3 Replies Latest reply on Jan 28, 2015 2:54 PM by Peter Spier

    Why does the frequency of lpi not match what was entered?

    SweetonDesign

      I'm doing several variations on the same document where I change the frequency of the lpi. In case some do not know what talking about, you can go to the print window and under Output there is an option to change the color to Separations if you are printing to a laser printer. Once you have changed the color you can then change the halftone for either Process Cyan, Magenta, Yellow, or Black.

       

      I've turned off the print options for Process Cyan, Magenta, and Yellow and have changed the frequency of the Black. My problem occurs when I print in a sequence of 1-3 lpi, and 5-9 lpi. In the sequence of 1-3 lpi my halftone for 1 lpi looked like the halftone I had previously printed at a frequency of 15 lpi, while my 3 lpi looked like a 1 lpi that I had also previously printed. Then in the sequence of 5-9 lpi, the printed 5 lpi had the appearance of the 3 lpi that I mentioned in the last sequence, while the 9 lpi looked like the 2 lpi.

       

      What is going on?

        • 1. Re: Why does the frequency of lpi not match what was entered?
          Peter Spier Most Valuable Participant (Moderator)

          Halftone spots are made from printer dots, and if you have a limited number of printer dots, typically 600 dots per inch, you can only achieve certain percentage spots at any particular lpi setting you choose because the printer only prints 100% or 0% of each of those 600 dots.

           

          A really good reference on this subject is Real World Scanning and Halftones by Blatner, Fleishman and Roth  from Peachpit press.

           

          Most laser printers are also capable of printing only a handful of different screen values, if that , no matter what you tell them to do.

          • 2. Re: Why does the frequency of lpi not match what was entered?
            SweetonDesign Level 1

            So does that also explain why when I printed in a sequence of 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 that the 20 lpi looks like the 1 lpi from my 1-3 print sequence? And that the 15 lpi looks like the 9 lpi from the 5-9 lpi sequence?

             

            It seems that what you are talking about is the printer resolution and not the halftones of a color.

            • 3. Re: Why does the frequency of lpi not match what was entered?
              Peter Spier Most Valuable Participant (Moderator)

              The two are related. Halftone spots are made from a grid (square in the simplest case, but rarely that in the real world) of printer dots. If all the dots are "on" (inked) in the grid, the halftone spot is 100%. All "off" is 0%, half on is 50%, and so forth. A 600 dpi printer at 100 lpi uses 36 dots (6x6) for each halftone spot, so you can have 37 steps in your halftone. Since most people can't see the difference in adjacent 1 % steps this is a good compromise screen value for the tradeoff between fidelity of detail and tonal range, and most lasers default to something in this range.

               

              Imagesetters typically have resolutions over 2000 dpi and are therefor capable of a much broader range of values (remember, you can only have lpi values that divide evenly into the printer resolution because you can't have half a printer dot any more than you can have half a pixel in your image).

               

              I have no idea what you are seeing in your test series, but it's likely the printer is not delivering what you think it is.