On Sep 18, 2015, at 4:52 PM, Robin Myers <robin@rmimaging.com> wrote:
To use it for a white reference requires making a stack of many layers.
It's been some time since I last did this, but I know I didn't do anything exotic and still got as good a measurement with an i1 Pro as I do off of Spectralon. Don't quote me; experiment...but I think all I did was fold several layers, wrapping into a square roughly the width of the tape...and I just used Elmer's to glue the bottom layers together and to affix to the chart. I'm sure I must have used tweezers or something similar to do all the folding. I might or might not have wrapped the tape around something like a square piece of the same Canson Platine Fibre Rag I used for the printed patches on the same chart. Anyway, it's not that hard to work with, even though the end result is, granted, on the delicate side. I think, today...I'd probably get some super-wide (1" or wider) tape, thickest I could find, and wrap it around a white-painted block, layer by layer, gluing each layer independently to the back side. When done, I'd probably mount the block in an hole cut to size, or something like that, and use it all by itself without trying to integrate it into a chart. Call it a $1.00 DIY Watch Your White. Fragile but cheap enough to be disposable.
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.
I have a 44" x 50' roll of Canon's official Tyvek Banner Media. The front, printable side is the exact same coating as Canon uses for their Heavyweight Coated paper. The back is the same uncoated textured surface as the envelopes. The back will _not_ take ink; it wicks and smears and makes a miserable mess. But, it's also the same 98%+ flat reflective spectrum as the envelopes. It's a good banner material, and the back side is good for situations where you want a large sheet of spectrally flat white.
Styrofoam is a very good white, but it also has the issues of Tyvek in relation to translucency and texture.
You remind me...two more properties about styrofoam cups. Their reflectivity is in the 80% range, if I remember right, but still flat, flatter than any of the commercial products other than Spectralon. But the translucency of the cups offers another very interesting possibility: you can put the cup over the lens and get an integration of all the light in the scene. Plus, in my experience, the resulting transmission gives a meter reading that's likely right about where you want your exposure to be. Obviously, this can vary from cup to cup, so experimentation and caution would be called for. But, again...that coffee cup outperforms the expensive commercial alternatives....
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.
This is true...to an extent. Well, it can get you in the ballpark if you've nothing better. But, in practical terms...most white fabrics are a bad idea both because of the fluorescent whiteners in laundry detergent and the translucency letting skin tones through. Papers, especially office papers on the desk or tacked to the wall, are bad because of the optical brighteners and low opacity. White walls are almost always intentionally off-white. White ceilings, even if actually white, tend to reflect the tint of the surroundings. Ideal is to get a spectrophotometric measurement of the actual illuminant and work from there. But I'm having some luck in some experiments making some educated guesses. In theory, if you know the nature of the illuminant but don't know the exact color temperature, you can nail it down from a picture of a ColorChecker. That's certainly the case for single-source illuminants, such as anything incandescent or daylight; there'll be an idealization of the spectrum that minimizes the errors between predicted and actual chart values, and that'll be very close to the same thing you get from a spectrometer. I haven't started experimenting with fluorescents, but I'm hoping I'll be able to get to the point where, if I know the scene has fluorescent lighting and the lighting is one of the standards, I'll be able to identify <i>which</i> standard -- again, just by trying them all and seeing which fits best, and if that fit is "close enough." Mixed lighting should also be possible, but much more difficult since you've got multiple variables. But...at some point you have to do a sanity check. There's no point trying to do reprographic work in real-world scenes with uncontrolled lighting. You need to abandon that approach at some point and instead either go for reportage, for reproducing the scene as a spectroradiometer might (which is trivial; create your test values with simulated D50 and use the white balance predicted by your simulation for D50 regardless of the actual scene lighting) or instead go "full Adobe" and try for "pleasing" subjective color instead. Cheers, b&