ABSTRACT
Although it is true that a camera lens can be used as a projection lens, a projection lens does have several differences from a camera lens:
They must be able to withstand the high power densities encountered in projection. (This assumes film projection. See discussion for Figure 22.6, an liquid crystal display (LCD) projection lens.) This is particularly true when a high-wattage xenon arc is being used. There is usually no problem with coatings; the problem is with optical cements. Canada Balsam is totally unsuitable and, fortunately, this is rarely used today. “Lens Bond,” a thermosetting optical cement made by Summers Labs (Fort Washington, PA) works very well. It is a polyester resin. Versions are available for room-temperature curing, oven curing, and curing with a UV lamp. They meet the requirements of MIL-A-3920 with N d = 1.55. A similar material is made by Norland Optical (New Brunswick, NJ). LCD projection. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315222295/bd2ad0d9-a48e-40ac-8986-c51474069cbd/content/fig22_6.tif"/>
Epoxy cements are also sometimes used. These materials can withstand very high temperatures. A material that the author has found to be very useful is Hysol OSO-100. Parts must be cured overnight at 100 °C. N d is 1.493. Lenses can withstand temperatures to 125 °C. Another useful material is TRA-BOND F114, Tracon Inc., Medford, Massachusetts. The N d for this compound is 1.54. Useful over the temperature extremes −60°F to 130°F after a 24-h room-temperature cure.
An iris is not needed. These lenses have a fixed aperture stop.
Sufficient back focus must be provided to clear the film transport mechanism. This generally eliminates a field-flattening lens near the film plane ANSI (1982).
The lens exit pupil must correspond to a source image of the lamp. This is typically 4.0 in. from the film plane for xenon arc 70-mm projectors. About 2.2 in. are necessary for academy (35-mm) format projection.
Because the projection distance is always at least 100 focal lengths, projection lenses are frequently designed for infinite conjugates. This can sometimes be a dangerous mistake because, for wide-angle lenses with appreciable distortion, the lens aberrations are strongly influenced by conjugate distances. See comments regarding this in Chapter 9. All designs in this section are presented for an infinite conjugate and the user is advised to check performance at the actual conjugate distance.
The screen is generally a cylinder and curved toward the audience. This radius is usually between 0.8 and 1.5 times the lens-to-screen distance. On a first-order basis, the film should be on a radius same as the screen radius.
266 267However, because this radius is very long as compared to the film width, a flat film surface is generally used.
Projectors made by Pioneer have a 50-in. cylindrical radius (center of curvature is toward the arc). Likewise, the IMAX projector has a 18.78 cylindrical radius that requires a cylindrical field lens on the projection lens side. Strong–Ballantine projectors have a flat gate.
268 269ANSI 196M calls for 16 ft. Lamberts screen illuminance. Although the eye resolves 1 min arc at 100 ft. Lamberts, its resolution drops to 2 min arc at 16 ft. Lamberts screen illuminance. If an observer was in the rear of the theater (at the projector) his angular resolution, θ, would be θ = 1 R F , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315222295/bd2ad0d9-a48e-40ac-8986-c51474069cbd/content/eq109.tif"/>
where R is the resolution at the film in lp/mm and F is the focal length of the projection lens (in mm). This applies to an observer at the rear. At the front of the theater, an observer will always see an aberrated image (if the lens was perfect, he would see the magnified image of the film grain). A compromise then is to achieve a resolution at the middle of the theater of 2 min of arc. This is equivalent to 1 min arc= 1/RF at the center of the field of the projection lens. At 30° off-axis (as seen by an observer at the center of the theater), the resolution could be reduced to 5 min arc. See SMPTE (1994) as a guide to projected image quality.
