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  • Standard Line Screen

    What line screen do you run? do you run a different line screen based on what paper the job is being printed on?

    we run a standard 200 here, and just recently started running 150 on some jobs, depending on paper.

  • #2
    Re: Standard Line Screen

    Hi Smith,
    In my opinion its very important to see the type of paper and its surface before printing, to decide the LPI to be used.
    As a thumb rule we can say that, "For a rough paper like NEWS PRINT one should use coarser screen, While running a smooth surface paper like ART PAPER one should go for finer screen rulling".
    I would suggest
    75 to 85 LPI for NEWS PRINT PAPER
    133 to 150 LPI for MAPLITHO
    175 to 200 LPI for ART PAPER

    Reason is, if we use finer LPI like 150 on news print paper, the dots of lower percentage like 5 to 10 will not come on the paper, on the other hand if we use coarser screen like 75 LPI for ART paper, the white gap between the dots will become visible due to the smooth surface of the paper.
    I hope this is right....

    Edited by: umesh on Sep 21, 2007 2:08 PM

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    • #3
      Re: Standard Line Screen

      We run 175 lpi for uncoated stock and 240 lpi for coated stock.

      Abe Hayhurst
      Director of Color and Technology
      We Do Graphics, Inc.

      Comment


      • #4
        Re: Standard Line Screen

        Hi, we currently run 175 for most of our work, 150 for some jobs also depending on stock. We just recently installed an Agfa Acento so we are looking to experiment with 200lpi Sublima screening, hopefully it will produce good results and we can move up to that for our higher end colour work...

        Cheers, Tony

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        • #5
          Re: Standard Line Screen

          We run 175lpi on uncoated stock and 200lpi on coated. We now use Hybrid Screening (Heidelberg) which is great!

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          • #6
            Re: Standard Line Screen

            HI,
            it's nice seeing that you all use different line screens to improve you printing quality. Please with all this keep in mind that lpi (Lines per Inch) and ppi (Pixel per Inch) goes hand in hand.

            For 150 lpi you need your pictures to be saved in 300 ppi. For 200 lpi you need your pictures in 400 ppi. If you use 200 lpi with 300 ppi pictures you just have a finer screen but not a better picture quality because you simply do not have enough pixels available for this kind of fine screen.
            You can use this small chart:
            ppi lpi
            150 75
            160 80
            240 120
            300 150
            400 200
            480 240
            The problem is that a picture saved in 400 ppi is much much bigger than a 300 ppi one.
            If you want really to improve your printing having only 300ppi pictures you need using FM screening with about 20 ยต.

            Best regards,
            Gerhard

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            • #7
              Re: Standard Line Screen

              The Pixels Per Inch (ppi) to the Lines Per Inch (lpi) used to be a 2:1 ratio like what Gerhard said. But you can get the same quality with a 1.5:1 ratio.
              So the chat would be something like this:
              ppi.......lpi
              112.5..75
              120.....80
              180.....120
              199.5..133
              225.....150
              262.5..175
              300.....200
              337.5..225
              360.....240
              375.....250

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              • #8
                Re: Standard Line Screen

                Since Gerhardt mentioned 20micron stochastic, does anybody use 10 micron?
                Do you have different setups for coated and uncoated?
                What would be good "default" for different papers, like 10 micron coated and 25 micron uncoated?

                Does it make sense going smaller than 25 micron on uncoated paper?

                We are commercial shop and we use 10 micron extensively. So far we did not have special rules for coated vs uncoated but now we are looking into it so I figured I'll ask for opinions.

                Thanks

                Zoran

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                • #9
                  Re: Standard Line Screen

                  From the old days (I figure it still rings true), the smallest resolution you can get without getting pixelazation is 1.2 pixels per printing dot. So if your linescreen is 175, you'll need at least 210 dpi (175 x 1.2 = 210) to not get pixelization (blurry images where you can see the stair step).

                  Don

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                  • #10
                    Re: Standard Line Screen

                    Of course, 300 dpi is what we recommend, but just saying the least dpi we can get away with without seeing the "stair-step".

                    Don

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                    • #11
                      Re: Standard Line Screen

                      Hi everyone,
                      this thread is really becoming interesting,starting with LPI we discussed about the type of paper and then image resolution(pixels), now one more important thing should be considerd, ie. Imaging resolution or dpi used by the CTP for exposing.
                      Not much attention is given to this part, because most of the RIP's ask you to enter only the required LPI and then the corresponding RESOLUTION or dpi is automatically choosen by the RIP.Basic rule says that there should be atleast 256 GRAY LEVELS for the image to looks smooth, ie. without any BANDING or STEPS.
                      The number of steps for a particular combination of LPI and DPI can be easily found out using

                      *Gray levels= (DPI/LPI)^2^*

                      Why i'm telling this ???
                      because just shooting the file for higher LPI, will not give good results, if the imaging resolution of the exposing device is not high.

                      i dont know how it works when FM screening is used, if any one knows please share it ..

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                      • #12
                        Re: Standard Line Screen

                        umesh,

                        Guess I should be using 150 ls for both coated and uncoated then? I have images at 300 dpi (most comes in that dpi) and platesetter resolution 2400 dpi (set to 2400 instead of 2540 because of problems encountered that were solved by making the LW and CT evenly divisible), and have been using 175 ls for coated and 150 ls for uncoated. So using the math you provided, 2400/150=16x16=256 levels of gray, so 150 ls is fine for uncoated. But using the same math, 2400/175=13.714285x13.714285=188.08161 levels of gray, so looks like I need to make both coated and uncoated 150 (because I'll still get mostly 300 dpi images and need the ratio of LW/CT res to be an even whole number, which would necessitate 2400). What do you think I should do?

                        Don

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                        • #13
                          Re: Standard Line Screen

                          Hi,

                          The formula you quoted: Gray levels= (DPI/LPI)2 only applies to very basic halftone AM screening as might be used in B&W low resolution laser printers. Most modern halftone screening (since 1990 at least) use a form of "supercell" screening. This gets around the limitations implied by the gray levels formula. In simplest terms, it does not matter what gray level a specific halftone dot represents - what matters is the grey level of an area of dots. I.e. If you cannot create an individual 11% dot you can create the effect of an 11% dot area by alternating 10% and 12% dots. Bottom line, modern AM screens that use supercell algorithims do not have the grey scale limitations that the formula suggests they should have.

                          best, gordo

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                          • #14
                            Re: Standard Line Screen

                            Hi

                            Using differing screen rulings for differing papers is just an very basic method of process control for TVI. Running 150 screen for uncoated and 200 for coated will reduce the TVI on the uncoated to levels close to a 200 line screen from common files. But if this is not measured and controlled....

                            This is not the best, or the correct way. I have clients running 300 screen and FM on uncoated papers. By using process control to measure TVI and using ISO 12647/2 based profiles, the results are fantasic.

                            Regards

                            Paul Sherfield
                            The Missing Horse Consultancy

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                            • #15
                              Re: Standard Line Screen

                              Hi Mr.Pritchard,

                              Agreed that this is a very basic formula, and now super cell technology is used.But i guess supercells means grouping of number of cells for obtaining accurate screen angles and screen frequency.
                              And another thing in offset, variable densities cannot be printed, so number of _gray levels per pixel_ has to be maximum of 2 while imaging the plate, as against the case of digital printers, where number of gray levels per pixel are more than 2,so even low imaging resolution will do.
                              I may be wrong sir but if possible please put some light on this part... this may also solve Don's query..

                              Regards
                              Umesh

                              Edited by: umesh on Sep 25, 2007 2:31 PM

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