Overcoming Highlight Moiré with Wasatch Precision Rosette Hybrid Screens

Mark A. Coudray / Coudray Graphic Technologies

Screen printers face a difficult time controlling highlight moiré. The situation becomes progressively more difficult as the frequency (line count) of the halftone increases. Here is the problem: as the dot diameter decreases as the tone percentage decreases, a point is reached where the physical diameter of the halftone dot is smaller than the screen mesh thread diameter. At this point, the thread blocks the mesh opening of the halftone dot and a void in the pattern appears. The situation is compounded by the use of conventional AM halftones (shown below).

Conventional AM Screen
Conventional AM Halftone

Conventional AM halftones are defined as an ordered array. This means the dots are aligned at a defined frequency and angle. The screen mesh is also typically aligned on a 0° - 90° grid on the screen frame. The ordered rows of halftones present the ideal opportunity for the mesh threads to interfere or block the dot openings. This situation is particularly pronounced when there are large areas of constant or gradually changing tones. As the dots get progressively smaller, the chances of the mesh thread blocking the dot opening increases.

Depending on the thread diameter, mesh count, and halftone frequency, the appearance of moiré will begin below the 10% tonal values. Initially, the moiré will be barely noticeable, but it will increase as the dots get smaller. Very quickly, the smooth gradients become a checkerboard moiré, or banded moiré, and the gradient is destroyed.

Below is an example of a conventional AM halftone screen printed image that exhibits highlight moiré. Take note of how the smooth gradient of the sky begins to break up and moiré as the sky meets the mountains. Click the image for a larger preview.

Conventional AM Halftone Screen Printed Image
Conventional AM Halftone Screen Printed Image

To solve this problem, printers had four choices. The first is to clip the tone at the smallest non-moiré value. The tone would be accurately reproduced to this value, but no values below the printable dot would be achieved. This is a very serious problem if the image has pastel or light tone colors. Many of these subtle colors would simply appear as white.

It is also a significant problem when trying to maintain pastel and tertiary neutrals. Neutral colors require a small component of a third color in order to maintain their neutrality. An example would be beach sand where a very small 1%-3% cyan dot is necessary to achieve the correct color. When this small percentage is not achievable, due to thread blockage, the color shifts from a neutral to a chromatic. In the case of the beach sand, it becomes a pastel orange, which is clearly unacceptable.

The second choice would be to increase the very light tonal values to a dot percentage that is reproducible. While all colors are achievable using this approach, the image can experience a tonal shift in the highlight area that results in an overall loss of contrast in the final print. This is typically evidenced by a dark, muddy look.

The third approach involves splitting the process color into a dual component. Typically, Light Cyan, Light Magenta, and Light Black. By lowering the color strength (density) of the ink, it is possible to print a larger dot that is not subject to moiré. While this approach delivers excellent results, and near continuous tone, it does require 1-3 more screens to be printed. The economic cost of this is often prohibitive.

The fourth alternative is to switch from conventional AM halftones to FM stochastic halftones. Stochastic or Random Dot halftones work differently than conventional AM halftones. They’re based on a random generation of single size dots. The frequency, or density, of the dots generated, is determined by the tone in the image. FM halftones vary throughout the image and are not fixed to a specific ordered grid like AM halftones.

FM halftones are an excellent choice for controlling highlight moiré and moiré in general. This halftone approach is used very effectively with inkjet output because the spot size can be very small. The image effectively looks like a continuous tone which is the desired objective for any halftone reproduction.

FM Halftone
FM Halftone

The challenge for screen printers is that we must print a fairly large random dot in order to minimize the thread interference we experience as the dots get smaller. We are limited to the smallest printable dot for any given halftone frequency. This will change depending on the mesh count and thread diameter we’re using as well.

Another aspect of FM halftoning is that the larger dot screen printers must use can give the reproduced image a granular or grainy effect. There are instances where this is highly desirable, but when it comes to very smooth tones and gradients (i.e. automotive paint or flesh tones), a disruptive surface texture is undesirable.

Wasatch Precision Rosette Hybrid Screens

Wasatch Computer Technology has solved this dilemma with the introduction of Wasatch Precision Rosette Hybrid Screens. This remarkable solution combines the best of both AM halftones and FM stochastic halftones by merging them into the same image. The printer can control precisely how the two screening methods are combined as well.

Wasatch Precision Rosette Hybrid Screens
Wasatch Precision Rosette Hybrid Screens

This solution allows the printer to select the size of the FM dot they’ll be printing and at what point in the tonal gradient it will appear or merge with the AM dot. This can be controlled on both the highlight and shadow regions of the tone scale.

Highligh and Shadow Controls for Wasatch Precision Rosette Hybrid Screens
Highlight and Shadow Controls for Wasatch Precision Rosette Hybrid Screens

The ability to precisely determine the exact size of the FM dot is a significant benefit for screen printers. Most stochastic or FM halftone software limits the selection of the dot size, typically by limiting the dot to the output resolution of the output device being used to image the screen (computer-to-screen) or the film positive via inkjet or laser imagesetter. Wasatch Precision Rosette Hybrid Screens let you select dot size based on multiples of your output device resolution. This means the printer has much more control of which dots will work for them based on mesh count, thread diameter, and halftone frequency.

Beyond dot parameter selection, Wasatch Precision Rosette Hybrid Screens effectively solve the highlight moiré problem in both gradients and solid tones. By combining with AM halftoning, the typical surface disruption or graininess of a pure FM halftone is minimized.

In the left hand image below (a close up view of the AM halftone screen printed image shown earlier on this page), you can see the smooth gradient moiré as the cyan dot becomes too small to print and the mesh begins to block the opening. When Wasatch Precision Rosette Hybrid Screens are applied to the cyan screen (as shown in the right hand image below), the gradient reproduces accurately and smoothly to the horizon. You can clearly see the conventional AM dot rosette in the balloons and in the mountains. Click either of the images below for a larger preview.

Moiré occurs when the cyan dot becomes too small
to print and the mesh begins to block the opening
With Wasatch Precision Rosette Hybrid Screens, the gradient reproduces accurately and smoothly to the horizon

Until the introduction of Wasatch Precision Rosette Hybrid Screens, screen printers faced a limited selection of marginally effective solutions for controlling highlight moiré. This cost effective RIP-based screening technology will improve image quality while reducing the number of screen remakes and minimizing press setup time.


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