Hybrid AM Screening/XM Screening

Hybrid AM screening (a.k.a. XM screening) is a method to compensate for resolution issues in the print production process in either plate, plate imaging, processing, press condition or a combination of those areas. When there is an issue with resolution it typically reveals itself as an inability to hold small dots in highlights and/or shadows. For example, in this image:Poor resolution has caused tone clipping - a loss of highlight dots on the girl's cheek and hair as well as plugging and loss of detail in the shadows.
This screening method began in flexography as a way to recover the loss of highlight and shadow dots resulting from the low resolution rubber-like plates and plate exposure methods used in that process.In flexography, small highlight dots either fail to image on plate, or if they do, they may not have the strength to hold up under pressure on press and simply bend over, creating "scum" dots and harsh tone breaks.
This screening method has recently been marketed to offset printers as a way to recover highlight and shadow tones that might otherwise be lost.
The underlying screening technology is typically the vendor's conventional AM screen and is indistinguishable from it as this image shows – top gradient AM, bottom gradient Hybrid AM/XM:The only differences occur at the extreme highlights (1%-3%) and shadows (97%-99%).

How Hybrid AM/XM screens overcome resolution issues

Here is an unscreened gradient:If we look at just the highlights, this is what the 1%-3% dots should look like when it's screened (in this case at 240 lpi):However, if the plate has low resolution, or the CtP device has problematic resolution, or if the plate processing has issues, or the press condition is not optimal then there may be a loss of highlight dots. In this example, the 1% dots are lost:Hybrid AM/XM screening recovers the lost part of the tone range by constraining the size of highlight and shadow dots so they they never get smaller than a size that can be held through the plating/printing process. For example, if the smallest reproducible dot is a 2% or 98% dot, then that is the smallest the system will image. Recovering the 1% tone when only 2% dots can be used, is done by imaging 50% of those 2% dots in the 1% tone area. The result looks like this:Hybrid AM/XM screens are so called because they leverage a technique borrowed from FM screening (see February 26, 2009 blog entry). Dots are all the same (2% in this example) size, placed in pseudo-random fashion with their frequency (number) changed to vary the tone.
Here is a 4/C conventional AM screened image:
To compare with a Hybrid AM/XM image:Although these are at different lpis you can see the important difference which is at the extreme highlights of the gradient.

Possible issues with Hybrid AM/XM screening

The dots used in flat tone areas are discontinuous as is shown here with a 3% AM tone on the left and 3% Hybrid AM/XM tone on the right:This can result in grainy appearing flat tone areas, pastels, and light screen tone values of black. The larger highlight dots are subject to more dot gain as solid ink density varies and hence may become more visible in the reproduction. Gradients may appear "noisy" at the transition from gradient to unprinted page.

Evaluating Hybrid AM/XM screening offerings

• Only compare AM and Hybrid AM/XM screening at the same lpi (i.e. 175 lpi AM to 175 lpi Hybrid AM/XM or 240 lpi AM to 240 lpi Hybrid AM/XM).
• A 1% dot a 240 lpi is a single pixel imaged at 2400 dpi (10.6 micron).
• A 1% dot a 150 lpi is two pixels (10.6 micron each) imaged at 2400 lpi (21 micron).
• Do not assume that the inability to print a single pixel is the fault of the press. A press in reasonable mechanical/chemical condition can print a 240 lpi AM screen. Separate the print production process to plate, plate imaging, processing, and press condition to determine where the resolution limitation is taking place.
• In offset printing, the tone scale is typically identical to the vendor's conventional AM screen offering - only the size of highlight and shadow dots are constrained. The range of tones that are constrained may be predetermined/preset by the vendor, or many be adjustable by the customer, depending on the vendor's implementation.
• Vendors differentiate themselves by how well their screens transition to the XM tone area and the smoothness of those tones.
• Vendors also differentiate themselves by whether they allow the printer to set the minimum dot sizes themselves or whether it is fixed at a certain value by the vendor.