Re: White references
Re: White references
- Subject: Re: White references
- From: Robin Myers <email@hidden>
- Date: Fri, 18 Sep 2015 16:52:17 -0700
>> 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.
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> 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.
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> 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.
The calibration plaque for the i1Pro, and its predecessors (e.g. Spectrolino, SPM, ColorEye) use a white material which has been mixed with a binder, I suspect an epoxy, it is not a ceramic. If you closely examine the underside of an i1Pro calibration plaque (not the i1Pro2 because the plaque bottom is hidden) you will see that it has been formed while liquid into the plastic holder, allowed to harden, then the top side has been smoothed and polished.
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> 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.
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> <blockquote>PTFE is the only white material I recommend for neutral balancing cameras.</blockquote>
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> 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.
Plumber’s tape is skived from a solid PTFE rod into a very thin layer. While it has a very uniform reflectance, as you note, it is very thin and thus translucent. To use it for a white reference requires making a stack of many layers. This makes it difficult to put into a re-usable form. Taping the layers does not work well since PTFE is Teflon®, notorious for being non-stick. I found one adhesive which worked with the PTFE tape, but the adhesive had a slight violet color, which showed through the PTFE tape and affected the color. Mechanically binding the tape layers into a holder is one way I was able to use the tape but it was inconvenient.
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> But...there're two cheap alternatives that every photographer should be aware of.
Thanks for jogging my memory about alternatives.
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> 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.
Tyvek® is an alternative with some caveats. It is available in two forms, Type 10 and Type 14, that might be useful to a photographer. Type 10 is a hard, paper like, stiff material, used, as you mentioned, for mailing envelopes, DVD sleeves, building moisture wraps, etc. It is created by heating and pressing fibers of high-density polyethylene (HDPE). It has a very high reflectance. However, because it has a highly textured surface the material varies in brightness across the sheet. It is also translucent, so it requires stacking many sheets to prevent showing the backing. Further, the texture makes any point sampled white balance vary, depending on the density of the material. If Tyvek is used for white balancing, I recommend averaging the reflectance across a large area. Depending on your camera and workflow this may, or may not, be possible.
There is another version of Type 10 Tyvek. It has been produced in a form suitable for printing in inkjet printers. It is used as a water-proof paper for maps, drawings, labels and other purposes. It has a very smooth matte finish and is more opaque than the envelope Tyvek. Unfortunately, it is loaded with fluorescent whiteners, so it is very bluish and cannot be used for a white reference.
Type 14 Tyvek is a cloth-like material which is soft and flexible. I have one sample of this material but it was crumpled before I received it so my testing has been hampered by all the wrinkles. It also appeared to have variable density across the material, so I need further samples before I can recommend it for neutral balancing.
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> 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.
Styrofoam is a very good white, but it also has the issues of Tyvek in relation to translucency and texture. It is soft so easily damaged, but I have used it in various imaging situations where the foam would not be subjected to handling. Styrofoam cups can also pass their contents into the cell structure, thus coloring the foam, so be sure to get an unused cup. The benefit of styrofoam is very low cost and easy availability.
While we are on the subject of cheap neutral balance materials for photography, often an object within the image can be used to balance the image. In some situations I have used clouds as gray references, although this is only with clouds that do not show any coloration, so usually this is for mid-day shoots.
Robin Myers
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