On Sep 18, 2015, at 2:26 PM, Robin Myers <robin@rmimaging.com> wrote:
While PTFE references such as Spectralon® and Fluorilon® do not exhibit perfectly flat spectral reflectances, they are the best materials presently available for white reflectance standards and are used as such for calibrating spectrometers by many companies and NIST.
PTFE makes for a great reference standard, but, best I know, outside of photographic click-to-balance workflows, it is not used in a manner that assumes that it is pure white. If you measure the calibration tile included with an i1 Pro -- the one that you have to put the instrument on before every session -- you'll see that it's decidedly less white than PTFE. It's got a curved spectral reflectivity significantly less than 100%. But it's a beautifully smooth spectrum, and presumably the material is very stable. It's plenty reflective enough that it presents an high signal-to-noise ratio for the electronics. PTFE is also stable and has an even higher reflectivity, making for marginally better absolute properties. But it's less durable than the ceramic used for the i1, and I'm sure much more expensive, which would explain why X-Rite used the ceramic instead. You don't need an absolute white reference for spectrometry. The instrument (or software) knows what the combined reflected spectrum of its lamp and reference is supposed to be; whatever it measures at the time of calibration is used to create an offset to correct for whatever momentary conditions from the environment or whatever are causing drift. You could use a lump of coal or a colored ink or whatever for that, save the readings would get noisy. <blockquote>PTFE is the only white material I recommend for neutral balancing cameras.</blockquote> If you're doing the click-to-balance method, PTFE is as good as it gets. Thread seal tape (like what plumbers use) is a good, cheap source; layer it up to ensure opacity. BabelColor used to sell an affordable 1" circular Spectralon (or equivalent) target, but that was years ago. Actual Spectralon targets are available for insane prices. But...there're two cheap alternatives that every photographer should be aware of. First is Tyvek. It's nearly as good as PTFE, and you can buy it cheap, usually in the form of envelopes, at your local office supply store. It's nearly indestructible, which is why it's used for envelopes. It's got a bit of specularity to it, including grain in the specularity, so you have to be careful with lighting. At the same time, if it throws no specular highlights, neither will whatever you photograph. Second...is styrofoam. I'm not aware of any commercially-available white balance target other than Spectralon that's as good as a styrofoam coffee cup. As a bonus, the conical shape lets you get directional samples in mixed lighting. Or a styrofoam sphere from a crafts store could do the same. Again again, best is to predict the RGB values for the scene's combination of the camera's spectral response plus the illuminant's spectra and the spectral reflectivity of a number of patches. Click-to-balance is a shortcut for an unique class of circumstances that doesn't actually exist, though there're a number of easy-to-attain not-entirely unreasonable set of approximations that are "good enough" for non-critical work. Cheers, b&