With CTP there is little difference between dot shapes in the plating process but there are some subtle differences on press.
In film workflow, mechanical gain in the imaging and plating process exacerbates mechanical gain on the plates and cause as non-linearities where neighboring dot structures touch, causing a tonal jump on the plate. This problem is further compounded by mechanical gain on press, leading to even larger visual tone jumps where the dots meet.
Thermal CTP eliminates all mechanical gain in the plating process. Even where neighboring dot structures touch, there is no bleeding of the pixels or dot shape into one another like there is in an analog-and Gaussian-based exposure system. The result is accurate tonal reproduction onto the plate, so that Round, Euclidean, and Elliptical all produce the same physical dot area on the plate. However, there are subtle mechanical differences between the dot shapes, based on how they respond on press, because the compounding effect of plate gain has been eliminated and the tone jumps are not nearly as obvious.
The accuracy of pixel-for-pixel reproduction, and therefore the edges of each halftone dot, is wholly dependant on the optical resolution of the device and media. For CTP devices that do not deliver accurate reproduction of each pixel, differences will be less subtle than on a CTP device.
Dot shape descriptions
Figure 1. EllipticalP
Figure 2. Heavy Elliptical
Figure 3. Light Elliptical
Figure 4. Line
Figure 5. Line1
Figure 6. Rhomboid
A commonly used dot shape that gives a smooth appearance in highlights and midtones. It is commonly used in imaging flesh tones and images with high and medium key detail. Dot gain and tonal jumps can be a problem in the shadow areas, because the white space at the center of four adjoining circles can easily become filled with additional ink as the dots grow and begin touching. However, with accurate and stable imaging, shadow detail is preserved remarkably well on press.
Figure 7. Round
Figure 8. Euclidean
Figure 9. Square
Dans la procédure CtP, il n'y que très peu de différence entre les formes de point de trame lors de la confection des plaques. Cependant, de subtiles différences apparaissent sur la presse.