Why use halftone screen angles?

Go back in the history of halftoning and you'll discover that for multi-color presswork the halftone screens were always angled relative to one another. Why not just have all the screens at the same angle and be done with it? Well, when halftone dots are at the same frequency and angle then they will either print on top of each other (dot on dot)or they will print partially overlappingor they will print beside each other (dot off dot).
When dots print on top of dots you have wet ink sticking to wet ink (wet trapping) and white spaces between the dots. When dots print beside dots you have wet ink sticking to dry paper (dry trapping) with less white spaces between the dots. What does that mean? If you look at the images above you'll see that the dot on dot color looks darker and less vibrant than the dot off dot. The result is that the final blue hue, in this example, of dot on dot will also be different than the dot off dot blue because wet trapping inks reduces the ink's efficiency at filtering light. The white paper surrounding the dots also contaminates the perceived color by adding a graying effect and therefore the dot on dot printing will have less chroma (vibrancy) than the dot off dot.

The biggest issue though is that when there is slight misregistration on press the screen will shift from dot on dot to dot off dot causing the presswork to shift color and tone dramatically. However, by rotating the screens relative to one another, this wet trap/dry trap effect is randomized and therefore the color becomes more consistent when slight misregistration occurs.

The second major issue that occurs if all the screens in an AM/XM halftone have the same angle is that of moiré. Here, one of the colors has been slightly rotated - perhaps because of a small imaging problem, or because of a small press problem.
When this happens a very strong moiré appears when all colors have the same angle. However, by rotating the screens so that they are 30 degrees apart, there is some tolerance for small angle errors and moiré will not appear.

Using rotated screen angles for AM/XM halftones overcomes the dot on dot/dot off dot issues.
With FM/Stochastic screening, the same problems are overcome by using a different screen pattern for each of the process colors.

Moiré

A moiré pattern is an artifact that occurs in the print reproduction process when any two, or more, repeating patterns overlap each other.
Moiré in the print reproduction process is similar to the distortion effect on television when a presenter's clothing includes a striped or crisscross pattern as in the gentleman's shirt in the short video below from the WhatTheyThink.com website:
Click image to play
It that case, the presenter's stripe patterned shirt is "harmonically beating" i.e. has a similar frequency and angle to the video camera's sensor and/or the pixels on the computer's display. This results in the appearance of a secondary pattern or "moiré."

The most common types of moiré encountered in the print production process.

Scanning/sampling moiré
These artifacts are caused by the frequency/angle of the scanner sensor (flat bed and drum scanners, or digital camera sensor) harmonically beating with a pattern in the object being scanned. In this case the artwork ends up having the moiré embedded in it and is part of the image. For example, the original pattern in the pinstriped shirt below (left) acquires a moiré pattern when scanned (right).
If you encounter this type of moiré, there is a Photoshop technique developed by John Wheeler that may help you eliminate it. The tutorial is here: http://tinyurl.com/3mmzv4h

Moiré can also be introduced when a halftone printed image is scanned. In the picture below, the top image is how the photograph in a magazine appears to the eye and below it the result when the image is scanned.
In this case, the moiré is caused by the halftone dots in the magazine reproduction harmonically beating with the scanner's CCD array.

Subject moiré
These artifacts are caused when the halftone screen that is being used to reproduce the image on press harmonically beats with a pattern in the image being reproduced as in the example of the striped shirt below:This artifact is sometimes referred to as "screening moiré" since it is the halftone screen that is causing the problem. However, I use the term "screening moiré" to refer to a different problem - see below.

Screening moiré
Screening moiré, which is a term that is sometimes confused with subject moiré, is actually an artifact caused by either an inappropriate or incorrect halftone screen angle within a CMYK image. With modern screening systems this is rarely a problem. What is most likely to happen is that a screening moiré that is already present is somehow made more visible. For example, the image at left below is a blow-up of a screen tint made of Yellow and Cyan. Because the Y and C screen angles are less than 30° apart they create a moiré. However, because Yellow is so much lighter than Cyan the moiré is not normally visible.However, if the Yellow printer becomes contaminated, as at right above, the existing moiré can become very visible.

Another cause of screening moiré can occur if a prescreened (bitmap) graphic is imaged on a device that has a different resolution than the original art. In the below example a prescreened image that was created at 2400 dpi (standard for North American imagesetters) has been imaged on a 2540 dpi device (standard for the rest of the world):The result is a severe moiré in what should be a flat background screen tone.

