Resource: media representative, with assistance from Kodak service representatives.
Plate values are different than mask exposure values
The LAMS digital platemaking process does not directly engrave a photo polymer plate. Digital flexo plates are created by first ablating the carbon mask, and then exposing the ablated plate to UV light. Although a finished flexo plate is a three-dimensional object, digital workflow only indirectly influences the creation of the final three-dimensional form through the ablation of a two-dimensional bitmap. This is an identical concept to using a film negative to UV expose a flexo plate.
When a ThermoFlex device is set up to specifications, the two-dimensional bitmap is accurately represented on the exposed mask. A 5% dot in the image will measure as 5% on the ablated mask. However, digital plates produce a sharper dot on plate than was exposed on the mask, as a result of the oxygen inhibition process. A 5% exposure on the mask may result in something like a 1% dot on the final plate.
Results will differ, depending on the photosensitivity or other characteristics of a particular vendor's plate material, UV unit intensity, and washout unit performance. There are no universal rules as to how much a dot will sharpen, and sharpening tends to increase in highlights and decrease in shadows.
Figure 1. A traditional film dot (left) and an oxygen-sharpened digital dot (right)
How much a dot sharpens in comparison to the original mask exposure value can only be determined through the fingerprinting of test plates (after UV exposure times have been finalized). The goal of fingerprinting is to determine which exposure value results in the smallest dot that can be plated and printed reliably with a given print condition. The exposure value that creates the smallest usable dot is often called the minimum dot value. This term is often confusing to customers, as they may confuse the minimum mask exposure value with the resulting dot on plate. Many new customers express surprise that the smallest dot that they think they can create on a new digital workflow system is only a 5% dot. Kodak representatives should educate you on the effects of oxygen sharpening and resulting plate values. A 5% mask value can result in a dot smaller than 1%, which is typically smaller than any results obtained with a film workflow.
You may also have concerns that a minimum exposure value higher than 0.4% represents a potential loss of grey levels, and a corresponding loss in quality. You should be aware that film workflows also result in grey level loss in highlights, due to the dot gain inherent in film workflows. Digital highlight dots are smaller, and reproduce highlights with more precision, which results in an increase in quality. Additionally, oxygen sharpening opens up shadow areas that normally fill in with analog workflows, and this gain in grey levels also increases the quality of digital plates. The effective use of bump curves and tone reproduction curves will result in the maximum amount of tone separation throughout the entire tonal range.