Reverse Blue Screen

By Erland, Jonathan | American Cinematographer, September 1982 | Go to article overview

Reverse Blue Screen


Erland, Jonathan, American Cinematographer


The advent of the recent wave of motion picture special effects films, and the growing sophistication of the audience that the genre enjoys, has put an enormous strain on current travelling matte technique. Readers Of AMERICAN CINEMATOGRAPHER will recall recent discussion of the subject by David Samuelson in his column, "Talking Technically" (May and June 1982). Also in the literature is a comprehensive survey by Walter Beyer (Journal of S.M.P.T.E., March 1965, Vol. 74, No. 3 pp. 217-236), and Raymond Fielding's excellent book, "The Technique of Special Effects Cinematography" (Hastings House, 1968). There have been some nineteen patents issued in connection with this one facet of cinematography.

Most of the solutions, such as blue screen and sodium vapor, et al, were directed at the need to capture both the matte and the live action foreground scene simultaneously. In the realm of model photography, the introduction of sophisticated motion controls permitted the shooting of registered sequential passes in what is known as the front lit/back lit system. At Apogee, we use a highly refined narrow-band blue transmission screen with our specially developed fluorescent lamps for both conventional live action blue screen and front/back lit photography.

All systems, however, which depend upon an illuminated screen beyond the subject to produce a matte, are prone to the problem of reflected light known as "blue spill," especially when using shiny models or a wide angle lens. Additionally, very thin sections of the model tend to be "wrapped" by the light and drop out of the matte.

These problems, among others too numerous to mention, were all brought into harsh focus when Glint Eastwood approached Apogee, Inc., to produce the special effects for the motion picture FIREFOX. Not only did the model (a gleaming black aircraft) promise to be difficult to matte, but the backgrounds against which it would appear are the most extreme test of any matting system-namely arctic snow and ice and stunning cloudscapes.

REVERSE BLUESCREEN

To counter the above problems, Roger Dorney and I sought to turn the blue screen process "inside out." That is, instead of endeavoring to photograph an opaque subject against an illuminated screen, we proposed to photograph an illuminating source against a black background. We use the phrase "illuminating source" deliberately, because where this process differs from the familiar "front/back lit" process is that the subject itself is made to become a source of radiation rather than merely a reflector of radiation.

This is accomplished as follows: A subject model is prepared in the usual way for photography, except that some of the usual constraints can now be eliminated. It is permissible by this method to incorporate highly reflective surfaces such as glossy paint or metallic materials or to use any color, including blue. In addition, fine elements such as mesh, thin wires, struts and wings of very narrow section can now be incorporated.

When the model is complete in all its detail, it is coated with a transparent medium, such as lacquer, containing a phosphor which is invisible in the absence of ultra-violet radiation. The subject can now be photographed, illuminated by normal stage lighting sources, with the possible addition of ultra-violet blocking filters such as Wratten 2B if the lamps are of a type that emit more than a minimal amount of ultra-violet radiation.

A second pass is now filmed, on the same film load, but consecutive to it. This time the stage lights are extinguished and the subject is irradiated with ultra-violet radiation of a wave length of approximately 360 nanometers. The ultra-violet radiation striking the subject is converted by the phosphor incorporated in the coating on the surface of the subject from 360 nanometers to about 430 nanometers and thus re-emitted as visible blue light.

The subject is now functioning as an illuminating source rather than as a reflector of light falling upon it. …

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