Golden Nugget #5 Density & TVI or L*a*b* originally appeared on JimRaffel [dot] com in 2005. the topic is one that still comes up frequently in conversations with customers.
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.
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.