Jump to content
UltravioletPhotography

Measurements of WB reflectance targets.


Recommended Posts

I have lately stared to try to measure reflectance with my spectrometer setup.

As this kind of measurements are new for me it is likely that my methods and results are not optimal.

I hope to lean more and look forward to comments to improve the method.

 

Equipment:

Ocean Optics Flame-S XR1 array spectrometer, range 200nm-1000nm

Ocean Optics DH-2000-BAL, Balanced Deuterium Tungsten Source, 210-2500nm

Ocean Optics Premium Grade Reflection Probe

Custom distance/angle-piece, for the probe tip, for a 6.0mm probing distance during normal probing angle, (90°).

Reference: Virgin PTFE sheet matted with 220 grit sand paper under flowing water.

 

Test Objects:

Top row: Reference PTFE, Two DIY references based on this thread: https://www.ultravio...__fromsearch__1

Bottom row: PTFE-based 1µm membrane filters, processed and fresh.

(Sempe-BLACK 2.0 on the rear side of a filter, dito seen from the front-side, fresh filters in their container)

post-150-0-23063200-1559207690.jpg

Note that the membrane filter is very thin and translucent.

 

Resulting spectrograms #1:

post-150-0-93838400-1559208325.png

Top line (black): the probe lifted and put down again directly after the calibration. Shows sensitivity to probe angle and target consistency.

Second line (blue) DIY target (UW) medium.

Third line (green) DIY target (UW) dark.

Fourth line (grey) Seple-BLACK 2.0. This measured reflectance is higher than Jonthan found here: https://www.ultravio...dpost__p__16925

I think the reason is that I applied the paint differently.

 

Resulting spectrograms #2:

post-150-0-90169800-1559209734.png

(Blue line) PTFE-Membrane filter measured from the front PTFE-side.

(Red line) PTFE-Membrane filter measured from the rear substrate-side.

 

The reflectance values >100% must be due to fluorescence.

The optical power of the light source is significant for the shorter wavelengths

The PTFE membrane is very thin and mounted on a different substrate material.

It is difficult to explain the results here fully.

This type of target might still be OK to use in daylight as there is no UV-C and very little UB-B to excite the fluorescence in the UV-A by the substrate material.

It is also possible that substrates for different makes of membrane filters behaves differently.

One thing is clear, the PTFE membrane is thin enough to be affected by the substrate and materials behind.

 

Resulting spectrograms #3:

post-150-0-16003900-1559209720.png

Purple line: A PTFE-membrane filter doped from the rear with Semple BLACK 2.0

I wetted the membrane filter well with tap water, placed with the PTFE-side downwards.

Then I applied slightly diluted Semple BLACK on the substrate side.

When dried the PTFE side was slightly marble patterned grey in colour.

It has a rather constant reflectivity with no signs of fluorescence.

Link to comment
Andy Perrin
Can you go into the infrared? It looks like both your light source and the spectrometer are ok to 1000nm?
Link to comment

Hi Ulf, you're DIY ones look nice and flat.

 

Not surprised you're getting different results to mine for the Semple Black. When I did it it was using a commercial lab grade Perkin Elmer UV/Vis, and it was setup for diffuse reflectance. I'm guessing from you're setup you're doing specular relfectance, although please correct me if not. That will have an impact on the overall reflectance measured. I painted mine onto a paper which had a rough surface too, to help scatter any reflected light.

 

Using a roughened PTFE plate as a standard, wouldn't the roughness scatter the light and reduce the reflectance? I know they use different standards for diffuse and specular reflectance.

Link to comment

Can you go into the infrared? It looks like both your light source and the spectrometer are ok to 1000nm?

Yes I can go slightly above 1000nm, but the combination of light source and grating efficiency in the spectrometer is lower in that end, giving a much worse SNR.

That could be improved by averaging several measurements. In thee current graphs I opted to truncate at above 700nm even if I reached 1027nm.

I had to set the intensity of the tungsten source in the light source to maximum fot this measurement.

Normally I avoid that to not shorten the lamps life.

 

What are you looking for?

Link to comment

Hi Ulf, you're DIY ones look nice and flat.

 

I'm guessing from you're setup you're doing specular reflectance, although please correct me if not. That will have an impact on the overall reflectance measured. I painted mine onto a paper which had a rough surface too, to help scatter any reflected light.

 

Using a roughened PTFE plate as a standard, wouldn't the roughness scatter the light and reduce the reflectance? I know they use different standards for diffuse and specular reflectance.

 

As all targets are not that specular, but in some way matted they scatter reasonably well, but not identically.

I assume that the measurement is semi-diffuse, but the probe does not collect all reflected light as does an integrating sphere-setup.

As there is a difference in how well each target scatters the light, there will be differences in the absolute detected light.

 

I will redo the measurements with an angled probe at 60° and see if the results are the same. The spacer tip is designed to make that possible in a reasonably controlled way.

The spacer(s)- tips for the probe were the the very first things I printed when I got my 3D printer last summer. Their shape might be improved.

 

I do not expect this to change with wavelength and think that the membrane filters might be less optimal as WB-references.

It is difficult to know how much they are affected in normal sunlight. When I used them, the WB result looked reasonably OK and better than other parts found in the scene.