Resampling moiré
Moiré artifacts can be introduced when images are resampled (have their resolution changed) somewhere in the production process.
Original resolution car grill at left. Resampled car grill at right.
Moiré, caused by resampling, usually occurs if the image is resized in a page layout program, or when the document is exported as a PDF, or as a result of the RIP settings when the document is processed by prepress.

Obscure moirés
These moirés are fairly rare, but do happen. When other explanations fail, these causes may be worth investigating.

Demosaicing moiré
On rare occasions you may encounter a "demosaicing" moiré. These occur when images with small-scale detail near the resolution limit of the digital sensor in a digital camera sometimes cause the demosaicing algorithm to produce repeating patterns, color artifacts or pixels arranged in an unrealistic maze-like pattern. On the left a properly demosaiced image, and on the right one in which the demosaicing algorithm has caused colored moiré artifacts in the fence and side of the building:Click image to enlarge
Single channel moiré
The standard photomechanical screen angles do not work best with digital screens. As a result some output device, halftone dot shape, screen angle and frequency combinations can result in moiré within one screen resulting in "single channel moiré."
One solution to avoid this problem was the development of shifted angles. The angular distance between screen angles remains more or less the same however all the angles are shifted by 7.5°. This has the effect of adding "noise" to the halftone screen and hence eliminating the moiré. For that reason, some individual screen sets may vary the requested screen angles slightly in order to overcome the potential for single channel moiré.

Paper related moiré
During the paper manufacturing process the side of a sheet paper that is in contact with the wire or forming fabric of the paper machine is the wire side (also called the reverse or bottom side). The wire side is usually not quite as smooth as the top or felt side and may carry a subtle impression of the wire pattern. If that pattern harmonically beats with the halftone screen pattern a subtle moiré will appear in the presswork. It often appears, and is confused, as a mottle. The difference is that mottling appears as random splotches while wire side paper related moiré appears as splotches that form a periodic pattern.

Avoiding moiré
One of the unfortunate effects of the use of inkjet proofing is that moiré artifacts are no longer detected during proofing cycles where there is an opportunity to deal with them. Instead, they are usually first seen on press at which point the job may need to be stopped resulting in increased costs and schedule disruption.

While the geometries of moiré formation are well understood, I'm not aware of any prepress system that incorporates moiré detection/prediction during document processing. So, the key thing is that print specifiers and prepress technicians have to take responsibility to reduce the likelihood of moirés occurring in the first place as well as being aware of image types that have the potential to form moiré artifacts.

Tips for avoiding scanning/sampling moiré
1- Use descreening software if your scanner application has this option.
2- Try scanning at a resolution equal to the halftone lpi used for the printed image.
Left image scanned at 600 dpi and rescreened. Right image scanned at 150 dpi (same as printed) ready for rescreening.
3- Try scanning the image by placing and scanning the original at different angles.
Left image scanned at 90° shows moiré on face. Right image scanned at 30° - no moiré on face.
Tips for avoiding subject moiré
1- Identify images with the potential for moiré. If the prepress workflow allows it, export the processed halftoned bitmap files of the suspect images. Proof them by viewing at 100% on a monitor or outputting to a laser printer at a magnification equal to the resolution of the printer. (e.g. if the bitmap is 2400 dpi, and the laser printer is 600 dpi, then output the bitmap at 400% (2400/600 = 4)). View the proofs on screen or on paper from a distance to see if a moiré is present.
2- Use FM/stochastic screening. Because this type of screening has no frequency or angle it avoids subject moiré completely.
3- Use FM/stochastic screening for the screen that is causing the subject moiré - typically it's the black printer.
4- Swap screen angles usually the black for magenta.
5- Change separation method to UCR instead of GCR.

Tips for avoiding screening moiré
1- Use FM/stochastic screening for the yellow printer. If you're using a 150-200 lpi AM/XM screen then use a 35 micron FM/stochastic since it will have a similar dot gain curve.
2- Make sure that incoming halftone screened bitmap files have a resolution that is equal to or an even divisor of the resolution of the output device. Make sure that those bitmapped images have not been resized in a page layout application.

Tips for avoiding resampling moiré
1- Import images into page layout applications at 100% - do not resize in the application.
2- Images should have a resolution that is an equal divisor of the output device. E.g. 300/400/600 dpi are even divisors of 2,400 dpi.
3- Make sure that PDF creation applications are set to not resample images.
PDF creation settings
4- Make sure that prepress RIP settings are set to not resample images.

