HDS screens

Screen Structure

Although the screen structure of HDS contains a random component, there is an underlying ordered, though dispersed, structure. Dispersed screens produce structures with a high perimeter to area ratio. This allows for sharper reproduction, even at relatively low scanner resolution. HDS is ideal for reproducing subjects with moiré issues and where detailed reproduction is required.

Speed and Performance

While most FM and stochastic screening methods typically reduce the speed and performance of the RIP, HDS can be implemented with no performance and speed penalty.

Higher Quality Images

HDS eliminates the grainy highlight effect experienced with some FM screening methods that are particularly apparent in smooth areas of contone pictures, flat tints, and vignettes. These are the result of purely random screening methods that cluster dots into odd sized and shaped structures. HDS uses a dispersed but still clustered technique to produce a screen that has the benefits of both stochastic and conventional screens. The clustering improves dot gain characteristics, plate lifetime on the press, and ability to produce a representative proof while the dispersed component eliminates moiré and improves image quality.

Since individual spots do not randomly cluster into structures with odd sizes and shapes, graininess is reduced compared to other FM screening methods. At the same time, the underlying structures are so fine in scale that the benefits of increased sharpness are maintained. Though there is no regular and periodic structure, there is a certain regularity of form and scale. If you examine HDS tints closely, you will notice that the tints have a certain homogeneous structure that changes smoothly up and down the gray scale. This consistency in structure at a particular gray level or dot value eliminates many of the problems of controlling printing conditions associated with purely stochastic screening methods that contain structures of many different sizes at the same dot value.

Family of screens

HDS provides a family of screens, tailored for different types of reproduction and different output device capabilities. HDS includes standard four colorant screens for use with the standard CMYK process colors as well as variants developed for hexachrome and photo ink devices. HDS is also available in five varieties of dot structures or settings making it compatible with many different types of output device:

• HDS Super Fine: this screening is a solution for low resolution output devices.

  

• HDS Fine: this screening produces extremely smooth grain-free results and has the finest structure of any HDS setting. It is intended for use with high quality output devices capable of recording single isolated laser spots on separations. HDS Fine works best for high quality commercial printing environments with tight prepress and printing controls.

  

• HDS Medium: this screening offers a slightly larger dot structure in the midtone range. It can be used in intermediate commercial and publications printing, either sheet fed or web.

  

• HDS Coarse: this screening makes use of somewhat larger structures to increase printability and is best selected on presses which cannot hold very fine detail such as in high speed web printing.

  

  Note: The HDS Coarse screening has a double version as well, for even coarser results.
• HDS Super Coarse: this screening uses a larger structure to increase printability and the capability to retain highlighted areas of an image. It is suitable for use on mid- to low-range image setters or plate setters with resolutions between 1000 and 1600 dpi.

  

  Note: The HDS Super Coarse screening has a double version as well, for even coarser results.

Which dot to use?

The selection of which type of HDS dot to apply depends on the physical characteristics of the output device, the type of ink and paper. The physical size of the HDS dot created with each of these options will vary both with the device laser spot size and the resolution at which it is driven.