The choice of FM halftone dot shape is important because it has an impact on plate choice, plate imaging, the aesthetics of the final presswork as well as on the lithographic performance of the press. Because there can be a wider variation of FM dot shapes than AM, this post will just illustrate several representative dot shape options. Each vendors' offerings can then be evaluated by using the information in this post as a guide.
In an FM screen each dot is formed in a halftone cell, typically based on a grid of 16 by 16 pixels. The pixels within the cell are "turned on" in pseudo random fashion in order to form the FM dot shape or tone area. The cells are then stitched together, like a mosaic, in order to form an area of dots or tone area.
On the left (enlarged) is a single halftone 16x16 pixel cell with several pixels turned on. On the right (reduced) is a tone area defined by a mosaic of sixteen individual 16x16 halftone cells.
Following are some basic FM screening dot shapes/patterns and their performance characteristics in use.
Click on the images to enlarge.
First Order FM: Dots are all the same sizeBenefits:Image has a photographic quality even when viewed under a loupe. Often used for fine art reproductions.
Issues:Grainy as well as mottled looking flat tone areas, small dots are more difficult to image consistently on plate and hold on press. Because the issues usually outweigh the benefits, this FM dot pattern is seldom used today except for specialized work.
Second Order FM (a.k.a. Hybrid FM): Dots grow in the tone scaleSecond order FM screens are the de facto standard in today's print production. With this type of halftone screen the dots grow in size through the tone scale. Dot growth can be in one direction – perhaps forming worm-like features as in the first example (Kodak Staccato), or grow in both directions – forming more conventional looking dots as in the second example (Screen Spekta).
Benefits:Depending on the specific vendor's implementation, graininess in flat tone areas is eliminated.
Issues:Thin one or two pixel wide worm features, as in the first example, may demand higher resolution plates, and/or imaging in order to maintain consistency. Dots shapes, as in the second example, may cause a propensity for shadows to plug on press.
Second Order directional FM: Dots grow directionally in the tone scaleDots have a strong directionality. Sometimes this dot shape in a vendor's FM offering is used for only one of the process colors in order to reduce "clumping" or secondary patterns when process colors overprint.
Benefits:Eliminates secondary patterns when process colors overprint.
Issues:Directionality of the dot shape can exaggerate directional issues, such as slur and doubling, on press.
Nasty FM: Dots are plain uglyThis graphic is just intended to emphasize the fact that there is a great variety in FM screen patterns. Therefore one should not apply general statements such as "FM screening is grainy" – quality will vary according to each vendor's implementation.