Jump to content
UltravioletPhotography

Cheap UV white balance substrate


Recommended Posts

Sharing this in case it is of interest. I found myself wanting a substrate I could use for a white balance with the UV camera, while I have access to a Spectralon disk I wanted something I could keep with the camera and use as and when needed. I did not however want to buy another Spectralon sample. So I got to wondering whether a simple PTFE plate would do the job. A few minutes on eBay and I found someone who could sell a 100mm x 100mm PTFE plate described as being "PTFE Teflon Plate, high temperature, low friction engineering plastics".

 

I bought 2 samples of this material. One I just cleaned with soap and water as there was a small amount of surface contamination. The cleaning worked ok but did not remove everything (I'd say about 90% on visual inspection). On the second one, I gently rubbed the surface with 12 micron lapping film to remove surface debris, which did remove all visible contamination but left small scratches all over the surface.

 

Once this was done I ran both of them on a Perkin Elmer Lambda 650S UV-Vis spectrometer (150mm integrating sphere) to measure surface reflection between 250nm and 800nm (1s collection time per nm). The graphs are shown below, both the full range and the 300nm to 400nm portion.

post-148-0-89343100-1496663806.jpg

 

Both the samples showed similar behaviour at the longer wavelengths, but started to deviate slightly below 400nm. However interestingly the the 'cleaned' sample looks to be flat reflectance between 300nm and 400nm, at about 90%. Now this isn't as high as some of the Spectralon samples, but being optically neutral, I'm presuming this should be good as a white balance material for use in that range.

 

Overall cost £3.65 per piece, with larger pieces available if needed. I understand PTFE is used for this purpose, but I'm not sure if the reflectance of a sample like this has been directly measured before. I'd be interested to hear peoples comments or thoughts.

Link to comment

Nice evaluation Jonathan, you clearly show the expected reflectance profile of PTFE. Many of us have long used PTFE to WB especially in the field where one would might not wish to contaminate $pectralon. One caveat is that unlike Spectralon a PTFE sheet or plate is not a diffuse reflector and may easily flare with specular reflection depending on angular orientation.

 

PTFE can indeed be sourced quite inexpensively online. Conventional practice cautions one to be careful to get "virgin" PTFE to avoid some of the engineered forms that may contain other materials. One can also use PTFE in transmittance mode like a WB lens cap, here is a discussion of that approach.

Link to comment
Thanks John. Yes, the issue of specular reflection is a very valid one, so diffuse light only with this. It's one of those things - I'd love to be able to say "all PTFE would be fine". Problem is, without a spectrometer to verify it, how do you know it is, and that you haven't got a grade which has other materials present? If it's of interest I can put up the link to where I got mine from.
Link to comment

..... If it's of interest I can put up the link to where I got mine from.

 

I am well supplied but there would likely be interest.

Link to comment

If you need a UV white target simply for adjusting white balance of your photos, then virgin white PTFE works just fine. You simply must be careful not to over-expose it. Over-exposure of any white target can lead to inaccurate* white balance. It is easier to over-expose PTFE than it is to over-expose Spectralon. Make sure your illumination is evenly spread over the Spectralon and take care with your exposure times. Bracketing some exposure times is useful. Then you can choose the exposure whose histogram is closest to but has not hit "the wall".

 

However, if you need to judge the UV reflectivity of your photographic subject, then only a diffuse Lambertian calibrated white target (such as Spectralon) should be used.

 

*White balance in a photograph is affected by so very many different variables, that it would take at least a book to explain them all. Using a uniformly white target is just the beginning. And keep in mind that for UV photographs, it is all false colour anyway. There is not an accepted definition of "UV white balance". And there is no imperative to white balance a UV photograph. WB in UV is a choice, not a rule.

 

In the reflected UV photography field, one sometimes has to spend money. Scientific accuracy does not come cheap.

 

[sometimes I'm telling you something you already know. But maybe other readers don't. So I err on the side of wordiness. :D ]

 

I've read somewhere about sanding PTFE, but the link escapes me at the moment.

Link to comment

With a sheet, you might get light contamination from behind the target. A "slab" of PTFE is better.

 

Added: It is recommended that you wear gloves to handle your Spectralon. Blow the dust off it before storage. Spectralon can be sanded underwater with an extremely fine grit. I had a lovely slab of Spectralon from Edmund Optics which got badly damaged. I cleaned and sanded it for use as a white balance target. But the calibration is no longer valid.

Link to comment

https://www.onlineme...214&top_cat=181

My PTFE disc is about 3.5" in diameter and about 1/4" thick. Was sliced off a rod like shown in the link. I don't know what you'd use to cut PTFE. Table saw??

 

http://www.professio...TRODVIRGINPTFE

Here is a foot long rod of virgin Teflon, 3.5" in diameter for (US)$166. You could cut 14 quarter-inch rounds from that for $11.85 per plate. (But I didn't include any shipping!)

Link to comment
Andy Perrin
I bought my PTFE from the business of one of our own forum users, UVIR Optics. (Actually what I'd love to know is what is the best way of cleaning the PTFE? Mine has gotten a bit dusty.)
Link to comment
Andy Perrin

Anyone ever tried the PTFE powder?

 

That is an interesting idea because I bet it would have a lot less specular reflection in a powder form.

Link to comment

I always have to go look that up: https://en.wikipedia.../wiki/Sintering

The white-coated trees near the hot geysers and pools in Yellowstone are referred to as "sintered", and I have that stuck in my head.

 

Yellowstone National Park, Wyoming, USA

"Sintered Trees" 01 Sept 2015

whiteSinterTreesNearGrandGeysUpGeysBasin_20150109yellowstoneNpWY_11886pn.jpg

 

whiteSinterTreesNearGrandGeysUpGeysBasin_20150109yellowstoneNpWY_11947pf.jpg

 

whiteSinterTreesNearGrandGeysUpGeysBasin_20150109yellowstoneNpWY_11958pn.jpg

Link to comment

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
×
×
  • Create New...