—1-1-1-1-1-1-1-1-1-1 1 1_1_1_L

90 80 70 6 0 50 40 30REFLECTANCE To

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

0 5 IO 15 20 25 30 35 40 45 50 55 60 65 70 TRANSMISSION %

0 5 IO 15 20 25 30 35 40 45 50 55 60 65 70 TRANSMISSION %

ing light back to its source. A reflex reflector can be made by using corner mirrors or glass beads. The 3M screen uses glass beads. The limited angular distribution of reflection is illustrated in Fig. 3. Because of the controlled angle of the reflected light, the screen has a very high gain. If one observes the screen from a vantage point not more than about lA° from the axis of the projector, it will appear to be nearly 1,000 times brighter than would a matte white surface receiving the same illumination. Because of this high gain of Scotchlite, very little illumination is required from the projector; therefore front projection can provide backgrounds of almost any desired size. A 130-A arc lamp projector can easily illuminate a 30,000 sq. ft. screen of Scotchlite to balance a 200-fc-key foreground scene. Thus, screens up to 120 X 250 feet can be used.

Figure 3. Relative angular distribution of reflection of 3M Scotchlite reflective sheeting Type 7610.

Figure 3. Relative angular distribution of reflection of 3M Scotchlite reflective sheeting Type 7610.

The ability to use large background screens is one of the principal advantages of front projection. By comparison, rear projection was limited to a screen size of 20 to 30 ft., even when illuminated by three high-powered projectors. Even considering the losses of the semi-mirror, one need only use about one footcandle of illumination on the screen to balance a foreground key light of 200 fc. This is a net ratio of about 200:1 and is more than adequate to result in invisibility of the image projected on foreground objects — even a white shirt. If one considers a white shirt to be nearly 100% reflective, and the reflectivity of black velvet to be approximately 2%, this represents a ratio of only 50:1. Thus a white shirt is so dull compared to Scotch lite (200:1) that it appears to be blacker than black velvet when the Scotchlite is illuminated to the brightness of the foreground scene.

The Scotchlite material is available in two-foot-wide rolls. The screen can be constructed by simply papering the material onto a wall-like surface or wooden backing or hanging it in horizontal strips. It is only necessary to cover all of the screen area. Butt edges are not required, and pieces may be overlapped. It is advisable, however, to prepare a screen from the same production batch since a second batch may differ slightly in brightness gain.

Tesselating The Screen

Irregularities in reflection of the Scotchlite material may be minimized by cutting or tearing the Scotchlite into small pieces, scrambling the pieces, and reassembling them into a mosaic. This, however, is wasteful of material and is labor intensive. Apogee, Inc. has designed a die which cuts Scotchlite into symmetrical hexagons with curved edges; with the aid of a template the tiles are mounted on a prepared Dacron and Mylar sheet with a 3% overlap. The completed screen is checked by photographing it using a ring light and high-contrast film in order to exaggerate any imperfections that might exist. (Apogee, Inc. holds a patent #4,548,470 covering this method of screen fabrication and supplies either the complete screen assemblies or separate tiles for the user's application.) It is not necessary that Scotchlite be absolutely flat or square to the camera since its gain is quite uniform over a rather wide angle of incidence, as shown in Fig. 4.

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