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  • G7 SIDs

    I couldn't find the answer I was looking for in the search so thought I would post a new thread.

    I have been reading through as many G7 documents I can find and I don't see one thing. What SID's should I shoot for to start? Everything I read says SID's aren't important and every press is different, but where is my first step??

    I have 2 brand spanking new Epson 9880's that have the Gracol 6 ICC profile assigned to each, so I know my proofs are Gracol 6 calibrated. But how do I get my press there?

    And then, what TVI's should I be shooting for? Again, I read where everyone's different press conditions will affect the curve needed for press, but where do I start? I know my TVI's for CMY should be within about 3% of each other and K will be about 6-8% higher.

    Trying to profile in Staccato on a 28" Mitsi Sheetfed press.

    Any guidance would be GREATLY appreciated. My brain hurts from trying to figure this out.

  • #2
    Re: G7 SIDs

    Hi Aaron,

    the SIDs are determined through hitting the LAB solids on the CMYK and trap/overprint values.

    Once you have these you will have a density range for your particular ink/press/paper combination.

    Page 15 of the current How To has more details on this. The idea is to make sure the solids are correct (ie the color is the right color) and that the traps/overprints are correct indicating optimum printing -

    Ron Ellis

    Comment


    • #3
      Re: G7 SIDs

      >I have 2 brand spanking new Epson 9880's that have the Gracol 6 ICC profile assigned to each, so I know my proofs are Gracol 6 calibrated. But how do I get my press there?

      By "Gracol 6" are you referring to the Gracol2006_Coated1v2.icc profile? This would be from characterization data optimized for Gracol 7 (2007 specfication). "Gracol 6" is an outdated specification.

      >what TVI's should I be shooting for?

      TVI in G7 is a secondary metric as the NPDC (neutral print density curve) replaces it. The required TVI is whatever values allow you to hit tonality and gray balance defined in the NPDC. The G7 How to guide (www.gracol.org) spells this out pretty well, and explains how to get there.

      >Trying to profile in Staccato on a 28" Mitsi Sheetfed press.

      You'll likely have a fairly hefty cut back curve to reduce tonality to match the G7 NPDC. Again, the G7 How to guide explains the calibration steps.

      Comment


      • #4
        Re: G7 SIDs

        Hi Aaron,

        Let me just add to the remarks by those two very capable gentlemen. G7 is really a method for optimizing press and proofing performance and therefore tells you the "how" more than the "what." However, you do obviously need some target values on press to get started, and density is a lot easier (and more familiar to your pressmen) than Lab. For coated sheetfed, #1 or #2 stock, the neighborhood you want to be in is

        C: 1.4
        M: 1.5
        Y: 1.05
        K: 1.75

        You can run a little higher or lower and of course on other stocks the numbers may be lower. Once you're up to color you can use a spectro to implement G7, while density can continue to be used for process control.

        The current "official" from GRACoL is available on their website: http://www.gracol.org/resources/iccaccept.asp

        Best regards,

        Mike

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        • #5
          Re: G7 SIDs

          During my G7 press runs I note density readings, as well as TVI, after I achieve proper solid LAB values. Being a narrow web flexo printer I think we will have better QC if we continue to use density and TVI until we can figure out how to even out some of our variances in the press sheet. Small changes in TVI on a press sheet can play havoc with the LAB values of the grey patches.

          This said, when measuring density (and LAB) I always include paper and when measuring TVI I always subtract paper. We have just purchased a number of densitometers for one of our locations and they have set them up to minus paper on density readings. I suspect that either including or not including paper when reading density is probably not that important, as long as all instruments are set up the same.

          Does anyone know if the suggested density settings in the G7 spec include paper? If this is defined I missed it.

          Anyone care to support either with paper, or without paper as the "proper" method?

          -Bill-

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          • #6
            Re: G7 SIDs

            I am really green to color management. Should the LAB colors be read wet or dry? Also, what tolerance do I have with the readings? More are within 5%+ or -. And if a single value is a bit off but the others are pretty color, how do I know what to do on press? Increase density...decrease density?

            I'm starting to understand all this but have a lot to learn.

            Comment


            • #7
              Re: G7 SIDs

              >Does anyone know if the suggested density settings in the G7 spec include paper? If this is defined I missed it.

              The densities are given as "equivalent" to the Lab values which would include paper, so its safe to assume that the nominal density is including paper.

              >Anyone care to support either with paper, or without paper as the "proper" method?

              For TVI, it should be negligible either way. Zeroing on paper white (subtracting the paper) subsequently removes that value from the solid and tints you're measuring, so the reported percent TVI should be the same. One of the reasons an NPDC definition is more valuable when defining tonality....

