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Film Positives and Accurate Image Translation

(December 2002) posted on Sun Jan 05, 2003

Coudray examine the different types of imagesetters and offers advice for getting consistent, high-quality output.

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By Mark A. Coudray

Obtaining accurate color and image reproduction with screen printing requires careful translation of image information at each step of the design and production process. Today's original artwork is most often created in a digital environment, imaged digitally to film (or similar materials), transferred in an analog manner to the screen, and reproduced onto the final substrate by an analog printing press. For the process to work, the materials and methods used to carry and transfer the image must be stable, predictable, and capable of passing along all of the image information as the image undergoes the digital-to-analog translation.

Each step represents a single translation, bringing with it the potential for gain or loss of image quality. By far, the most common situation is image-quality loss. This month, we'll look at the issues to consider in order to avoid this problem.

Types of imagesetters

Four primary types of digitally imaged film positives are used today. The first group is toner based. Positives created in this manner usually take the form of vellum or polyester substrates imaged on high-resolution laser printers. Image quality on the film is typically enhanced with a separate processing system or solvent-based aerosol fusing agent.

The second type of positive is thermal film. This involves a special polyester-based film that becomes opaque to UV light wherever it's exposed to heat from the thermal imagesetter's heated nibs or lasers.

The third option, which is gaining in popularity, is polyester film imaged with a high-resolution inkjet printer. For this approach, special high-density opaque inks are applied to treated polyester film to create the positive image.

The fourth type of film imaging uses conventional, silver-based, photographic films exposed on high-end digital imagesetters. The resulting films can be used straight from the imagesetter to expose screens or enlarged through a projection system for large-format applications. Because silver-based materials require chemical development after the materials are exposed and reclaiming of metallic silver from the waste stream, their popularity is in decline. Nevertheless, photographic systems offer the best film positives available.

The suitability of any particular film-imaging system depends on a job's resolution and printing requirements. What might be a minimum tolerance for a CD printer would likely be overkill for a textile printer. What may work for a textile printer doing spot colors would be unsatisfactory for process color. Let's look at the capabilities and limitations of the common systems.

Film stability


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