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Light sources have three main attributes. The optimization of each determines whether stencil quality is excellent, good, or poor.

Light sources have three main attributes. The optimization of each determines whether stencil quality is excellent, good, or poor.

Spectral output The wavelengths of light, or spectral output, of an exposure source should match closely the spectral sensitivity of photostencil materials. The closer the match, the more efficient the light source, and the better it will cure or crosslink the molecules of the stencil material so that the stencil will be more durable. Common sensitizers used in photo films and emulsions have specific sensitivity ranges. It’s a common misconception that photographic stencil materials are UV-sensitive. In fact, they are sensitive to light ranging from upper ultraviolet through the visible blue part of the spectrum. That’s why yellow lights make good safelights—photostencil materials don’t react to the longer wavelengths of yellow.

Light intensity Photostencil 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 Light geometry concerns the angle of incidence of light rays as they strike the artwork and pass through its clear portions into the photostencil material. Light rays emitted from a small area or point source—at a minimum distance of 1.5 times the diagonal of the stencil—are more nearly perpendicular as they meet the artwork and stencil (perpendicular light incidence). This affords better transfer of image and non-image information from the artwork into the photostencil 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).

Light sources are categorized according to how well they provide these 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, flood 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.

The essence of screen printing is a stencil that blocks the flow of ink to non-image areas of the print and is open in image areas, allowing ink to flow freely through the mesh. The integrity of the stencil material and the uncompromised openness of the image areas are equally essential. In addition, there should be a clear demarcation between image and non-image areas. These attributes originate with the artwork.

Artwork refers to any imaged media placed between a light source and photostencil film or emulsion during exposure. Most photographic stencils are contact-exposed in a vacuum frame, so the dimensions of the artwork are the same as those of the stencil and the final print. Imagesetter-exposed silver film (right-reading photographic positives), laser toner (usually on vellum), inkjet on inkjet film, thermal images, and knife-cut masking film are examples of commercial media used for contact exposure.

Photostencils can never be better than the artwork used to generate them, nor can the final print be better than the stencil. If the dark areas of the artwork lack sufficient density to block light during exposure, or if they contain pinholes or thin spots, stencil material will harden in what should be the open image areas of the stencil, and ink will not flow readily through the image areas of the stencil during printing.

Similarly, if clear areas of the artwork are, at best, translucent rather than transparent, or contain artifacts such as dirt or dust, light energy will be filtered or blocked before it reaches the stencil material; non-image areas of the stencil won’t be fully cured or crosslinked and may break down during the printing run. Finally, if the delineation of image and non-image areas is pixilated, fuzzy, or lacking in acutance (edge sharpness or definition), so will the image edges on the final print.

This article was provided by Don Marsden, Ulano Corp., Brooklyn, NY.

Quick Tips

• For imagesetter-exposed silver film, any text or other information should be on the emulsion side of the positive and appear as you will want it to look on the final print; the emulsion side of the positive is placed against the stencil material during exposure.

• Projection exposure requires silver film positives that are considerably smaller than the final size of the print. Such positives must be of very good quality, as projection by its nature enlarges imperfections. Computer-to-screen systems don’t use artwork as we’ve defined it here. These systems either apply an image directly to the stencil material via inkjet deposition or with a digital-light-processing system to expose a screen pixel by pixel (digital direct exposure).

• The dark and clear areas of artwork can be measured on a densitometer. The opacity of the dark areas is referred to as D-max (maximum density) followed by the numeric densitometer reading. D-min refers to the minimum density, which is a measure of the clarity of the artwork. D-max 4.0 is ideal for stencilmaking, as it allows full exposure of non-image areas even if the artwork medium is cloudy or fogged.



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