On Sep 18, 2015, at 9:34 PM, Robin Myers <robin@rmimaging.com> wrote:
I tried various adhesives, all failed to hold the PTFE tape permanently. Some, such as Elmer’s were not used because they fluoresce or they turn yellow when dry.
I'm remembering more now...I'm sure I wound up wrapping the tape around something opaque and gluing the back side. At that point, even something like pitch would work and not affect the optics. Consider this: use spraymount or the like to securely glue a sheet of Tyvek to some white foamcore. (Top-quality OBA-free fine art inkjet media might be an alternative to Tyvek worth considering. I suppose in theory the foamcore would ideally have an OBA-free paper surface, but, in practice, even black foamcore is likely to work fine, so I wouldn't worry.) Use a razor to cut out a square the same width as the PTFE tape. Cut a single length of PTFE tape about three times its width. Lay the foamcore square with the Tyvek down on top of the tape, in the middle. Wrap one end of the tape to the back, gluing it with whatever ugly gunk you've got that'll hold. Do the same with the other end, only use enough tension to smooth out the surface. Repeat with another piece of tape at 90° to the first, and keep adding layers until you've reached the desired opacity. If you've started with Tyvek at the bottom, it shouldn't take more than a few layers of PTFE. Whether or not you mount the finished reference in some sort of frame is up to you and what you plan on doing with it.
Consider yourself very lucky. Canon discontinued this media some time ago.
Thanks for the heads up! I'll not use any more for banner-type stuff, and save it for the Tyvek.
Trying to get a material with the right spectral reflectance, good cost, material availability and production abilities is not trivial. I am still looking.
It's a good part of the reason why I started looking for alternatives to profiling based off of reflective media. It started just as an attempt to expand the gamut past the relatively low saturation possible from pigments compared with monochromatic light sources...but I eventually realized that depending on a reflective sample for any sort of absolute property that doesn't actually exist is not such a good idea. I still use ColorCheckers for reference, but I could do everything with just a single patch of literally any color that I had a spectral measurement of. Using (and averaging) all 24 patches of a ColorChecker is overkill, but trivial to do...and the surface texture of the ColorChecker's patches is reasonably lambertian, the original cardboard has nice big patches for the studio and the Passport its protective case for the field...they're fantastic products for a color reference; I just don't use them to build profiles, or assume that any of the patches are perfectly neutral, and so on.
I started with tungsten-halogen lighting, then, because of the poor emission in the short wavelength part of the spectrum, and its emission inefficiency, only about 16-25 lumens per Watt with the rest of the emission being infrared and the potential thermal damage to the artwork,
These days, it's worth considering regular dim household incandescent lights, and averaging however many long exposures you need to get the noise under control. Even if the blue channel winds up underexposed by, say, four stops compared to the red...well, just take 2^4 = 16 exposures and perform a median blend on the lot. The blue winds up with the same noise as the red originally had -- and, of course, the red has four stops less noise as well. Or, in other words, by blending 16 exposures, you increase effective ISO by 4 stops, making an ISO 100 exposure equivalent to an ISO 6 exposure. With that sort of approach, you could get superlative results from candlelight...assuming you also did some sort of flat field adjustment since there's no way you're going to get even illumination across the scene from a candle or three.
There is a fully spectral art reproduction solution available; ColorPony from ColorYoke. It uses a proprietary set of calculations that require spectral measurements of the lighting, spectra of the artwork’s pigments, spectra of a white card used to even out the cosine-fourth and lighting falloffs, plus the spectral response for the camera.
I'll bet you a cup of coffee that my approach bests that...if for no other reason than that they're probably not including the spectral efficiency of the lens in the model -- and you wouldn't believe how much variation there is from one lens to another. Nobody does, except in the naive sense that, if you use the same lens to photograph a work as to photograph the chart you use to build a profile, the lens gets "baked into" the profile. But the chart approach falls short on so many other grounds.... What I've got figured out now is likely going to keep me very happy for quite some time. There're three directions I'm thinking of taking it in the future. The first...the camera's spectral response isn't a perfect match for the standard observer, so there're some metameric mismatches in both directions -- colors humans can distinguish but the camera can't, and colors the camera can distinguish but humans can't. In theory, there should be some way to use colored filters to make the camera better simulate the standard observer -- perhaps even a single filter, but more likely multiple filters, and then taking three exposures with each filter and extracting and re-merging the appropriate channels. And, since there's no way I'm going to start designing color filters with specified transmission characteristics, that'll depend in no small part on the luck of the draw and whether or not any readily-available filters are "close enough" to what I'll need. (I could already trivially generate a graph of the ideal filter, but there's no point until I'm ready to tackle the project.) Second, Dr. Berns and his crew at RIT have published lots of papers on true multi-spectral imaging, including approaches that accurately reconstruct spectra using multiple filtered exposures from a digital camera. It might be worthwhile to implement some of that research...but only if I move past the proximate goal of making a print on my own large-format inkjet. The third would be designing an actual spectral-based capture workflow. I already have some ideas for how to make an oversized spectroscope that could use a scanning motion to image something with a DSLR at arbitrary spectral resolution, as fine as 1nm or smaller. But the postprocessing would be a real nightmare, and the mechanics of the setup...and, again, it would be gross overkill for general reproduction work and only of interest to curatorial and preservation work.... Cheers, b&