Hi all,
It's fascinating how much discussion this has brought up. As Pat mentioned, we've been using the polarizing filter, selectively, at CHROMiX for at least 17-18 years.
A few responses to questions / comments in the thread so far
I wonder whether the SpectraScan could use both UV + polarized filtering in one measurement
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Only one filter condition can be used at a time so, M3 +M2 combined can't be measured
First, both M3 standard and the Pol filter for the 'Lino include UV filtering. So, by definition, M3 is really M3(+M2)
The first question I would have is whether reading matte profiling targets with such a filter buys one a profile that would actually cause an ink lay-down that returns a minimum value of L*5 (Maximum Black) or so on a matte paper
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Theoretically, if the B/W range is extended with a deeper black point it should lead to more separation
As postulated above, using a Pol filter does not make the printer print to a higher density. For the measurement numbers it decreases the L* value as well as increases saturation, especially in the darker colors.
The physics are fairly simple. "First surface reflection" is the key here. In the case of a glossy surface there's very little light scattering and typically a spectral highlight reflection that we avoid viewing directly. The more bumpy the surface (and it increases as inks dry into the media) the more scattering that occurs from this first boundary. This scattering occurs before any ink colorants have a chance to do their filtering and often before the color of the media itself has an influence. So it's white light and it dilutes our perception of the color.
Polarization works in a least two ways. Light is naturally polarized when it reflects off a surface. This is why polarized sunglasses effectively cut the glare from car windows and the surface of water. Additionally, if the light source is polarized prior to reflecting off the sample, then there's a greater chance of the cross-polarized filter cutting down the first surface reflection. Just like sunglasses allow seeing fish in a river, polarized readings allow discrimination of shadow detail in print.
...I don't think M3 is all that useful a mode for making ICC profiles with most photographic...
There are a lot of mental calisthenics that can go into this. For us, as I expect for most of you, the proof is in the pudding. Over the years we've found that Pol measurements greatly improve some profiles while not improving others. Surprises like significant improvements in some toner profiles compell us to try it in many different situations.
The reality is that neither instrument is likely "correct". Unfiltered i1's may measure 23 L* while a Pol filtered 'Lino or Barbieri spectro LFP may measure down to 2-5 L*. The reality is somewhere in between.
Here's an interesting test: . .
- Print a graduated gray ramp on glossy paper and on the uncoated paper in question
- Measure both and Write the measured L* value beside each patch
- Place them under controlled lighting (not just color but angle of illumination as well)
- Visually determine the match between the darkest patch on the uncoated paper and visual equivalent patch on the glossy paper
The L* value of the matching glossy patch will be lower than the uncoated patch and what the instrument probably should have measured, if it weren't for all the surface scattering. You can numerically stretch the uncoated paper's measurements down to that lower value and it will likely produce a profile that's better at proofing it's expected output. We've just added a feature to our Curve4 software that does this very scaling. It helps create a match between an uncoated press sheet (measuring too light) and a proof on proofing paper (Curve4 is for G7 calibration of digital and conventional presses, not really appropriate for this crowd)
But will it produce a profile that actually renders output better? Probably not.
Why?
When measurements are used to generate a profile and it's used for rendering output, it's mostly about relative mapping. The lightest point will map to paper white (even if you're using the wrong profile) and the blackest point will likely map to the blackest print point (due to the behavior of the perceptual intent or black point compensation with rel col intent).
So, the effect of Pol measurements is felt in two separate ways:
1. As mentioned, the lower L* and saturation creates a profile with a larger gamut volume and this will create soft proofs with a higher dynamic range. They are probably eggagerated somewhat, though we have scaled measurements to correct for this in the past.
2. The greater dynamic range of the measurement allows for the detection and encoding of more color gradation. This often translates into a profile that can address such detail and create prints that have fewer problems with washed out shadows. We've built thousands of profiles over the years and I can attest to it working, and working well, in some printing situations.
The situations in which it makes the difference is something we've arrived at through experimentation.
As for the instruments? Gretag MacBeth's SpectroLino/SpectroScan and the Barieri Spectro LFP are the two automatic instruments that we've used over the years - and the 'Lino's long out of production. Some hand-helds like the eXact, KMS FD-7 and SpectroDens will take M3 measurements and can measure strips in a manner similar to an i1Pro or be mounted in the ColorScout table. None of these solutions are inexpensive. - the polarization filter alone for the LFP is around $800 (!)
As Pat mentioned, we hound manufacturers continuously to add M3 to their instruments like the i1Pro, iSis and FD-9 but they haven't so far. M3 can cut down the light significantly so it slows the measurement process down, but for us it's worth it. Our SpectroScan tables with the Pol filter can take an hour and a half to measure a single target - which is why we have a bank of them. But again, when it's worth it, it's worth it.
I hope this helps.
Steve