PTFE as Background for Photo

[Andrea B.]

Dave & John -

Thanks for raising & settling the issue of UV damage to Spectralon. ;)

 

In retrospect (...always the best 'spect' because things become clearer later of course...) I probably should not have purchased the 5 calibrated Spectralon reflectance rounds. As it turns out, uncalibrated would have served just as well for my initial investigation of white balance efforts in Ultraviolet photography. And would have served just as well for typical profiling. And, it certainly seems now to me that less expensive PTFE materials could have been used - as so many are doing.

 

Another thing to note is that while Spectralon is hydrophobic, it does easily get nicked, dinged and gouged. And is a superb dust magnet. So, using the Spectralon rounds in the field has rather scuffed them up over the last 3 years. From sad experience on another larger piece of Spectralon, I can tell you that it is very easy to discolour if anything other than water comes into contact with it. I do have known from the start that Spectralon can be wet-sanded. But you cannot really sand away a deeper nick. So if true diffusion is required, those nicks probably interfere with it.

 

Well, all that 'tis neither here nor there, as they say. Just some observations in retrospect that may save some folks a lot of money, I hope.

 

I now keep the Spectralon "pots" in a foam-lined pelican-like case along with the CC Passport. (That Passport is another thing which has had its share of scratches, mars and nicks in field use.) This helps prevent further damage.

 

OTOH, I must say that having the 5 reflectance standards spurred my "investigations". I also did a lot of experiments trying to match their reflectivity in UV photographs. That goes against the typical human (and thus camera) brightness curve (sometimes called gamma curve). But also gives one a feel for the true reflectivity of some subjects in UV light.

 

I still have one idea in mind for using the 5 standards arrayed around a flower. I hope to be able to try it out soon.

[JCDowdy]

Yes,

I have always advised folks here and on the other site not to buy the calibrated reflectance standards. Unless you are a lab selling certified reflectance measurements you really don't need the certificate. New uncalibrated ones are expensive enough, but deals on used stuff are out there if one is patient and attentive. ;)

 

We need to shoot these things and look at them with that RawDigger software you have been showing us, but that will be a different topic.

 

I was thinking about the implications of using a PTFE background and how some of the rather UV dark specimens would be rendered almost black to keep the background from being blown out. I am not sure if that is such a bad idea. I mean shouldn't very dark specimens look dark? If you included your 99% and 5% spectralon in the photo, essentially would you have the same effect?

[Andrea B.]

I have rather a lot of shots of the standards with various filters. I'd be happy to post some Raw Digger versions if anyone is really interested??

 

I was thinking about the implications of using a PTFE background and how some of the rather UV dark specimens would be rendered almost black to keep the background from being blown out. I am not sure if that is such a bad idea. I mean shouldn't very dark specimens look dark? If you included your 99% and 5% spectralon in the photo, essentially would you have the same effect?

 

You asked an excellent question and Bjørn & I once thought about it and decided what we would do. I've written about this before, but I have no idea where it is/was. (Skip the parts you already know. I'm never sure when I write, who knows what?)

 

The bottom line is this: if we presented our specimens with their actual brightnesses, it would make for an extremely bad, dark, uninteresting photograph lacking important details in the darker areas.

 

Why? Well, our human eyes do not see/interpret brightness in a linear manner. Our brain applies a "gamma curve" to what our eyes sense which raises midtones considerably. We set our cameras to also have a (raised midtone) gamma curve so that the photographs we make are as close as we can come (given proper exposure) to the way our eyes see.

 

In discussing this, remember that to move between actual light shining on an object and a photographic capture of same, we have to make the assumption that a standard's percent reflectivity of light equates to the human perception of percent brightness, as in the HSB model. It seems fair enough to do so.

 

Anyway.....if we photographed 5 standards of 99/75/50/25/2%, then the resulting photograph would not exhibit brightnesses of 99/75/50/25/2%. We can take the photograph into an editor and try to hammer on it and make the brightnesses match the reflectivities, but it does not work out well. And if we remove the gamma curve applied by the camera to the photograph, then the photo becomes very dark. [There is an additional complication because the camera cannot typically record the wide range of brightness that the human eye can perceive. One reason why I'm always looking for a cam with largest dynamic range possible!]

