Tuesday, May 12, 2020

About White Balancing and Spectralon (R) Standards

Quite a while ago I had written about white balancing reflected UV images. This meant to use a standardized white or grey standard to balance images, here "white" means that its reflection in visible light, but also in UV and ideally NIR is about the same. A very well know material for that is SPECTRALON (R) made by company Labsphere. It is "the" industry standard in laboratories and is used for spectrometry very often. It is made of specially prepared and cleansed Tetrafluorethylen  (PFTE) or TEFLON (R) powder. It is a rather pricey material. I had previously presented possibilities of cheaper acceptable options for photographic use HERE for instance using virgin white Teflon.

So today it is about what can happen to Spectralon standards, and which influence this may have.

I have used a Spectralon multi-step target, which consists of four zebra-striped panels of 99, 50, 25 and 12% reflectance which are mounted side-by-side.

[click on image to see a larger one]

Measured reflection of Labsphere SPECTRALON multi-step target (the white one was used as a 100% reference):
It gets obvious, that this multi-step standard still works very well and that the 99, 50, 25 and 12% reflectance values are still working.

Now in comparison a white Spectralon 99% target I bought, which may have been used for quite a while. As it did not look that great, I had sanded it down and cleaned it (light blue line approaching the 100% line):
(double lines are repeated measurements using a different distance)

But it gets rather obvious now, that this used standard must have aged quite a bit or some substance had penetrated its once white surface, as it still looks a bit yellowish and the reflectance measuerement clearly shows that, especially in UV.

I may have to repeat the sanding down process, but this time I will have to take away much more from its surface, using clean sanding paper, ending with 240 and 480 grit, to keep it matted.

Never touch that surface with bare fingers, only rinse with clean alcohol and or purified water and let it dry, don't use pressured air as it often contains oil droplets.

Labsphere has some information on their products as well as about proper handling HERE.

I have written about white balancing before HERE

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos