EVALUATING LIGHT SOURCES
Fiat lux! (“Let there be light!”), one of the earliest proclamations in the Book of Genesis, should be one of your first considerations in the creation of photographic stencils. Light sources have three main attributes. Insofar as a light source optimizes each attribute or not, it will produce stencils ranging from superb to poor.
The wavelengths of light, or spectral output, of an exposure source should match closely the spectral sensitivity of photo stencil materials. The closer the match-up, the more efficient the light source, and the better it will “cure” or cross-link the molecules of the stencil material, so that the stencil will be more durable. The common sensitizers used in photo films and emulsions have specific sensitivity ranges, as shown on the “Sensitizer Absorption Chart” (below). It’s a common misconception that photographic stencil materials are “UV-sensitive.” In fact, they are sensitive to light ranging from upper-ultra-violet through the visible blue part of the spectrum. (That’s why yellow lights make good safelights, because photo stencil materials don’t react to the longer wavelengths of yellow.)
Photo stencil materials, even those comparatively fast ones formulated for projection exposure, are not nearly as fast exposing or sensitive as camera film. If photographic stencil materials were made that fast, they would be virtually impossible to protect from “pre-exposure.” Thus, light sources need the power, the intensity, to expose stencils in a reasonably productive amount of time.
Light geometry concerns the angle of incidence of light rays as they strike the artwork and pass through its “clear” portions into the photo stencil material. Light rays emitted from a small area or “point source”--at a minimum distance of 1½ times the diagonal of the stencil--are more nearly perpendicular as they meet the artwork and stencil (“Perpendicular Light Incidence,” back page). This affords better transfer of image and non-image information from the artwork into the photo stencil material.
By contrast, with a widely dispersed light source such as fluorescent tubes, light rays enter the stencil material at oblique angles, quite literally angled behind the dark edges of the artwork, reducing the fine detail or resolution of the stencil (“Wide Angle Incidence,” back page).
We categorize light sources according to how well they provide the three key exposure attributes. The highest quality light sources are metal halide or carbon arc. Mercury vapor and pulsed xenon are of medium quality. Quartz lamps, fluorescent tubes. Grolites®, fotoflood bulbs, etc. have problematic deficiencies, though they can be quite adequate for low resolution work, or in shops that don’t need high stencil throughput.
All is not lost if you have a poor light source, but you will have to work smart to compensate for it, and understand and accept the limitations imposed by it. For help in making the most of your light source, contact Ulano Technical Services, Brooklyn, at 1-800-221-0616. Fiat lux!