Archive for the 'L*a*b*' Category

Kuler is not cooler - by Michael Jahn

Published
by
Jim Raffel
on April 28, 2008
in L*a*b*, blogging, calibration, color, gray balance and web-to-print
. 0 Comments

Michael Jahn, a JimRaffel.com reader replied to me via email about a previous post and I found his comments enlightening about how scary the world of color has become for those of us in the graphic arts that have to actually print this stuff.

So, for the first time ever (I think) a guest contributor at JimRaffel.com….

Hi Jim,

Read you blog about Kuler;

As I suggest in my subject like, I do not think Kuler is cooler.

Okay, I will give you it has a sweet looking thing to look at, but so was that Brazilian chick I dated.

example - go to Kuler - in the search tool, enter “swop”

notkloseinkuler.jpg

See attached (this is made from several screen captures - Kuler runs in a browser, so that is RGB - I did my screen captures in Photoshop building while in CMYK, profile was SWOP version 2 - not wanting to debate the fine points, as Kuler seems to have no real notion of CMYK that one wonders why they offer it at all…

So, my gripe is that if one were specifying in CMYK

– 50k should look fairly close to 50c, 38m, 38 y, 0k - in Kuler (TOP) they don’t, and in Photoshop, (BOTTOM) they do

and 50c, 50m, 50y 0k should look a bit warmish brownish, in Kuler it is neutral (incorrect) and in Photoshop it is a bit warmish brownish (correct)

I have no explanation as to why 19c, 0m, 0y, 38k should look like 50c, 38m, 38 y, 0k - in Kuler, or anything that might create, display, report proof, print or plate.

Clearly - This is not ready for prime time for color specifying in the world of print

–
Michael Jahn
Jahn & Associates
PDF Color Conversion Specialist

#48: Make Proofs That Match Your Press

Published
by
Jim Raffel
on February 16, 2007
in GRACoL, L*C*h*, L*a*b*, TVI (Dot Gain), color, color bars, delta E, measurement, process capability and proofing
. 0 Comments

Since #45 in December we have been discussing resolutions to improve the quality of your proofs. In order to gain the full benefit of this installment one should have read and implemented the suggestions in #46 & #47.

Proceeding forward it does not matter if you are utilizing a methodology like GRACoL G7, or a more traditional color management approach. In either case you will include a target on each print job with the same color bar swatches that you output on the proof (#46). This will allow you to measure the press ok sheets and compare them to the proofs, thus building even a larger statistical database to call upon.

When utilizing software like our ColorMetrix and ProofPass.com products for process control and print certification purposes it does not matter if you are using a methodology like G7 or traditional color management. While there are some new formulas out there to run the collected data through the tried and true colorimetric data (L*a*b* and derivatives along with some version of Delta E), do a great job comparing two similar or dissimilar imaging systems.

Unlike density and dot gain values which must be used with pigment sets designed for 4/color process printing, L*a*b* values allow the comparison of an inkjet proof and an offset press sheet. This is possible because we are looking at the actual colors, not values derived from a formula which assumes a certain pigment set.

Not only will you be comparing the measurements of the proof to the press sheet, but you will be building a history of what is a “normal” print condition of each press and paper combination. Some refer to this as finding the “sweet spot” of the printing press.

Overall, as your volume of collected data grows you will be able to refine the system in small steps by reviewing the proofing and press information both independently and together. Using numeric results, charts, and graphs you will be able to see small differences in color that can be adjusted for over time. No system is stable over time, so continuous monitoring is a must in order to maintain stable color.

Golden Nugget #5…Density & TVI or L*a*b?

Published
by
Jim Raffel
on July 13, 2005
in L*a*b*, TVI (Dot Gain) and density
. 0 Comments

*** Introduction ***

Next, I want to let you all know that since the inception of the golden nuggets idea both of the Mikes at ColorMetrix (Michael Litscher-CTO & Co-founder; Michael Woods-Technical Services Manager) have been after me to blog the golden nuggets as well as sending them out as an e-mail. While I thought it was a great idea from the start, I just did not have the time until this week. So, from this point forward after I send the golden nugget e-mail I will also post in it on the http://www.jimraffel.com blog. The blog has two other significant advantages; readers can post comments to each message; and I can post follow-up information or addition topics between e-mails.

Well, once again our customers and my associates in the field have made writing this week’s e-mail both easier and more difficult all at the same time. I was provided with many great ideas this week, but can only write about one. I also believe one of the ideas is significantly technical that I will need to write a white paper in the future and publish that on the blog site. The topic I will not be covering is closely related to the evaluation of inkjet proofs and press sheets with Density/TVI vs. L*a*b*. Eric Magnusson (www.leftdakota.com), explained to me this week how he uses the L* value of CMYK on inkjet proofs to derive approximate Dot Area values. While I find the idea interesting, I need more time to test it, and run the math by Michael Litscher our CTO.

*** Jim Raffel’s Week 5 Golden Nugget ***

I settled on the topic of using Density & TVI vs. L*a*b* to evaluate the difference between inkjet proofs and press sheets not only because it came up several times this week, but it keeps coming up every week in on-site visits and telephone support calls. This topic stems from the reality that densitometers use very specific filters defined by international standards to measure the CMYK ink set. These filters are tuned to the spectral response of the process color ink set. The inks used in ink jet printers do not have the same spectral response as the CMYK inks used on printing presses in our industry.

So, while the solid color bar patches of CMYK on the proof and press sheet look visually the same to our human eye (capable of seeing the entire visual spectrum at all times), they can and often do look considerably different to a device which simply has three filters (utilized to filter the visual spectrum into thirds). This also means that any value derived from a potentially flawed density value (i.e. – Dot Area, Dot Gain-TVI if you prefer, Print contrast, Trap, etc…) are equally unreliable as evaluation tools concerning the match of an ink jet proof and a press sheet.

Michael Woods and I have seen time and time again in the field inkjet proofs that do not visually match, but have density and TVI values within tolerance (we have seen the reverse also). As recently as last year I know of at least one inkjet proofing manufacturer who still utilized density trends to verify proof consistency over time. One of our mutual customers complained to me that while the manufacturer’s software and ColorMetrix both reported the proofs being in specification, the visual variation was not acceptable. I asked them to view the trending data in Delta E, and was not surprised to hear them say that shifts or 6-8 Delta E (CIE L*a*b*) in the balanced 3 color grays appeared to be the norm not the exception.

So, is there any value in measuring the density of inkjet proofs? I am not convinced there is. On the other hand, since it is easy to collect and store the data, why not do so. Also, one of my customers pointed out to me this week that they monitor Magenta density of their inkjet proofing system to catch changed changes in ambient conditions like humidity and temperature. While I have not seen this first hand, I am told by the customer that they can get shifts of .07 in Magenta, and still have Delta E variation of less than 3.5 (acceptable level for this instillation). But the movement in the Magenta density warns them they are having humidity problems that could cause the production of bad proofs soon.

The above example is what process control is all about. If you do not measure it, you can not control it. Then, when problems do occur you can go back and look for shifts in the measurements. After identifying a key metric, as our customer above did, you can add that to your daily monitoring, and improve quality and productivity while at the same time reducing waste.