              I prefer all measurements to be absolute myself. You could use either with or without paper for density measurement, but I think people predominantly report absolute density values.

              Comment


              • #8
                Re: G7 SIDs

                >Should the LAB colors be read wet or dry?

                Good question. The target Lab values are given as dry and the tolerance is 5 delta E. This can be a rather wide window, and yes, its more difficult to track three values (L*, a* and b*) than one density metric. There's a forth coming amendment to the G7 How To that addresses this...it's likely not possible to optimize all L*, a* & b* simultaneously given you can only increase or decrease ink film thickness, but if you concentrate on the important values for the color you're measuring (a* for magenta, b* for cyan and yellow, L* for black, ect) you can get an idea of whether to increase or decrease ink film thickness, keeping the desired L* in mind. I've found it helpful previously to perform a hi/lo density test with a given ink set prior to calibration to determine the optimal wet ink density that results in the optimal dry Lab values, but this can be overkill sometimes (more important to ensure proper tonality and gray balance).

                Comment


                • #9
                  Re: G7 SIDs

                  Bill:
                  The only densitometer that I ever met that would only read with subtracting paper was the Gretag. What brand are these new devices? To avoid confusion, as if that's possible, you should always include the paper in the density measurement. I think that is spelled out in CGATS documents and definitions of Status T.
                  John Lind
                  Cranberry Township, PA
                  724-776-4718

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                  • #10
                    Re: G7 SIDs

                    Hi John

                    >The only densitometer that I ever met that would only read with subtracting paper was the Gretag. What brand are these new devices?

                    Xrite instruments can be set up to do this, but not set that way for default. I believe you're correct regarding CGATS and absolute density.

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                    • #11
                      Re: G7 SIDs

                      Greetings John,

                      The new devices are X-Rite 500 series densitometers, which in the Options>mode section of Density allow one to select either absolute or -paper.

                      -Bill-

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                      • #12
                        Re: G7 SIDs

                        Aaron,

                        We hired a consultant to guide us through the G7 process. When we first read the G7 guide it seemed overwhelming but taken one step at a time it was cake. We started our testing on press and had to switch inks to a G7 set because we could not match the recommended LAB values.

                        The "Standard Values" are,

                        Cyan- L* 55, A* -37, B* -50, at 1.45 +/- 0.10 Density
                        Magenta- L* 48, A* 74, B* -3 at 1.45 +/- 0.10
                        Yellow- L* 89, A* -5, B* 93 at 1.00 +/-0.07
                        Black- L* 16, A*, 0, B*, 0 at 1.70 +0.2-0.05

                        We use Xrite 528 units set to Staus T.
                        These are values for a number one coated sheet, 95 L*, 0 A*, -2 B*.
                        We also needed to set a custom profile for our proof output to match our presses. We do not use the supplied profile. It was close but not close enough.

                        Comment


                        • #13
                          Re: G7 SIDs

                          Todd

                          Do you know what the tolerances are for the G7 Lab values. I was also wondering if anyone uses a cheat sheet for the pressroom so they know what direction to go when not hitting the Lab value tolerances.

                          Comment


                          • #14
                            Re: G7 SIDs

                            5 Delta E tolerance for Lab values is the standard. We measure with paper density and coating, because we coat the majority of our work. We also take wet readings and compare to dry readings to see if our numbers shifted abnormally. This is important to keep in mind if you run on stock that you don't have a profile for. The color shift can be dramatic as we all know. We are profiled to our house gloss coated and matte stocks. We have experimented with an uncoated curve but don't have it perfected yet, so we use our matte curve for uncoated stocks and it works good enough not to push for a new curve.
                            For the G7 process, Lab values for solid CMYK are only four parts of the "7". You also need to match the standard for the other three colors. MY, CY, MC and CMY. These colors will be impacted by press conditions and materials. Lab values of solid ink density will not tell the whole story and you should only rely on it as a point of reference. When there are issues matching color you need to examine the overprint colors to help determine the problem. It is extremely important to have a G7 color bar on every job. It makes diagnosing color issues simple. You either have numbers or you don't.

                            Comment


                            • #15
                              Re: G7 SIDs

                              Great info guys. Thanks a lot for your help. Next week we will be profiling to our uncoated stock. And words of advise implementing G7 with an uncoated sheet? I take it my LAB and overprint values will be different. Does anyone have those values?

                              So how many of you guys use a separate plate curve for matte stock? We've always used the same for gloss and matte, then a separate for uncoated stocks. Of course, we have always had problems matching proofs so targeting each stock would probably be best when talking gray balance and G7 standards.

                              Comment

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