BRIDG'S/Idealliance Printing Guidelines confusion

[J]

Hi,

I'm new to this forum and am trying to understand some things about color management. I'm a pressman and was given the BRIDG'S printer's survival guide for printing to specifications and standards to digest. In it there is a poster of printing guidelines. The info on the poster is a bit confusing and also I think there some mistakes in the text which adds to the confusion. Where it gives info like LPI, SID, TVI, Print contrast, etc. it calls them "suggested input variables" but are the numbers on the poster actually target values? It also has something with values called NPDC excluding paper. But it's not clear what the numbers mean or where they come from or how do I measure them. For example it says for #1 & #2 paper: 50% gray is .54 CMY and .50 K. Both of those numbers change depending on the paper. for example Premium paper the numbers are .53/.49 .
Can someone explain (simple) this please.

thank you J

PS, One of the mistakes (I think) is in the notes on the poster that are supposed to explain the question I have - it says 25% gray is 50C, 40M, 40Y and that 25% gray is 75% gray. Really.

 

[meddington]

Re: BRIDG'S/Idealliance Printing Guidelines confusion

>LPI, SID, TVI, Print contrast, etc. it calls them "suggested input variables" but are the numbers on the poster actually target values?

The Bridgs chart attempts to meld together legacy metrics of solid ink density, print contrast, TVI, with new metrics like NPDC, Solid Lab, gray balance, ect, and yes, it comes off as confusing. Whether or not these are "suggested input variable" or actual targets isn't made clear. It is stated that the Lab values (where they exist) take precedent over the density values, but the solid ink Lab values are listed under "suggested input variables". Further, if solid ink density is secondary to Lab values, than TVI and print contrast, being derived from solid density, would (should) be secondary variables as well.

>It also has something with values called NPDC excluding paper. But it's not clear what the numbers mean or where they come from or how do I measure them.

The NPDC values given are density values of the respective CMY patches (50C, 40MYfor the 50% gray) with the density of the paper removed. So if your paper measure 0.08 density, add that to 0.54 to determine your target. The NPDC values come from G7, which actually defines the NPDC throughout the tonal scale. The NPDC is essentially static from highlight to near midtone, but is dynamic at the shadow end, to allow for higher quality papers to receive thicker ink film/higher density.


The Bridgs guide has some good information in it, but its obvious that the data was thrown together by two camps who do not necessarily agree on which metrics should be included and which shoud take precedent. In my opinion, the values that should be "official" are the solid labs values, and NPDC (density and gray balance). Solid ink density, TVI and Print contrast are useful legacy metrics, but should be informational only.

>PS, One of the mistakes (I think) is in the notes on the poster that are supposed to explain the question I have - it says 25% gray is 50C, 40M, 40Y and that 25% gray is 75% gray. Really.

Yes, really. I cannot image how this made it to press like this, but someone dropped the proof reading ball. It should read 25% gray is 25C, 19M, 19Y and 75% gray is 75C, 66M, 66Y (rounded up from the official fractional values).

 

[J]

Re: BRIDG'S/Idealliance Printing Guidelines confusion

Hi Michael,

Your explanation helps very much. You mention that the NPDC values come from G7, which actually defines the NPDC throughout the tonal scale. Do you have examples of the typical density values of the rest of the tone scale that you could share? Are they a standard?

thank you again, John

 

[meddington]

Re: BRIDG'S/Idealliance Printing Guidelines confusion

>Do you have examples of the typical density values of the rest of the tone scale that you could share?

Well, the 25% and 75% are given in the chart... 0.25 and 0.90 respectively for coated Grade 1&2 paper. The entire tone scale is shown on the G7 NPDC graph paper, part of the G7 How To guide available free from www.gracol.org.

>Are they a standard?

They are a "specification".