Eliminating show-through in scans and/or photocopies

Paper is a fairly translucent substrate. The thinner/lighter weight or higher quality the paper the more translucent it is. The result is "show-through" - seeing a ghost image of what was printed on the reverse side - whether reading the page itself or scanning/photocopying it.
Scan with color bar and text on the next page showing-through.Close up view of show-through.
To eliminate show-through in scans and/or photocopies, simply place a piece of black paper behind the sheet that you are scanning. This evens out the tonality of the page and effectively eliminates show-through.

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.

Image resolution for printing - LPI vs DPI a.k.a. LPI vs PPI a.k.a. LPI vs SPI

Background - pixels make the original image

A digital "raster" image acquired from a scanner, a digital camera, or created directly in a "paint" application like Adobe Photoshop is made up of a mosaic of "pixels" (picture elements)."
Here is an original image at actual size:

Here is a close up view showing the actual pixels that form the image:

The physical size of the image is described by two numbers which can be expressed two ways:

1) The number of pixels per inch/centimeter.
and
2) The number of pixels in both horizontal and vertical dimensions.

Or:

1) The number of pixels per inch/centimeter.
and
2) The horizontal and vertical dimensions expressed in inches/centimeters.

Those are just two ways of saying the same thing.
Here is the original image with a dialog box showing its dimensions:

Note that the dimensions have a "lock" icon beside them. This is because the relationship of pixels per inch (ppi) and vertical/horizontal size are "locked" together. Changing one changes the other as you can see in the below dialog boxes (click on image to enlarge):

Note that as the resolution is changed (from 600 to 300 and 300 to 150 pixels per inch) only the density of the pixels changes, not the number of total pixels in the image, in this case 1412 pixels x 2028 pixels, therefore the file size remains the same. Put another way, each time the resolution in ppi is increased, or lowered, the physical image size changes but the total number of pixels forming the image (and hence the detail) remains the same.

Note that I use the term "pixels per inch" - ppi. Very often the term that is used is "dots per inch" or dpi. Technically the terms are not interchangeable - however, in daily usage, when speaking about digital images the terms are considered as meaning the same thing. You may sometimes hear the term "spi" - samples per inch. This refers to a scanner's resolution - i.e. it ability to acquire an image at so many samples per inch (e.g. 300 spi). Again, in practical usage, when speaking about digital images - ppi, dpi, and spi can be understood as meaning the same thing.

Interestingly, digital cameras typically do not have a resolution assigned to them.

Instead a digital camera captures data based on the "megapixel" ability of its CCD sensor. For example, a 14.2 megapixel camera might capture an image that's 4592 pixels by 3056 pixels, which equals 14,033,152 total pixels. When you open the file into an image-editing program a resolution must be assigned to the file. Most programs, including Photoshop, use 72 ppi as the default resolution.

Background - halftone dots make the image reproduction

Because printing presses can only lay down 100% ink or 0% ink, digital images acquired from scanners, digital cameras, or created directly in "paint" applications need to be converted into a binary (on/off) format. This is done through a process called halftone screening. The result is that the image will be converted to dots of either 100% or 0% ink with the original tones being simulated, in this case, by the size of the dots. Bigger dots represent darker tones - smaller dots represent lighter tones:

The fineness of the screen, and hence the level of detail in the original that can be preserved, is determined by how densely packed the dots are and is indirectly described by how many rows - or lines of dots are used per inch (or centimeter) to create the image. These virtual lines are highlighted in red below:
In this example the image is made up of 85 lines of dots per inch – expressed more commonly as an 85 lines per inch halftone - or more simply stated: an 85 lpi halftone image.

The key thing to remember is that although the halftone image is made up of dots - the level of detail that it can reproduce is described in terms of lpi NOT dpi.
So, original image pixel density/detail = ppi, spi, or dpi. Halftone reproduction dot density/detail = lpi.

Of course, in order to pack more lines of dots into an inch - the smaller the dots become and hence the greater amount of image detail that is preserved.

40 lpi halftone:

100 lpi halftone:

200 lpi halftone:

It is the relationship of how densely packed the original pixels are (see part 1) compared to the frequency of lines per inch of the halftone screen dots that determines what image resolution is appropriate for its reproduction in print.

The relationship between dpi/ppi and lpi for
grayscale images

The guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the image pixels should always be more densely packed (ppi/dpi) than the detail resolving ability (lpi) of the halftone screen that is used.

To illustrate this principle I'll take a section of the same image at different resolutions (ppi/dpi) and reproduce it using the same 150 lpi halftone screen:

Original 75 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is one half of the halftone screen resolution (lpi). As a result the halftone reproduces the individual pixels of the original. This visible artifact is termed "staircasing," the "jaggies," or "pixelation."

Original 100 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is two thirds of the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are less visible.

Original 150 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is equal to the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are much less visible.

Original 225 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is 1.5 times greater than the halftone screen resolution (lpi). Although some original image pixels may still be visible, in general, the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This minimum required original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.

