There are three different categories of printable images: full color, line art black-and-white, and continuous tone black-and-white. I’ve covered full color image preparation in a series of posts starting here, and methods of printing continuous tone black-and-white images on digital presses here. I’ll cover line art in a future post. This post is about preparing black-and-white images to print in black ink on an offset or digital press.
Continuous tone black-and-white images include black-and-white photographs, as well as artwork that has been photographed in black-and-white, such as paintings or lithographs. A black-and-white photograph can have an infinite range of tones, from solid black, through the grays, to the brightest white, though not all photographs contain the entire range. The halftone process converts that range of grays into an image of solid black ink dots of varying sizes so that it can be printed on a printing press that only has one color of ink. When printed and viewed at normal reading distance, the dots appear to the eye as a range of grays.
In the darkest areas of the photo, the dots will be so big they run together; here the image looks like white dots of varying sizes on a black background. In the lightest areas of the photo, the halftone consists of tiny black dots in a field of white—or more accurately, in a field of whatever color the paper is. In the middle-gray areas of the photo, the area of white and the area of black is 50% black and 50% white.
Formerly, a halftone image was created by taking a picture of the original photograph in a giant camera called a process camera. The halftone dots were created by placing a finely engraved transparent screen directly over the film in the process camera. This special screen resolved the different intensities of light in the image that the process camera’s lens focused on the film into dots of varying sizes.
Now the dots are formed by software, and imaged via laser or LED directly onto an offset printing plate or onto the imaging drum of a digital press.
Theoretically, the best image reproduction quality is obtained when perfect halftone dots are reproduced perfectly on the paper. In reality, the dots are never created or reproduced perfectly. The dots created by the software may not be perfect to begin. Imaging the dots on an offset plate or digital imaging drum involves some further degradation—though much less than in the days of all-film processes! Offset web (roll-fed) presses have some slippage of the fast moving ribbon of paper as it moves through the press, deforming the dots on the paper. Finally, when a liquid or semi-liquid dot of ink is pressed onto paper it spreads out, both by physical smushing and by seepage out through the fibers of the paper.
The spreading out of halftone dots is called dot gain, and and it is the primary factor that has to be taken into account when creating a halftone. For best quality, a halftone shouldn’t be created with a one-to-one mapping from grayscale density to dot density, but such that it will look right after dot-grain has occurred on press. In general this means decreasing the size of the halftone dots in different tonal areas of the image.
Dot gain depends on the quality of the paper and the precision of the press. Coarse cheap paper like newsprint produces huge dot gain, while fine coated paper will have minimal dot-grain. High-speed newspaper presses where the paper is fed through on a roll produce the lowest quality dot. Offset presses that feed the paper in sheets have better registration, or positioning, of the paper as it is printed, resulting in better-formed dots. A similar dynamic applies with digital presses: sheet-feeding allows better print quality than roll-feeding.
Dot Densities for Offset Printing
In a halftone, the darkness of an image is expressed as the percentage of the paper in a given area that is covered by ink: 100% is solid ink, 50% is half ink and half background paper, 0% is no ink. On a cheap newsprint a 50% halftone dot may turn into a 70% dot as a result of dot gain, drastically darkening the photograph and eliminating detail from the image. An 80% dot will approach 100% on cheap newsprint. The trick is to create dots of such a size that when they are printed on a particular press, they undergo dot-grain to just the right density. So for a newspaper photo, you might adjust the grayscale image so that the darkest area of the image has 80% density. If you do this with the Photoshop curves tool using a straight line, this will automatically decrease the dot density in the middletones as well.
At the other end of the tonal range, the lightest highlight areas, it looks weird to have the edge of photograph exactly the same density as the background paper. In fact, it looks like a mistake. Best practice is to create enough dot in the highlights so that there is a visible edge to the photograph. Depending on the press and paper, this could be 4-10%—higher quality presses on coated paper can reproduce a clean 4% dot, which would disappear on newsprint printed on a web press. The optimum densities for printing on newsprint will be something like 80% maximum dot, 10% minimum dot. Printing on a sheetfed press on coated stock it will be something like 97% maximum dot, 2% minimum dot, and on a web press 95%/3%. But you should ask your printer what the minimum and maximum dot densities are appropriate for your particular book. They’ll take into account the paper and the press when giving you an answer.
Digital presses print either using toner (dry pigmented powder) or liquid ink jets. Toner and inkjet presses have very different dot gain characteristics.
