What is the best status density for controlling a process? First of all, what do we mean by 'best'? I would say that for process control purposes, 'best' should be the status density that allows the most sensitive response to ink film thickness (IFT) variations. Depending on the application (metallic colors, fluorescent colors, process colors, wet or dry readings) this might vary. With that in mind, we should be looking for the density response that amplifies the particular wavelenghts that correspond to the spectral reflectance of a particular color. Le'ts focus only on process colors.
Examining status T and status I densities, it was obivous that status I densities are far more sensitive to unveil variations in IFT than status T. Status T densities were developed in the film era to make the separations. Thus, they needed to be broadband, and capture a good deal of the wavelengths involved. For process control however, the spectral reflectances of process inks are pretty much known, and thus this 'spread out' is not sensitive enough to amplify the response and allow the user to see IFT variations, at least not as much as status I does, where we have a much higher peak. Status E densities are almost the same with status T densities, with the exception of a higher sensitivity of the yellow filter.
Another issue is polarization. Polarization was developed in order to be able to make the dry densities look like wet, by taking out the diffuse reflectance of the light from the substrate. When the ink film is wet, we have much less diffuse reflections, and higher gloss... the beam of light that returns to the densitometric sensor is more focused. The problem with polarization is that it is complicated in its calibration. It's not always convenient to have a wet sample as a standard.
Why am I going here? US is on status T, Europe is on status E. We are talking about global process control standards (ISO, GRACoL7 to name a few)... but we have different measuring practices. In that sense, having the 'best' status density would mean to be using the status density that adheres to the regional standard. In this way we would be able to communicate efficiently with our suppliers and customers.
The situation gets a little more complicated when we are dealing with press manufacturers. A lot of them are in Europe, yet they are selling their presses in the US. Do they, or their subcontractors, adhere to the US standards? Should they? In order to answer this question (without wanting to go to the obvious, long-term solution of finally setting global standards) they should ask and answer the question what is best for their customers.
If I was the President (as we used to say in my village), I would setup either status I, or status E densities for all. I am a UV printer, so I do not care that much about polarization -easy way our, right ;) But we are status T in the US, and I am not, neither want to, be the President. So, that necesitates me having status T, unpolarized, even if it is worse that status E or status I.
To be more particular, and after having briefly talked about the different technologies, in my company we have online press controls that are status E, polarized, and they are pure old densitometers (no spectral densities here). The reason we do not have spectral readings converted to densities is that we have to be able to take fast readings of the color bar for process control. A spectral reading, sampling the entire range of wavelenghts at 10 or 20 nm would be slow. Our software (which is in every other aspect absolutely amazing and the best I've seen in the industry, check out www.lithec.de) offers us the opportunity to calibrate our polarized density readings to the spectral density unpolarized readings of our handheld instruments (500 x-rite series). Confusion builds up.
Overall, it works just fine, but when we have to move to a different range of densities, it fails... especially for the yellow and black filter readings. The easy solution is to recalibrate not from a given standard, but from the printed sheet itself. In this way it would be somewhat wet (to take care of the unpolarized-polarized confusion) and in the right range of densities. What do I mean by the right range of densities? Without having done any research on the topic, I would think that since density is a non-linear response, when you calibrate to a 1.30 dY, and then you print at 0.90 dY, the conversion factor in the software does not simulate the nonlinearities of the varying IFT. I guess there wouldn't be a nonlinear conversion factor for it either. The difference between spectral and filter density makes it even tougher.
(Standardizing the readings of the spectrodensitometers is an entire new chapter... even less probable to be solved. If I remember correctly, the falacy of color theory is in lacking a standard white point reference... I shouldn't be stating this without opening my books cause I do not remember exactly, but I think that that's it).
Anyway, explaining all these to our manufacturers' we were able to get them working on some status T, unpolarized density filters. Will they work? I hope so... but I do not know so. Would we be able to adhere to the US standards better, with the downside of having less sensitive process control? Might be.
We have to take our risks, do our research, and find out... otherwise we would still be using our eyeballs for process control...
-D
PS> Don't get me wrong, first and last for process control is the nice, old uncalibrated eyeball and these undetermined pathways to the V1 and V4 cortex areas and then to perception! We just need systems to support all these!
