D-max, D-min, and De-lineation: Evaluating Artwork
By “artwork,” we mean any imaged media placed between a light source and photo-stencil 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. Image setter-exposed silver film (right-reading photographic positives*), laser toner (usually on vellum), inkjet on inkjet film, thermal images, and knifecut masking film (Rubylith® brand)—all are commercial media used for contact exposure.**
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 fabric. 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.
Photo stencils 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 cross-linked 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.
*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 in inkjet wax directly to the stencil material (“inkjet CTS”) or utilize Texas Instruments’ digital light processing (DLP®) 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 stencil making, as it allows full exposure of non-image areas even if the artwork medium is cloudy or fogged.