Dot Densities for Toner-Based Digital Printing
To prepare for this article, I ran tests on both our one-color Océ presses and our four-color Xerox presses, preparing sequences of black and white images with different density adjustments. The results were quite clear: there is no need to compensate for dot gain on these presses. Even shadow detail was superior on the uncompensated images. The original images, optimized on screen for tonal range and detail, printed better than the same images with typical offset dot gain adjustments. I suspect, though I do not know for certain, that this is because 1) the toner is dry when applied to the paper and does not bleed out into the fibers and 2) the presses have closed-loop image quality monitoring so that the sheets coming off the press are constantly monitored and the press adjusted to maintain accurate densities.
Dot Densities for Inkjet Digital Printing
There are two kinds of inkjet printers/presses:
Drop-on-demand inkjet presses deliver precisely metered drops of ink to precise positions on the page. They can produce extremely high quality images, but because of their very slow speed they are rarely used to print books.
Continuous inkjet presses work by modulating thousands of continuous streams of ink coming from microscopic nozzles as the paper whips by underneath. The fastest continous inkjet presses feed paper at 1,000 feet per minute! Continuous inkjet presses are relatively new to the book printing market, but they are becoming a viable replacement for offset and toner-based digital for applications where high quality is not a requirement, such as textbooks. Continuous inkjet presses cannot currently come close to toner-based digital or offset presses in terms of quality, but they can provide an economical alternative at some run lengths for low quality work.
As with offset printing, ask the printer for guidelines on setting grayscale densities so you can optimize the images for a continuous inkjet press.
Adjusting Dot Densities in Grayscale Images
Here’s a simple—actually oversimplified—way to adjust the minimum and maximum densities in an image in Photoshop:
- Convert the image to grayscale.
- Use the curves tool or the levels tool to set maximum and minimum densities of the image to the percentages recommended by your printer. Let’s say that your printer recommends a maximum 97% and minimum 3% density. Using the curves tool, you can just grab the end point of the standard curve and move them so that an input of 100% has an output of 97%. Then grab the other end of the line and move it so that an input of 0% results in an output of 3%.
This method works, but individual images may require more extensive manipulation to print optimally. Some images, frankly, will never print well on commercial printing equipment because all the detail is in highlight or deep shadow areas where it is very difficult for presses to print distinct gradations. Often, there is a lot of detail in midtones that is easy to see on screen, and very difficult to actually print. For most photographs, this might not even matter. If you are doing a fine art photo or art book, printed proofs from the printer are an invaluable way to see the results of your tweaking, and make further adjustments as necessary.
Halftone Line Screens
Halftone dots are usually arranged in a grid, with precise spacing between the centers of the dots. That spacing is called screen ruling, and in North America it is denoted by lines per inch, or lpi. This not the same thing as dots per inch, or dpi, which is used to designate computer screen resolutions and the imaging resolution of a press.
Confusing, yes? Let’s clarify with examples from Bookmobile:
- Our Océ 6320 monochrome (black-only) digital presses create page images with a resolution of 600 x 1200 dpi. That means that everything on the page is made up of tiny imaging dots that are 1/600” x 1/1200” in size, including type, images, rules, tints, etc. We run halftones on these presses at 125 lpi, so the spacing between the centers of each of the dots on a halftone is 1/125”. Each halftone dot is formed by combining multiple imaging dots.
- Our Xerox Color 1000i and 800i presses create page images with a resolution of 2400 x 2400 dpi, so everything on the page is created with tiny dots that are 1/2400” x 1/2400” in size. These presses make halftones at up to 200 lpi, comparable to high quality offset presses.
In theory, using very fine line screens should result in better image detail. However, because of the drastic variability in the ability of a particular combination of press and paper to produce a high-quality halftone dot, finer screen ruling is not necessarily a better screen ruling. To use the extreme example, a 300 lpi printed on a web offset press on newsprint creates an image with all the clarity of a mud puddle, because the dots cannot be resolved. I used to shoot film halftones for newspapers. The editors always pushed for a 120 lpi screen thinking it would improve photo reproduction, but in reality the best images—crisp, with a good range of tones—were produced using a much lower 85 lpi screen.
Here are typical halftone screen values:
- Web offset, uncoated paper — 120 lpi
- Sheetfed offset, uncoated paper — 133 lpi
- Sheetfed offset, coated paper — 200 lpi
- Monochrome digital press (toner-based) — 125 lpi
- Four-color digital press (toner-based) — 200 lpi
Continuous inkjet press manufacturers claim up to 175 lpi, but based on the samples I’ve seen, that’s pushing it. Check with the printing company actually operating the press.
Ask Your Printer for Guidelines
Whatever kind of press your book will be printed on, ask your printer for guidelines for preparing grayscale images. They should have complete information available for you.
Printing Grayscale Images at Bookmobile
As mentioned above, we recommend optimizing the image on screen and not attempting to compensate for dot gain, because it is not a factor with our toner-based digital presses.
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Don Leeper is founder and CEO of Bookmobile, which has provided design, printing, eBook and distribution services for book publishers since 1982.