An optical printer is a device consisting of one or more film projectors mechanically linked to a movie camera. It allows filmmakers to re-photograph one or more strips of film. The optical printer is used for making special effects for motion pictures, or for copying and restoring old film material.
The first, simple optical printers were constructed early in the 1920s. Linwood G. Dunn expanded the concept in the 1930s, and the development continued well into the 1980s, when the printers were controlled with minicomputers. Prime examples of optical printing work include the matte work in Star Wars (1977).
In the late 1980s, digital compositing began to supplant optical effects. By the mid-nineties, computer graphics had evolved to rival and surpass what was possible with optical printers, and optical printing is now all but gone. Improvements in film scanners and recorders allow for a complete feature film to be processed by computers, have special effects applied, and then be processed back to film.
Today, optical printing is mostly used as an artistic tool by experimental film makers, for educational purposes, or for photochemical (as opposed to digital) film restoration. As a technique, it is particularly useful for making copies of hand painted or physically manipulated film.
As in any analog process, every optical "pass" degraded the picture, just like a photocopy of a photocopy (although the degradation can be greater with contact printing than with optical printing). Also, since a new, different piece of film was exposed and printed, matching the exact colors of the original was a problem. Usually the printer work was limited to only the parts of a dissolve needing the effect. The original footage was spliced mid-shot with the optically-printed portion, often resulting in an obvious change in image quality when the transition occurs.
Other problematic artifacts depend on the effect attempted, most often alignment inaccuracies in matte work. For this reason, shots intended to be manipulated via optical printer were often shot on larger film formats than the rest of the project. Otherwise obsolete formats, such as VistaVision, remained in use for many years after they had been abandoned for the conventional shooting of scenes because their larger frame size provided greater clarity, reduced grain size when reprinted and any alignment problems were not as conspicuous.
Scratches, abrasions, cinch marks caused by rewinding a film too tight, and other blemishes are a common problem with films that are candidates for restoration. Most of these scratches are in the back side of the film in the transparent substrate that forms the film base, rather than in the photographic emulsion on the front that holds the film's actual image. In the normal process of duplicating a film in an optical printer, collimated light passes through the film base on its way to illuminate the emulsion. Any scratches, abrasions, etc., in the film base cause the light to bend such that the duplicated film ends up with bright or dark (depending on whether copying from a negative or positive original) copies of the scratches. This bending of the light is being caused by the difference in the refraction index of the substrate and the air.
If these scratches and abrasions can be prevented from being captured on the new print, it eliminates one entire area of restoration work. Three methods address this problem. First, one can simply enlarge the portion of the film not damaged. This mainly has application where the damage is confined to the edges.
The second method is wet gate printing, a. k. a., "wet-gate printing" "wet gate printing," "wet-printing, or "liquid-gate printing." The "gate" in a projector is the apparatus that holds the film in place as each frame is shown. A wet gate is filled with a fluid that coats the film as it is fed through, constantly replenishing the fluid as needed.
Wet gate printing eliminates the refraction itself by filling in the gouges in the substrate with a fluid such as perchloroethene that shares approximately the same refractive index as the substrate. With the fluid temporarily displacing the air in the scratches and abrasions, refraction simply no longer occurs, so the defects are not reflected on the new copy. The light used is fully collimated, as with standard optical printing, and the film is undamaged by the process. This method does not work if the scratches on the emulsion side are deep enough to have removed some of the silver or dye image in the original film.
The third method involves inserting a diffusion filter in the path from the collimated light source to the film. It can eliminate the projection of small defects on the back side of the film onto the new copy because the light, rather than arriving at the back of the film in perfect parallel lines, instead enters the scratch from several directions. It is therefore still diffuse when it exits, ensuring an image of the scratch will not show up as strongly on the new print. (Only the desired image on the surface of the emulsion ends up being captured in focus.) This method results in the most finely-focused capture of the original. However, it cannot remove deep scratches, and is thus less effective than wet gate printing.
- Fielding, Raymond (1972). "7". The Technique of Special Effects Cinematography. Focal Press. ISBN 0-8038-7031-0.
- Couzin, Dennis (1988) "Contact and Optical Printing Sharpness, Some Ultimate Comparisons", Image Technology (Journal of the BKSTS), August 1988, pp=282–284. (online)