points. An adjustable base for the projector or camera facilitates this adjustment.

It is the virtual, or reflected, nodal point of the projector that is to be co-located with the nodal point of the camera lens. Thus any adjustment of the mirror's placement or angle shifts the position of the projector nodal point with respect to that of the camera. Since the nodal point of a lens is a single point somewhere within the lens, it is not accessible for making a direct mechanical alignment. Therefore it is necessary to make the alignment optically by using test targets located in the camera field. The degree of permissible error in the alignment of the lens nodal points is a function of several variables.

The principal variable is the separation of foreground objects from the screen. When the foreground objects are quite close to the screen, one may misalign the camera by as much as an inch in any direction without inducing a visible shadow line in photography. As foreground objects approach the camera, the alignment becomes more critical, until only V32 in. of alignment error can cause a visible shadow line. Thus, when alignment targets are used, they should be placed close to the camera to simplify the alignment procedure and to assure alignment accuracy.

The type of alignment target used can impose some problems. The use of white cards requires separate illumination, and balancing the brightness can be a bit of a chore. Small sections of the Scotchlite screen may be used, but since the brightness varies inversely with the square of the distance, they are over-bright when brought close to the camera. A good procedure is to stop down the projector and camera lenses to f/22, if possible, and tip the Scotchlite targets well past 45°. At a very steep angle, their brightness can be made to match that of the screen. Under these conditions, a misalignment of as little as Vyi in. can be readily observed.

The source of light that produces a shadow line originates from the projector lens, which in turn receives its light from the lamphouse and its optics. The alignment of the lamphouse and its optics should result in symmetrical illumination of the exit pupil of the projector lens. When the exit pupil is not symmetrically illuminated, the center of the emerging light bundle is not at the lens center. And while this off-center illumination in no way affects the background scene, it does result in shifting the shadows to one side or the other, just as though one had shifted the projec tor. Any change of the projector lens iris then acts not only to change light level, but produces the equivalent of a shift in x or y of the whole projector. An iris change on a projector with a poorly centered lamp can result in up to Vi in. of apparent misalignment.

A computer-generated table has been prepared to show the alignment error that induces a 0.0002 in. shadow line on the camera negative under a variety of conditions. This dimension (0.0002 in.) represents the threshold of visibility of a line projected on a large screen. The primary utility of this data, found in the appendix on page 413, is to show the relative influence of the object-to-screen separation, and to indicate the magnitude of alignment accuracy required. The actual alignment error that can be accepted is reduced by the halo effect, which will be discussed a little later.

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