FM screening differs from conventional screening by the absence of screen ruling and the use of small micro dots to represent the tonal values in an image. FM screens have a high dot gain which is comparable to the dot gain of very fine conventional screens. Some tests showed that FM screens are more stable on press compared to AM screens, that is, they experience less variation in dot gain when the solid ink density is increasing. The major questions of this study were: 1) What is the relationship between mechanical and optical dot gain for FM screens? Is the low dot gain variation of FM screens a consequence of FM screens having a large degree of optical dot gain? 2) What is the relationship between dot size and ink film thickness when printing at increasing solid ink densities? A possible explanation for the stability of FM screened halftone dots could be that they carry less ink than the larger dots in AM screens. 1) A test form with AM and FM screened tone scales was printed on a uni form, translucent, plastic substrate and measured both in transmission and reflectance mode. The substrate that was used in this study is more uniform than paper and will make transmission dot area measurements more accurate. 2) Images of dots of different sizes (21 um-147 um), printed at different solid ink densities were captured with a CCD camera. Images from the AM and FM screened tone scales in the test form were also captured with the CCD camera. It was found that the FM screen was slightly more stable on press than the AM screen, particularly at higher SIDs. The study showed that FM screens have more optical as well as more mechanical dot gain compared to AM screens. However, the mechanical dot gain of the AM screen increased more when the solid ink density was increasing. The CCD captures showed that a smaller dot has lower average density (core dot density) than a larger dot. The core dot density of a smaller dot increased less than the core dot density of a larger dot (85 um or larger) when the solid ink density was increasing. However, the effect of flare is not accounted for, it was not possible in this study to distinguish between core dot density and flare. Flare is light scattering from the substrate areas, causing the average density of the ink to be lower than it actually is. The study also showed that the average reflectance of the ink and the average reflectance of the substrate is a function of dot area, which explains in part the errors of the Murray-Davies equation. The stability of the FM screens could be explained by the higher optical dot gain and the lower increase in mechanical dot gain. A possible explanation for the lower increase in mechanical dot gain could be the lower core dot density of the FM halftone dots. The results are not fully conclusive due to practical problems with the experimental setup, further research has to be done in this area.
Library of Congress Subject Headings
Screen process printing--Evaluation; Color printing; Printing ink
Department, Program, or Center
School of Print Media (CIAS)
Gronberg, Eva, "A Study of the relationship between optical dot gain and mechanical dot gain of AM and FM halftones" (1996). Thesis. Rochester Institute of Technology. Accessed from
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