completely hidden from the camera when the camera and projector lenses occupy the same position. Since this is not physically possible, the axes of both lenses are made to coincide optically by the use of a semi-transparent mirror. The arrangement of the camera, projector, mirror and screen are shown in Fig. 1.

The function of the semi-mirror is to bend the axis of the projector in a right angle so that the light which reaches the screen appears to originate from within the camera lens. Since tine camera cannot see around or behind a foreground object, it will not see the shadow cast by that object if the shadow is confined strictly to the area behind the object. Placing the projector axis coincident with the camera axis accomplishes this objective within certain limitations that

will be described. Although the projector is located to the right of the camera in Fig. 1, it may be located on either side or may project into the mirror from above or below. It is also permissible, from a functional point of view, to interchange the camera and projector locations.

The mirror, at 45° to the projector and camera axis, reflects the projected image onto the screen; but the mirror, being semi-transparent, allows about half of the projected light to go directly through the mirror onto the nearest wall and be wasted. Such wastage is unavoidable since the mirror must be semi-transparent to permit light from the foreground scene, as well as from the background itself, to reach the camera lens.

The 45° mirror is also a partial mirror as seen by the camera, and provides to the camera a view of the side wall of the stage as well as a second image resulting from the projector waste light. To eliminate these secondary images a small, dull black screen is placed opposite the projector, as shown in Fig. 1.

Introvision (Hollywood) replaces portions of the black screen with a piece of Scotchlite screen. Supplementary lenses permit focusing the projected image onto the supplementary segments. When matched to black flats on the main set, it is possible to have actors appear to emerge from doorways and from behind objects in the projected background.

Another development by Courier Films Limited, the Zoptic Process, employs a zoom lens on the camera and the projector and interlocks the zoom controls. By simultaneous zooming of the foreground and background lenses in the same direction, objects in the field appear to move toward or away from the camera. This technique was used extensively in the 1978 production of Superman. The transmission/reflection ratio of the mirror is not critical; however, for best utilization of foreground and projection illumination, transmission should always equal or exceed reflection. Their relationship is shown in Fig. 2. The projector light that finally enters the camera experiences a reflection at the mirror to get to the screen, and then a transmission through the mirror to get to the camera. Utilization is therefore a function of the product of the reflection and transmission percentages. Even if one assumes no losses, the maximum efficiency cannot exceed 25% and it occurs at a 50/50 ratio. In front projection, one should expect two stops of light loss.

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Figure 2. Front projection mirror; transmission/reflection ratio and efficiency.

Since film exposure of foreground objects requires a given amount of light at the camera, any transmission loss through the mirror must be made up by increasing the illumination of tine foreground. Thus transmission should be as high as possible. The efficiency of utilization of the foreground (FG) illumination is a linear function of transmission and increases as the transmission increases. An increase of transmission from 50% to a value of 60% results in a 12% increase in the utilization of the foreground illumination. It can be seen from the figure that this change from 50 to 60% in the transmission results in a drop of only 4% (25 down to 24) in the utilization efficiency of the background (BG) illumination.

The scene being projected onto the screen is also being projected onto the foreground objects and actors. Whether or not the scene being projected on foreground objects will be visible in the photography depends upon the intensity of the projected light relative to the intensity of the foreground illumination. A specific high-gain intensity relationship is thus far solely a property of the Scotchlite screen.

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