G7/GRACoL 7 > 175lpi

[cmcfarling]

The official Calibrating, Printing and Proofing by the G7â„¢ Method doc states

"For commercial work, halftone screen ruling should be at least 150 lines/inch and nominally 175
lines/inch."

In addition the GRACoL 2007 spec is based on a 175lpi screen ruling. So my question is, can a press be officially G7 certified using the GRACoL 7 spec using a 200lpi or even a 250lpi screen? Or is it that once you're above 175lpi you're not within the GRACoL spec anymore?

 

[TerryWyse]

can a press be officially G7 certified using the GRACoL 7 spec using a 200lpi or even a 250lpi screen? Or is it that once you're above 175lpi you're not within the GRACoL spec anymore?

Point 1....It's a common misconception but there is no such thing as being "G7 certified". You can be G7 *qualified* ("G7 Master Printer Qualification") but there's no certification that involves tolerancing for pressrooms. Closest thing to "certifcation" is only for off-press proofing systems such as inkjet printers and RIPs.

Point 2...You can calibrate your press to G7 using any type of screening you want...FM, 250lpi AM, whatever...the only "requirement" is that you're hitting the G7 specification for gray balance and tone ("NPDC"). Even targeting specific Lab/density values for solid ink is not strictly a G7 requirement but is only part of the spec for GRACoL Coated1, SWOP Coated 3 and 5. Bottom line, use whatever screening you want as long as you can hit the G7 spec. I will warn you though...in my experience it's not possible to match the GRACoL/SWOP data sets when running either FM or high linescreen AM. You can "match" G7 but the color itself will change enough (higher saturation usually) with the higher resolution screening that you won't be able to match the actual GRACoL characterization data without employing color management such as device link profiles.

My 3 dE worth,
Terry Wyse

 

[cmcfarling]

Terry, does your advice apply to waterless offset presses as well, such as A Presstek 52DI?

 

[TerryWyse]

Terry, does your advice apply to waterless offset presses as well, such as A Presstek 52DI?

Sure. G7 is basically screening "agnostic"....in fact it's PRINTING PROCESS agnostic as well. G7 simply outlines some metrics for tone and gray balance ("NPDC") and not much else. It's a methodology more than it is a specification. It becomes related or tied to a specification only in the context of GRACoL and SWOP. If you target GRACoL Coated1 or SWOP Coated 3/5 as a characterization data, you will be "G7".....but just because you've calibrated your press using the G7 method/aims, doesn't mean at all that you'll be matching GRACoL or SWOP.

Clear as mud?

Regards,
Terry

 

[cmcfarling]

Yea, I actually have a fairly good understanding of everything you just explained, although I haven't actually done any hands-on calibrations at this point. My last question was geared more toward waterless DI vs. traditional offset in the context of your first answer. In other words, would a Presstek DI press have a better chance of meeting GRACoL specs using G7 at a 250lpi screen than a traditional offset press?

 

[TerryWyse]

Yea, I actually have a fairly good understanding of everything you just explained, although I haven't actually done any hands-on calibrations at this point. My last question was geared more toward waterless DI vs. traditional offset in the context of your first answer. In other words, would a Presstek DI press have a better chance of meeting GRACoL specs using G7 at a 250lpi screen than a traditional offset press?

On that question I would have no idea. Too many variables to consider. I would say since an offset press running 250lpi is going to have considerable "gain" that will have to be compensated for by the G7 calibration, if you were to tell me that 250lpi waterless has LESS gain that typical offset at that line screen, then the G7 curve correction will be less severe....but I think the chances are pretty slim that the Presstek DI with waterless plates imaged @ 250lpi will print to G7 specs without any curves at all, so it's really just a matter of degree.

I've done small Ryobi DI presses with Presstek waterless plates and the dot gain was pretty horendous even at normal screen resolutions when pushed towards GRACoL Lab solids.

Terry