Choosing the right screen angle for over-printing spot colors

To decide which screen angle to use when a screened PMS/spot color overprints a 4/C process image you will need to look carefully at the image that you will be overprinting.

The basic rule is to use the screen angle of the least prominent (or missing) screened process color that will be underneath the screened spot color.

For example, if there's no screened black under the spot color - use the Black angle, if there's no Cyan use the Cyan angle, etc.

Try to avoid using the Yellow screen angle because in standard screen angle sets yellow is only 15 degrees away from C or M. As a result moiré is always there but it is usually not visible because the yellow is so pale. The moiré can become visible however, if the yellow becomes contaminated - or if it is used for a dark spot color.

For example, in the graphic below, the left image is Cyan (at 105°) overprinted with process Yellow at the standard 90°. The moiré is barely visible. However, in the center image overprinting Cyan with PMS 144 – a very dark yellow/orange color – using the same Yellow screen angle results in the existing moiré becoming very visible. On the right, PMS 144 uses the Magenta screen angle (75°) instead which eliminates the halftone moiré seen in the center image.

Alternatively, you could try running the 5th color using a second order FM screen. If you're using a 175 lpi AM/XM screen then the FM should be about 35 micron because if it's any finer you'll need to create dot gain compensation curves for the FM. Because 35 micron is a fairly coarse screen it is best used for fairly light colors otherwise you may find that the screen is too visible.

Because spot/PMS colors are typically formulated to be printed solid and not halftone screened, make sure that your ink vendor knows that you are going to be screening the ink and the dot size range as well (either in microns or lpi) so that they can formulate the inks accordingly.

Halftone screen angles

With the exception of FM (stochastic screening), all screens consist of dots arranged in a regular pattern or matrix. The vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. When the screen is aligned parallel to the paper edges, the screen angle is said to be 0° or 90°. The rotation angle away from the vertical axis is known as the screen angle. The screen can only be rotated up to 90° before it repeats itself. For example, a screen rotated 15° is at the same angle as 105°, 195°, and 285°.
A black and white halftone image consists of a single screen. The screen pattern is very noticeable when positioned at 0° and is least visible when rotated 45° as illustrated below.For that reason, black and white halftones are usually printed with 45° angled screens – particularly with coarser screens.

When two (or more) screens are printed on top of each another, a visually objectionable pattern known as moiré may occur. The most serious moiré patterns occur at very small angles between screens. Below are two overlaid halftone grids angled at 5 degrees and 10 degrees apart with the resulting moiré pattern:
The best angle between two screens that is least likely to cause moiré, and is most forgiving to small degrees of error, is 45°. However, in four color process printing, four different screens must be superimposed and all four screens must be angled within the 90° limitation.

A set of standard screen angles has been established that is based on a combination of theory and experience. First the least visible color, yellow, is placed at the most visible angle 0° (90°). Then the most visible color, black, is placed at 45°. The cyan and magenta are then placed between these two. Cyan at 15° (105°) and magenta at 75°. These angles represent a best all around compromise for most pictures and represent the standard, most commonly used screen angles. They also form the least objectionable moiré – the rosette pattern (more on rosettes here).
Because the Yellow printer is only 15° from the Cyan printer it produces moiré. The visibility of the moiré can be exacerbated if the Yellow becomes contaminated by ink traveling into it from previous press units. To help reduce the visibility of the Y/C moiré, most screening systems run the Yellow at a slightly higher frequency (lpi) – typically 108% of the frequency of the C, M, and K printers.Left: Yellow at the same frequency as Cyan. Right: Yellow at a higher frequency to help reduce visible moiré.

These standard screen angles are based on analog photomechanical screens and do not work best with electronic screens. At angles other than 0° and 45° a type of moiré patterning within one screen "single channel moiré" may occur. To avoid this problem, some vendors utilize shifted angles of 7.5° to introduce "noise" around the edges of the dots in order to break up and eliminate the visibility of single channel moiré.

Most printers will have a standard screen angle set that is used for all their jobs. However, if certain jobs have images where two of the process colors predominate and where those two colors are less than 30 degrees apart, then that screen set should be avoided and a different one used instead.

