The Dish on Dots

There is another screen that is referred to as a “hybrid” but is really an FM screen with a less random pattern than a typical first or second order stochastic screen. These screens feature a “speckle” shaped dot instead of a pure circle or other geometric pattern. Because they have a less random distribution of dots, they appear smoother then many FM screens—particularly under less than perfect printing conditions. However, they are not as smooth as AM screens and don’t have the color saturation or press stability of true stochastic screens. Hence, while their benefits are real, they actually represent a compromise between FM and AM screens. The ideal screen would not be a compromise between AM and FM screens, but would offer the best of both screens: the smoothness of AM with the press stability and color saturation of FM.
ESKO-GRAPHICS: Transitional screening refers to the mixing of AM and FM screens within a tonal range. Transitional screening uses FM in the highlights and shadows, AM in the midtones, and has been shown to yield a printable line screen. In the midtones, AM screening is preferred because it generates less dot gain and a clean tint. However, in the highlights and shadows, where the dots become too small for easy reproduction, the AM screen transitions to a FM. These screens have shown particularly good benefits in flexo printing, but also are being used for offset.
FUJI/ENOVATION: The whole idea behind transitional screening is to improve the printability of FM. Transitional screens typically start with stochastic in the highlight areas, transition to AM in the midtones, and transition back to stochastic in shadow areas. The goal is to provide easier printability in the midtones and better detail in the highlight and shadows.
HEIDELBERG: It is possible to print high line screens with AM screening. The downside is the higher the screen ruling, the finer the highlight and shadow dots become. Eventually, they reach a point where they are too small to print.
Transitional screening solves this problem by maintaining a minimum dot size, but reducing the number of dots needed to achieve to proper tonal value. In this way the lightest and darkest of tones can be reproduced more accurately, avoiding harsh breaks in the highlights and “plugging” in the shadows. Transitional screening is easier to print than stochastic and requires less stringent process controls.
KODAK: Transitional screens capture the benefits of AM and FM screening while avoiding their deficiencies. Typically, hybrid screens are classed according to which screening method forms the largest proportion of the tone scale. A transitional AM screen, also known as a “cross-modulated” or XM screen, is primarily an AM screen with elements of FM. A transitional FM screen is primarily an FM screen with elements of AM.
Transitional AM/XM screens are useful primarily in flexographic applications, where the RIP, imaging device, plate, or print reproduction process cannot render dots smaller than a given size. Transitional FM screens use larger midtone dots that are less affected by imaging variations, are easier to resolve, and generally produce smoother tone renderings in flat tint areas than first order FM screens.