In the thread: Using an i1 to measure FWA content? this message mentioned some polymers that show equal reflectance over the usual spectral range:
Quote On 2010 Feb 9, at 5:12 PM, Graeme Gill wrote:> For the i1pro the reference tile certainly rolls over at about 410nm. It's
> reflectivity at long wavelengths is about 0.9, while it's about 0.7 at> 380nm (according to the reference information). Presumably the spectrolino
> is similar. > > [This seems typical of many "white" materials. The only common thing I've > come across that doesn't do this is white polystyrene foam.] I'm glad you mentioned this, and that it stuck in my head.Because it made me remember that, at the back of the paper swatch booklet that
came with the printer, there was a section of banner materials. And, forwhatever reason, I also remembered that one of the swatches was for something
with a name that sounded like it came from DuPont.Sure enough, Canon sells rolls of Tyvek for printing on. And not only does it have a flat response across the spectrum without the falloff you describe, it
measures D50 Lab: 98.946946 -0.283136 -0.241464. I think I just found the ``paper'' I've been looking for! (The other banner materials all have obviously over-the-top amounts of FWA.) A roll of it is a fair chunk o' change, but not as much as a single ColorChecker SG.... b& End of quoteFor some weeks I'm measuring the whites of several inkjet papers with a Spectrocam without a UV cut filter and its Xenon near daylight spectrum. Measuring the print side with the paper on a neutral museum matt board, the same with the paper on a black tablet and the rear of the paper with the front on a black tablet. That says something about the whiteners used, about the optical whiteners, where they are located in the paper and how opaque the paper is. I get some comments that the measurements are different to what other people measure. I think the Spectrocam deviates more between readings but it could also be related to the differences in the light source. I ordered and received an Eye 1 basic normal, the one without the UV cut filter.
The discussion on the numbers made me aware again that we do not have a white reference sample that could be used to give a standard for discussions like mentioned. And with that reference eliminating Spectros and Colorimeters measurements that are really off. And with the use of the right algorithm make readings of different spectrometers more comparable. Searching the way spectrometers are calibrated in labs I came across the pressed PTFE powder tiles that are used for that purpose. While not that accurate, natural solid PTFE = Teflon already has nice optical qualities for that purpose. That led to distributors that could supply natural PTFE sheets. To try it out I realised that there ought to be Teflon tape in the workshop. There was.
I measured some rolls with my Spectrocam, with a friend's Eye 1. Called a friend to measure a roll he had with his Spectrocam. I measured the rolls with the new Eye 1 that arrived last week. I have asked some people around the world to do the same. Buy a roll or some different rolls of plumber's Teflon tape. Preferably on a white core, 0.1mm thick tape, half an inch wide. Cut part of the plastic flanges away that the sensor can be put on the roll, on the curved surface. It is soft enough to adapt to the sensor hole. Peel one layer of the tape off. Measure with a calibrated spectrometer. Not too much light in the studio but no need to find a dark place. Typical value Lab 99.2, 0.0, 0.3, and a very straight horizontal line from 380-730nm. I have some reason to think that Teflon tapes around the world will give almost equal measurements, within the usual deviations of spectrometers. There are specifications for the purity of the Teflon to be used, the 9 mm thick layered Teflon tape sample difuses the reflectance nicely. There's a Wiki page on PTFE with a reference to the optical properties. The stuff is almost inert to aging and a fresh surface is created in an instant with the tape roll. There is usually some OBA/FWA in the white plastic core but I do not see that in the measurements on the Teflon. I could be wrong but it will be interesting to see more results. There's a promise that a lab will do more precise measurements of Teflon rolls I can collect globally but I like to have more measurements from more places and normal spectrometers first.
The calibration tiles as delivered with spectrometers do not have that same reflectance over the spectral range, they climb from the 380nm reading to the 730nm reading and are less straight on the curve. Like mentioned in the ArgylCMS message above. The reflectance differs between the Spectrocam calibration white and the one of the Eye 1.
I wouldn't mind if people send me measurements with the method I described above. If the theory is wrong there's little lost, you can still use the tape for its original purpose. If I'm right then a 2$ white reference roll is globally available.
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