Original 300 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is twice the halftone screen resolution (lpi). As a result the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This ideal required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.

Original 600 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is four times the halftone screen resolution (lpi). The image file size is about 7 times larger than the 225 ppi/dpi image but provides effectively no difference in the final reproduction.

The relationship between dpi/ppi and lpi for CMYK
images

As with grayscale images, the guiding principle for understanding what original image resolution (ppi/dpi) is needed compared to the halftone screen (lpi) that will be used is that the halftone screen should not reproduce the image pixels themselves but instead the tones the pixels represent. It is worth comparing these images to their grayscale equivalents in part 3.

To illustrate this principle, I'll take a section of an image rendered at different resolutions (ppi/dpi) that has been converted from grayscale to CMYK and reproduce it using the same 150 lpi halftone screen:

Original 75 ppi/dpi - halftone screen 150 lpi:Here the image ppi/dpi is one half of the halftone screen resolution (lpi). As a result the halftone reproduces the individual pixels of the original. This visible artifact is termed "staircasing," the "jaggies," or "pixelation." That being said, the jaggies are less severe than we saw in the grayscale image at the same ppi/dpi. Also the numbers on the sail appear clearer. This suggests that it might be possible to use a lower image resolution for reproducing a CMYK image than can be used for a grayscale image.

Original 100 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is two thirds of the halftone screen resolution (lpi). As a result the halftone still reproduces the individual pixels of the original - but they are less visible.

Original 150 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is equal to the halftone screen resolution (lpi). Because the CMYK image is a composite of four individual halftone images it tends to lessen the visibility of the individual pixels of the original.
This minimum required original resolution for a CMYK image can be represented by the formula: lpi = ppi @ 100% reproduction.

Original 225 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is 1.5 times greater than the halftone screen resolution (lpi). The halftone no longer resolves the individual pixels of the original - just the tones they represent.
This ideal original resolution can be represented by the formula: 1.5 X lpi = ppi @ 100% reproduction.

Original 300 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is twice the halftone screen resolution (lpi). As a result the halftone no longer resolves the individual pixels of the original - just the tones they represent.
This maximum required original resolution can be represented by the formula: 2 X lpi = ppi @ 100% reproduction.

Original 600 ppi/dpi - halftone screen 150 lpi:
Here the image ppi/dpi is four times the halftone screen resolution (lpi). The image file size is about 7 times larger than the 225 ppi/dpi image but provides effectively no difference in the final reproduction.

The below table provides image resolution requirements for a variety of typical print applications:Note that this table refers to conventional "AM" halftone screening where the lpi signifies the dot density and hence the resolution of the halftone screen. However, there is another type of halftone screen in use which does not have a traditional lpi. Instead, this type of screening organizes the halftone dots in random appearing patterns. Below are three different vendor's offerings (click on images to enlarge):
This type of halftone is called "FM" or "Stochastic" screening (covered in other posts in this blog). Rather than indicating resolution according to "lpi" - the average actual dot size, specified in microns, of the screen pattern is used instead. Typical dot sizes are: 10 - 20 micron for commercial work, 20 - 25 micron for magazine work, and 35 micron for newspaper work. Because this type of screening has a higher average resolution than conventional AM screening - it's a good idea to use images at a higher resolution to take advantage of this screening's detail rendering capability. Typically 400 ppi/dpi for 10-20 micron FM, 300 ppi for 25 micron, and 200 ppi/dpi for 35 micron.

Image resolution "gotchas" – where things can go wrong

Whether you are targeting your images for AM or FM screening, there are at least three places where the resolution of the images may be accidently altered:

1) If the image is resized/scaled in the page layout application – it may no longer have an appropriate resolution:
2) If the image is resized/scaled when the file is converted to the PDF format – it may no longer have an appropriate resolution:
3) If the printshop's workflow is setup to resample incoming documents – they may no longer have an appropriate resolution. Most prepress RIPs are set, by default, to downsample incoming files to 300 ppi/dpi.

Your hidden microscope

When a loupe is just not strong enough, a microscope can be a great help in analyzing press problems – especially if it can capture an image that can be shared with coworkers, or sent to vendors or consultants for evaluation. However few printers can warrant the cost of such a piece of equipment. Fortunately most shops have an excellent alternative in the form of a flatbed scanner. Even a very cheap one can do a very respectable job. The two samples below were captured using a very basic $59 (USD) desktop scanner.First, a black and white halftone (original size at left and enlargement at right - click on image to enlarge)Next, is a scan of a color bar which shows a lay down problem with the black printer (original size at left and enlargement at right- click on image to enlarge):For best results use a scanner with the highest possible native (non-interpolated) resolution. Scans at about 1200-2400 dpi seem to work best. Have the graphic to be scanned placed in line with the direction of travel of the scanning head, and use a black backing to prevent show through. Finally, scale the image in Photoshop by changing the resolution (lowering the dpi) with "Resample Image" deselected.