 

So you can choose to present a good viewable UV photograph or you can choose to (attempt to) present realistic brightnesses and many times get unviewable photographs. Bjørn and I choose to present good photographs.

 

Only actual examples will help illustrate this. I do not have access to my drives currently as we are traveling. But I do have some photos on the laptop which will serve for now to show the problems. Better examples to arrive later. (This photo isn't so great.)

 

Here is the experiment.

Shoot a 99% and a 2% standard in the same photo.

 

Photo with left black standard set to 2% brightness. The center white 99% reflective standard now measures 62% brightness (in the editor).

 

Photo with center white standard set to 99% brightness. The left black 2% reflective standard measures 18% brightness (in the editor).

 

Photo with left black standard set to 2% brightness and simultaneously the center white standard set to 99% brightness.

1) The camera cannot simultaneously record 2% and 99% reflectivities at 2% and 99% brightness because of the aforementioned lack of dynamic range. However, in the editor we can make these brightness settings and thus stretch the histogram from left wall to right wall. This does not, however, give a true expanded dynamic range. It merely redistributes the recorded data in such a way that gaps can appear in the histogram. This can be quite hard on some photographs, creating blocky patches or posterization artifacts.

2) In this stretched photo, the middle grey 50% reflective standard is at 85% brightness. There's that gamma curve at play.

 

Photo with curve adjustments to produce 2, 22, 50, 73 and 91% brightness for the 2, 25, 50, 75 and 99% reflective standards. This was as close as I could get the all 5 simultaneous photo brightnesses to the 5 reflectivities. It wouldn't go further because I was running out of data.

This photo looks almost OK at first. But I have attached the histogram to show the comb-like look it is taking on as the histogram is stretched further and further, creating dataless gaps. And then I have attached an excerpt from the photograph showing the resulting blocky artifacts beginning to show up.

 

 

[Andrea B.]

For the UV version, I repeated the process and eventually got 8, 22, 50, 74 and 97% brightnesses corresponding to the 2, 25, 50, 75 and 99% reflectivities with a similar comb-like histogram and some blocky artifacts. I also got some colour artifacts - probably because of the hard jerk on the colour wheel when a UV photo is (pseudo-) white balanced.

 

I saved the curve from this attempt and applied it to a flower photo made in the same time period with the same light. This will bring home the message about choosing between reality in reflectivity representation versus a good photograph.

 

Here a curve is applied to approximate linear brightness (i.e., to "undo" the customary gamma curve).

 

Here is a good photograph (well, good enough...) that does not represent the actual brightnesses.

 

The experiment would be better served by showing you a photograph containing both the standards and the flower in one scene. But as mentioned I cannot access those just now. I'll try to get back soon with such an example.

[colinbm]

Thanks Andrea

[Andrea B.]

You are welcome, Col.

 

I retry these experiments from time to time as I learn more or have newer or different converter/editors.

 

Any comments or questions or pointing-out-of-errors-or-misunderstandings is always welcomed. A lot of assumptions are/were made!

 

Somewhere back there I said that calibrated standards are not needed. But when I do an experiment like this one, I'm happy to know that my black standard is really at 3% reflectivity in UV rather than 2%. Not that this 1% could possibly make much of a difference in photo work. I just like knowing what it really is. This is, however, probably a character flaw on my part.

[JCDowdy]

Thanks for the detailed reply! Please excuse my ill timed pestering, I didn't intend to distract form your holiday.

 

I'm never sure when I write, who knows what?

 

Always best to assume ignorance on my part. I find it normally less bothersome than missing information.

 

Your previous study clearly shows what I was assuming could be a problem. I only mentioned RawDigger because I thought the RAW composites were rendered without the gamma correction and was curious how the brightness of an array of standards would correlate.

 

Happy 4th!