Techdocs Library > White papers > Greyscale and Registration Explanation

Document Author: Additional Author(s):	Donald Otis Tamsen Wiltshire		Document ID:	WP100333
Doc. Organization:	Techline Americas		Document Revised:	02/23/2009
Product(s) covered:

Abstract: There have been questions on how to understand image reproduction in digital printing.

Screen ruling refers to the number of rows or lines of dots used to render an image. You should understand that this has everything to do with images, and really nothing to do with text. While 600 DPI fonts, or outline fonts (which are resolution independent), will make for a finer edge and sharper text, the screen frequency does not become an issue until we refer to image processing. The dependencies in screen ruling are the resolution of the imagesetter, the paper stock, and the type of press. An application like newspaper, which is printed on low-grade paper on high-speed presses is commonly 85 LPI. A higher screen ruling would saturate the newsprint with ink and make the images look muddy. A four color magazine printed on coated paper might use a screen ruling of 133 LPI, where any lower value would make the images look coarse. The screen ruling determines the size of the halftone dot. The halftone dot is made up of printer dots, and printer resolution determines the number of dots available to create a halftone dot. It is the relationship between the screen ruling and the printer resolution that determines the tonal range that can be printed. The formula for determining the number of shades of grey is (output resolution divided by screen ruling)squared plus 1 = the shades of grey. Increasing the screen ruling creates smaller halftone dots and adds detail to the image, although it reduces the number of greys available to make the image. In our printers, the Picture Elements, or PELs, are either off or on. They are never "kind of on", so you have black or white pixels, and that is that. That is fine in text, but in pictures, there is a problem. The original photograph, like this one from Corbis.com, are continuous tone, or contone, in nature: That is, if you looked at the original image with a loupe, you would see that there are countless shades of grey, as well as black or white, all running into each other. Now, that is not easy to replicate in just two colors. In fact, if you do try, it looks pretty bad, unless you trick the eye. The way we perform this trick is called "dithering". What that is the use of multiple lines operating as a single line. If you look at printout really closely, you see that it is made of dots in diagonal arrangement: Now, if you look even closer, you will see each "line" is really a group of lines - often a group of 8 x 8 squares called a "super pixel" - where some pels are off and some on. The darker, the more of these are turned on, the lighter, the less: And that is really how it is done - no grey, just a pretend grey, made by faking the eye out. That offers quite a few "shades" - 8 x 8 = 64, plus 1 (all off), for 65 levels. There is a real science about how you place the groups of dots, and it comes down to what is called the "screen". The screen is the lines per inch. You get this by dividing the pels of the output device by the number of lines in the super pixel, dividing this by the square root of 2, and that is the LPI. So, with an 8 x 8 super pixel, a 600 DPI printer would give 600/6, or 75, * 1.414 = 106 LPI. This is the "screen" that, coincidentally is the best one for the Infoprint 2000. In PostScript Level 3, you get the addition of a pattern where the corners of the super pixels are filled in, causing even finer shading: A true greyscale printer can print multiple tones - not just black. This increases the number of available shades dramatically. For instance, a printer that can print four tones has the ability to provide 256 shades of grey. In some circumstances, this is very noticeable, in others it is irrelevant. In color, you overlay the colors of cyan, magenta, yellow and black, in different screen angles, in order to avoid moire patterns - diagonal patterns which appear across the image. Traditional angles are black, 45 degrees; magenta, 75; yellow, 90; and cyan, 105. In electronic dot generation, it is difficult to get the exact 15 degree and 75 degree screen angles used in offset lithography. In these cases, we have developed algorithms to provide similar function at different angles. Registration refers to the exact lining up of the different plates in offset lithography - that is, you have multiple plates, one for each color used. If the alignment of the plates is off even minutely, it is readily apparent to the eye. In this example, there is a four color series of "+", where each color is one size smaller than the next: The first example is aligned; in the second, the yellow is out of registration by one single pel. You can see that this would be very noticeable if extended across a large image - you probably have seen examples in the newspaper of this. In digital printing, the registration is handled at set up, and under normal circumstance does not need to be addressed run to run. Additional information in this regard can be found in the Print Publishing Guide, part number 0397 0719, available from Adobe, as well as in the Pocket Pal, from International Paper, available from PubOrder.

Classification:	Hardware; Software	Category:	Planning and Design
Platform(s):	Printing Systems
:		Keywords:	image printing, dro