Mathematical Evaluation of Color?

[Hopkins Printing]

I am curious if anyone is using a spectrodensitometer (we have X-Rite) to measure the L A B values of color on a press sheet. The idea being is that you could mathematically determine if a press sheet has good color or not versus just eyeing it.

If you are, can you tell me what is considered acceptable deviation in terms of L A B values (eg. if you have an established L A B standard for a given color, how far off can the L A B values be compared to the standard before that given color is considered to be...off)?

Thanks in advance,
Jon Morgan
Hopkins Printing

 

[Colorblind]

Re: Mathematical Evaluation of Color?

From what I know, when comparing LAB values between a sample an a reference, achieving anything less than 2 Delta E means that the human eye won't perceive a significant difference. I don't know if the tolerance is different for press sheets but that's what my integrated Sherpa proofing quality control uses as an acceptable value.
Here is a great link for such calculations:

http://www.brucelindbloom.com/index.html?ColorDifferenceCalcHelp.html

 

[gordo]

Re: Mathematical Evaluation of Color?

You may want to try measuring the color bar CIEL*a*b* values for all the jobs going through one press for one week. Assuming that the press operator completed the run with presswork color aligned to your proof, this will give you an idea of how much, or little, the delta variation is. You may find that, when the presswork "matches" the proof, the delta variation in color bar CIEL*a*b* values is so great, or unconnected with the live image area, that it is not a valid way to judge a "match."

best, gordo

 

[raffelj]

Re: Mathematical Evaluation of Color?

Jon,

To expand upon Gordon's comment: We have customers who do in fact measure the color bar and determine acceptability of print quality by the numbers. I agree there are situations where the color bar may not accurately represent the live work in the job but that is a matter of understanding your work. Our customers who have made this work do so with color bars that often times contain special color swatches to represent critical colors in the job.

In this way they can print the same job in multiple plants around the world and achieve a consistent result.

Jim Raffel
ColorMetrix Technologies, LLC
http://www.colormetrix.com