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Accumulative Density: A Powerful Color-Control Tool

(July 2008) posted on Tue Jul 15, 2008


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By Mike Ruff

What we learn here is that the accumulative values in the print area are relevant from print to color target. Individual density results in the image area can often exceed those of your solids on your color bars. Remember, this is accumulative. That means that if the value you read is an accumulated total of all the light re-flectance of what you’re measuring, then you’re adding some of all color values from all colors present. Look at Figure 1 again. Its cyan value is 1.44 cyan, printed over magenta. Wow! How could this happen? We’re only printing with a 1.30 solid! How can we have 1.44? Because the magenta solid and the yellow solid also contain cyan spectral values. The value we read here is an accumulation of all of the values. The substrate also adds to the values. So by knowing and understanding this, we realize that the measurement we’re comparing is very accurate and useful because it shows us everything going on the image area and compares our result to an intended color target.


Accumulative density in ink hue

You can use your spectrophotometer’s accumulative-density feature to evaluate ink hue reported in CMYK. As mentioned earlier, evaluating a solid in L*a*b* is not a problem, but a report in CMYK is more relevant to a printer. For example, let’s say we’ve reviewed a spectro’s L*a*b* evaluation and found the problem to be a reading toward the blue side of the b axis. Why is it blue cast? A look at the CMYK breakdown using the same data would reveal missing yellow, while the L*a*b* would report a blue shade ma-genta. Instead, we find the fix in the CMYK. What we learn here is that reporting the solids in CMYK is very useful and accurate—certainly not any more accurate than L*a*b*, but much more clear as to why the solids are different.


Accumulative density in substrate color

Substrate evaluation is accumulative density’s final, very useful, function. Again, we can read the substrate in L*a*b* and use the same data to calculate the substrate’s CMYK values. These CMYK numbers give us some help in evaluating color-matching issues caused by the substrate. A so-called neutral-white substrate adds no cast to the print. Neutral, according to ISO 12647-5, the ISO screen-print standard, is defined for #1 grade paper as L* 93, a* 0, b* -3 (Table 1).

