A client of mine is preparing a high school textbook for custom printing with an online printing company. The layout is complete, and she has turned her attention to adjusting the cover and text photos. Today she asked for the “dot range” the business printing provider could hold on press.
While this sounds esoteric, it really can be broken down into a relatively easy concept, and it is very important to understand this concept if you are preparing your own photos for an offset custom printing vendor (which in most cases you will be doing).
What about black-only photographs?
When I spoke with the prepress department at the digital printing company, I learned that they could hold a 2 percent dot for highlights and a 94 percent dot for shadows. What this means is that within a grid of halftone dots (an inch by an inch square), the total area covered by black halftone dots would be 2 percent. Since the line screen for this press would be upwards of 150 lines per inch (150 rows of halftone dots), the dots in this one inch by one inch space would be very, very small. The digital printing service could still maintain fine detail within the halftone screen at this level. Everything lighter than this would be the white of the book printing paper stock.
Conversely, the digital printing company could hold a 94 percent dot in the shadows. That is, within a one inch by one inch square, assuming a line screen of 150 lpi or higher, 94 percent of the area would be filled with halftone dots. As halftone dots go, these would be rather large. Anything darker than this would become solid black. One reason for this is that my client’s book will be printed on an absorbent, uncoated stock (70# Finch Opaque), which soaks up ink. Using a percentage halftone dot higher than 94 percent would cause the ink to spread on the press sheet and fill the entire area with black ink.
(Keep in mind that offset custom printing is a binary process. In any given area, there is either ink or no ink. Within a black-only textbook, this means either black ink is present or absent. When reproducing halftones, the business printing vendor can simulate shades of gray by printing halftone dots of different sizes within a regular halftone grid pattern. The percentages we are discussing pertain to this halftoning technology.)
How does this work for full-color images?
My client’s black and white textbook has a cover, which will be printed on a gloss coated press sheet. This paper will have better ink hold-out than the uncoated paper inside the book (that is, ink will sit up on the surface and not seep into the paper fibers). Therefore, the halftone screen for the four photos on the cover will probably be finer than for the text. More than likely, the business printing vendor will employ halftone screens with rulings upwards of 175 lines per inch.
The digital printing service informed me that the press can hold a 3-2-2 dot at 270 percent. Let’s break this down.
Full-color printing is done with halftone screens, but unlike the black and white images in the textbook, the cover is printed with four halftone screens at different angles to one another: one halftone screen on each of the press units (Cyan, Magenta, Yellow, and Black). Assuming that the 3-2-2 refers to highlights (because the numbers are so low), we can read these as 3 percent Cyan, 2 percent Magenta, 2 percent Yellow, and 0 percent Black (because the business printing vendor confessed that 3-2-2 was really 3-2-2-0; Black has no number and hence is zero).
At the other end of the spectrum (the shadows), the total ink coverage cannot exceed 270 percent. This gives the designer leeway. Let’s assume that all four process colors at 100 percent density would equal 400 percent coverage (C, M, Y, and K x 100 percent each). This would be far too much ink. The press sheets would stick together. It would be a mess.
Therefore, the business printing vendor has learned through experience that the press used to print my client’s book cover can hold a shadow within a 4-color image of up to 270 percent ink coverage. Depending on the color composition of each of the four full-color images in my client’s cover design, any one area of shadow might be made up of (for instance) C-80, M-20, Y-15, and K-120—or any other combination of screened percentages not exceeding a total of 270 percent.
Color halftones are still halftones.
Remember that color halftones are still halftones. Each of these numbers represents a screened percentage within an inch by inch space. The halftone screen is higher (175 lpi or above), and the screens are set at angles to one another, but they’re still screens, just like the halftone screens for the black ink images inside the textbook the book printer is producing.
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on Thursday, August 18th, 2011 at 8:11 am and is filed under Book Printing, InDesign Software, Prepress, Printing, Soft Cover Book Printing.
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