The following screen angle sets are all valid and are in common use. The sequence for the screen sets listed below is C, M, Y, K (i.e. the first screen set on the list is: 15C, 45M, 0Y, 75K). Remember that screen angles have quadratic symmetry so 0 degrees is the same as 90, 180, and 270 degrees.

Standard 4/C U.S. screen angle set:
15, 75, 0, 45 (possible moiré in greens since C and Y are only 15º apart)

Standard 4/C European screen angle set:
15, 45, 0, 75 (possible moiré in greens since C and Y are only 15º apart)

Other usable screen angle sets: Keep in mind that when two colors are less than 30º apart there is a risk of moiré
15, 45, 0, 75
15, 75, 0, 45
15, 45, 30, 45
45, 15, 0, 75
45, 75, 0, 15
75, 15, 0, 45
75, 45, 0, 15
75, 15, 60, 45

For 2/C jobs (e.g. duotones): Other angles can be used, however, the guiding principle is that the angles should be 30º or 45º apart and that the darkest color should be at 45º to reduce its visibility and lessen "sawtoothing" (see below)
Dark color at 45
Light color at 75

For 3/C jobs (e.g. tritones):
Darkest color at 45
Medium color at 75
Lightest color at 15

For 5, 6, or 7/C jobs (e.g. Hi-Fi color):
Use the angle of the unused color
Violet/Blue uses Yellow or Black angle
Green uses Magenta angle
Red/Orange uses the Cyan angle
Note that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V/B, G, or R/O.

Dealing with the Yellow printer moiré issue
Interscreen moiré becomes more visible when the angles of any two screens are less than 30 degrees apart. Yellow is usually allowed to be less than 30 degrees because it is such a light color that the moiré is usually not visible. Also, the frequency of the yellow printer is usually made higher than the other three colors (typically around 108% higher) to further minimize the visibility of the moiré. However, the moiré can become more visible if the yellow printer becomes contaminated/dirtied by the preceding process colors, or if its density is too high.

So, when skin color predominates:
15, 45, 0, 75 (avoids M/Y conflict/moiré but introduces C/Y conflict)

Or when light greens predominate:
45, 75, 0, 15 (avoids C/Y conflict/moiré but introduces M/K conflict)

Some printers use a coarse FM screen instead of a conventional AM screen for the yellow printer.This eliminates the moiré issue completely since FM screens do not have a fixed frequency or angle. For a 175-200 lpi AM screen an FM screen of about 35 microns would be used since that dot size will have a dot gain similar to the AM screened colors.

Other screen angle considerations
In certain circumstances, depending on the size of the graphic and the frequency of the halftone, the selected screen angle can distort the accurate rendering of images.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray lines have been changed.Note how the screen has affected the rendering of the gray lines at different angles. The artifact at 1, 2, 3, and 4 is referred to as "ribboning" and is fairly common in automobile images.

In the below graphic, the halftone screen angles have been changed to the standard 4/C process angles (K 45º, C 15º, M 75º, Y 0º) but the angle of the three gray lines have been kept the same (0º).Ribboning has appeared in the Cyan and Magenta angles while the Black and Yellow angles have caused the appearance of different dotted line effects.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray box has been changed in 10º increments.Note how the smoothness of the edges of the box changes as its angle relative to the halftone screen angle changes. The ragged appearance of edge of the last box is referred to as "sawtoothing."

Screen angles for more than four color - i.e. "Hi-Fi" printing (5, 6, or 7 colors)

Four color CMYK process printing is a good compromise that achieves a wide enough color gamut for most applications while using the minimum number of inks to achieve it. However, sometimes the printer needs to go beyond 4/C in order to achieve a satisfactory rendition of the image. Typically the gamut deficiency will be in the overprint colors - Red/Orange, Blue/Violet, Green.

Here is an original RGB image:And here is the CMYK version of it:To restore some of the original color impact, the printer may choose to use "bump" or "touch" plates to boost color back into areas where it was lost. However, adding extra colors causes problems since all possible screen angles have already been used by the C, M, Y, and K printers.
In this example, these are the four channels that make up the image:Note that there is virtually no Cyan in the Red/Orange areas, or Yellow in the Blue/Violet areas, or Magenta in the Green areas. Therefore, those screen angles become available for the extra bump inks. So the trick is to use the screen angles of these unused colors.
In this example Violet, Green, and Red:In short, the Violet ink would take the unused Yellow angle, the Green ink would take the unused Magenta angle, and the Red ink would take the unused Cyan angle. Note also that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V, G, or R.

