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FLEXO Magazine : June 2014
system. So a dot that is half magenta and half white will appear to be a pastel magenta or pink color. When we use a densitometer to assess properties like tone value increase (dot gain), the instrument software actually converts the density back into linear reflectance and then computes the ratio of the reflectance of the tone to the reflectance of the solid, usually with the contribution of the substrate removed. Other process control parameters, defined in terms of density values, have equivalent parameters in the reflectance domain that are in wide use in other forms of graphic reproduction and image analysis. This would include print contrast, defined as the ratio of the density difference between the solid and the three quarter tone to the density of the solid, but in almost all other graphic applications it is defined as the lightness or luminance factor of the test area to the reference area. In some cases, the test area may be a full tone or solid color and the ref- erence may be the background, or it may be that the test area is a mid tone value and the reference is the full tone value. Print contrast, for example, measures the optimal solid ink density to the optimal dot gain value for that ink on paper job. Another way to state this is: “How much ink can I lay down before I begin to plug up in my shadows?” High print contrast is often referred to as having “pop” or “snap” where the image “jumps off the sheet” because details in the image shadows are present. COLORIMETRY IN CONTROLLING PRINTING While the spectrodensitometer converts the linear spectral products to density by applying a logarithmic transformation, the spectrocolorimeter con- verts the linear tristimu- lus values to perceptual variables using a power function transformation. Image 3 shows a com- parison between density and lightness (L). The plot converts CIE L* to 100 – L* so that the curve has the same dependence on luminance as it does visual density. Luminous reflectance is CIE Y for a reflecting surface. As can be seen from the Image 3, density and lightness have a similar shape and pattern, as linear reflectance is converted into either density or lightness or into one of the other CIE attributes, which share the same power function dependence on the linear reflectance. So any property of a print that depends on the tone value or printed dot area can be tracked by the CIE color value, X, Y or Z, or the CIE L*, or even the total color difference (ΔE*). But what are the metrics that depend on film thickness? As noted, film thickness is generally considered to be the main press control aim. So how does one control film weight with a color reading? Image 4 shows the dependence of both density and color difference on changes in film thickness, for an offset application. Notice in the panels in Image 4 that while density is approximately linear with changes in film thickness (weight), it changes only very slowly, while the color can change dra- Image 3: Plot of CIE Lightness and ISO Density JUNE 2014 | FLEXO 41