Conventional halftone screening technology (AM screening)

Conventional halftone screening technology (amplitude modulated, or AM) screening breaks a continuous tone black and white image into a series of dots of varying sizes and places these dots in a rigid grid pattern.

For a continuous tone color image it is necessary to break the colors down into the four process color inks, typically cyan, magenta, yellow, and black. Each color is screened to create a color separation. To avoid color or tonal variation, the rows and columns of dots for each separation are usually rotated to print at different angles.

Gradations of color and intensity are reproduced by increasing or decreasing the actual size of the dots in a particular area of the image while the distance between dots remains constant. Harlequin Precision Screening, standard with the Harlequin RIP, is an enhancement to conventional screening that uses a proprietary algorithm.

The visibility of the dot pattern itself is controlled by changing the screen ruling, or the constant distance between the dot centers, measured in lines per inch. The closer the dots in a particular screen (the higher the screen ruling), the better the representation of image detail and the less obvious the dots become, but the more difficult it may be to print or reproduce.

Moiré

Probably the most serious quality issue with halftone screening is the unavoidable creation of interference patterns known as ‘moiré’. These patterns result from the angle offset of the screens for each separation.

Moiré can also result from interference between a single screen and structures within the image objects being reproduced. This “object moiré” can be especially problematic in images incorporating patterns such as venetian blinds, picket fences, tweeds and other fabrics.

Much work has been done to minimize the problem of moiré in conventional halftone screening by altering screen angles, dot size and shape, and by placing the most obvious colors (usually cyan, magenta and black) at angles calculated to minimize interference between them, moving the moiré to the least obvious color (usually yellow).

Different methods produced varying results but could not eradicate the moiré problem entirely.

Problems with Tone Gradation: Mid-tone Jump and Banding

Another way to minimize moiré is to increase the number of screen lines per inch. As the screen ruling increases, so the interactions resulting in moiré get smaller, making them less noticeable to the eye.

Unfortunately, the higher the screen ruling, the more likely mid-tone jump or banding in vignettes are to appear. Mid-tone jump occurs most often around the 50 percent range of gray levels, particularly with round or square shaped dots, where the area of white is roughly equivalent to the black area resulting in a checkerboard effect on the screen.

A dark band of color may occur where a smooth transition should be.

Registration and Dot Gain

Using a higher line frequency to minimize moiré patterns places greater demands on the system’s ability to preserve the fidelity of the output halftone image.

In traditional print this would include dot-loss or gain at plate making and on press; in digital printing the same issues arise as the ink or toner is applied to the media.

If not handled correctly, both dot gain and registration errors can contribute to distortions in the output and a degradation in perceived image quality.

Loss of Highlight Detail

Highlight detail often suffers because the small dot sizes used to reproduce highlight information are lost in the transfer to film or plate and then to press, or at the print head in a digital printer.

This effect is especially pronounced where high line screens are used, for example to reduce moiré.