Rosettes – everything you didn't realize you needed to know

Rosette basics

Printing depends on halftoning to simulate shades of gray, color, and image detail. In four color process printing, four halftones – one for each of the cyan, magenta, yellow, and black inks are overlaid to produce the image. Unfortunately, overlapping two or more halftone grids can create an objectionable pattern called a "moiré" which, interestingly is the basis of the rosette.
Here, the overlaid halftone grids are 5 degrees and 10 degrees apart:

Here, the overlaid halftone grids are 15 degrees and 20 degrees apart:
As you can see, the greater the difference in angle between overlapping grids, the smaller the resulting moiré and the less apparent it is.
Here, the overlaid halftone grids are 30 degrees and 45 degrees apart:

Once the second grid has been rotated to 45 degrees, the moiré pattern is at its smallest and at a sufficient viewing distance seems to disappear.

Because a halftone screen is a quadratic grid (e.g. 90 degrees appears the same as 0 degrees, 135 degrees is the same as 45 degrees) the largest angle difference possible between two screens is 45 degrees, while the largest angle offset between three screens is 30 degrees (90/3=30). As a result, the defacto standard in four color printing has the three most visible process colors 30 degrees apart (C at 105 degrees, M at 75, and K at 45). Since Yellow is the least visible color it is angled at zero degrees – just 15 degrees from cyan. To further reduce moiré, the yellow screen is usually run at a higher frequency – typically about 108% of the other process colors.

The two kinds of rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105, 75, 45) they form a moiré pattern called a "rosette."
To make the structure easier to see, here is the same graphic but with C, M, and K all black. Note that the yellow screen is not included since, because of its higher frequency, it does not form part of the rosette.

This type of rosette is called a "dot-centered" or "closed-centered" rosette because each of the patterns has a dot in its center.

Here is a gradient using the dot-centered rosette:
The second type of rosette is called a "clear-centered" or "open -centered" rosette. It is created by shifting one of the process colors one half row of dots from the other two colors.
Here it is in color:
And in black only for clarity:
And as a gradient:
In general, dot-centered rosettes:
• show a less visible pattern than clear centered ones
• have individual dots that land on top of one another - reducing chroma/gamut slightly
• produce color slightly differently than clear-centered rosettes
• tend to lose shadow detail
• with slight misregistration cause significant color shift
• are more popular with low screen frequencies - 100 lpi and lower

In general, clear-centered rosettes:
• show a more visible pattern than dot centered ones
• look slightly lighter due to more paper showing between dots
• produce color slightly differently than dot-centered rosettes
• tend to preserve shadow detail better
• resist color shifts better when slight misregistration occurs
• are more popular with high screen frequencies - 150 lpi and higher

Halftone dots are built inside halftone cells. Those cells have to fit together seamlessly. In order to rotate the screen, you have to rotate the cell – and there are only certain frequency/angle combinations at a given resolution where this seamless tiling is possible. The result is that at screen angles other than zero and 45 degrees, like cyan and magenta, the angles are not exactly as requested. As a result, the rosette can drift from being clear-centered to being dot-centered.

In this image the cyan is off by just two degrees and you can see the rosette going from dot-centered in the upper left to clear-centered in the middle and back to dot centered in the lower right:

In black only for clarity:
And reduced in size for clarity:
As it can appear in an image:
A well designed halftone screen will usually be able to maintain a clear-centered rosette across the largest diagonal plate that will be used. A less well designed screen may see "rosette drift" occurring over a distance of a few inches.

Rosette drift can also be caused by slight press misregistration caused by issues such as back sheet flare, web growth, or "waggle" (lateral sheet movement in the press). In this case rosette drift is not localized but occurs in the entire press sheet area.

In register - clear-centered rosettes:

Out of register by one half row of dots - now dot-centered rosettes with a subsequent tone and color shift:

With either cause of rosette drift, the problem can appear in presswork as:
• a moiré. Since a rosette is itself a high frequency moiré it is very sensitive to angular shifts.
• as "noise" or a grainy appearance in flat screen tint areas. This is because as the rosette drifts it has the effect of lowering the frequency of the halftone.
• as a shift in tone as the clear-centered rosettes are filled with a dot and then cleared again.
• as a color shift as the overprinting colors change their relationships with the shift from clear-centered rosettes to dot centered rosettes.