It looks like a nice cure to paint the rear side with Semple Black as it seems to eliminate the fluorescence effect.

 

With unlimited funds I would like to have one of the darker Spectralon targets. Alas, it is too expensive to be considered for my needs.

I like to avoid overexposure of the target and still have a reasonably correct exposure of the normal parts of the motive.

Now I know that my DIY targets are reasonably OK and the rear-painted membranes might be usable too.

Link to comment
The reflectance values >100% must be due to fluorescence.

Probably most likely due to light source stability and/or dark current drift. Make sure your light source has been on for about 30 minutes. When making critical measurements I found it necessary to check/re-measure dark current every few samples to avoid <0% or >100% readings.

Link to comment
Top line (black): the probe lifted and put down again directly after the calibration. Shows sensitivity to probe angle and target consistency.

I was never happy with the angled reflectance probe setup in a portable measurement situation and only managed to get good consistency in a permanent mount configuration. I typically used an integrating sphere whenever possible as it gave much better measurement consistency.

Link to comment

What thickness is your membrane? Must be thin (as you say) given the partial visibility through it.

I don't think all such membrane filters have a substrate. Some may be entirely made out of sintered PTFE fibers.

Link to comment

Probably most likely due to light source stability and/or dark current drift. Make sure your light source has been on for about 30 minutes. When making critical measurements I found it necessary to check/re-measure dark current every few samples to avoid <0% or >100% readings.

The light source was stabilised and both dark current and 100%-calibration was done really close in time. This is not a drift phenomenon.

It might be a difference in specularity though. I will try with another probe ≠ normal to the surface, as this is my only option, lacking an integrating sphere.

Link to comment

What thickness is your membrane? Must be thin (as you say) given the partial visibility through it.

I don't think all such membrane filters have a substrate. Some may be entirely made out of sintered PTFE fibers.

The filter membranes I have are 0.16mm thick measured with a good digital Mitutoyo Absolute caliper.

I didn't think there was a substrate either until I saw the measurement results. Then I noticed that one side was a bit fibrous and the other was more even and matted.

Do you have experience with such filters of different kinds?

Link to comment

The PTFE membrane I have is slightly over 1mm thick (1.08mm).

As you say, one side is more matted, what I call the top side, and the other side, bottom said, has more of a pattern to it, however, looking extremely close with a magnifying glass, I can see no material.

It may be that side was sprayed onto another material that had a pattern, which left a pattern on that side when the membrane was remove.

I will see if I can find out.

I also have a small sample of the thinner membrane you have, it is extremely thin, and you can kind of see through it slightly like in the photo up top.

Link to comment
The light source was stabilised and both dark current and 100%-calibration was done really close in time. This is not a drift phenomenon

I used Ocean Optics products for over 10 years, including their stabilized D2H sources, one of the reasons I switched to an Avantes stabilized D2H source. Unless you know you have an inconsistency in your system setup, I wouldn't be so quick to dismiss the light source, or dark current drift (had to return a couple of spectrometers for drift issues).

Link to comment
Ulf, I don't think there is any 'support' in the type I have. It is probably sprayed on some surface, sintered, and removed.
Link to comment

I used Ocean Optics products for over 10 years, including their stabilized D2H sources, one of the reasons I switched to an Avantes stabilized D2H source. Unless you know you have an inconsistency in your system setup, I wouldn't be so quick to dismiss the light source, or dark current drift (had to return a couple of spectrometers for drift issues).

I did suspect those types of errors when I saw the strange results >100% with the clean membrane filters and went back and verified and re-calibrated several times.

I also measured the other targets again in the same measurement sessions.

Only the membrane filters give strange results, and only when they are new and clean.

 

I might be wrong about the reason being fluorescence. It could be other things like difference in specularity etc.

It is not likely to be caused by drift issues as it only affect the measurements of the fresh membrane filters.

 

I have not seen any big problems with drift in the spectrometer or light source before, when I analysed the OD of filters like Baader U, SEU2, U-360+S8612-stacks etc.

Those kinds of measurements are more sensitive to drift and I humbly think I understand that part of measurement difficulty reasonably well.

Link to comment

Nothing in particular, it was just generally curiosity about what the Semple does in IR.

The next time I have the setup rigged I will do a measurement for Semple Black, focussing on that end of the spectrum too.

I hope I get enough light in the IR end to get meaningful results when the deuterium lamp is off.

The measurements above stopped making sense for >750nm

Link to comment

Even membranes are made up of an interesting pore structure, with many various mesh hole sizes. Since only fresh filters give the result, you may have refraction or back scattering in the filter. Once dust and oilly fingers touch it, this may be lost.

So could be a bunch of interesting different things going on here. If you have some plumbing tape, you could compare. That is non-porous thin ptfe, the stuff used to lubricate your connections. Some people think it's used to help seal, but that's not its purpose, it just acts as a lubricant to tighten up the pipes.

Link to comment

Spectralon targets are also quite porous, and get dirty very easily and need to be cleaned and wet sanded. They soak up water when wet sanded.

 

There is a difference between the 'membrane' filters and the thicker PTFE depth filters, I think the ticker ones are made of only PTFE.

To begin with, it doesn't make much sense to make a white balance target out of something you can partially see through, what is behind the target will slightly mix with the target...

like the membrane target shown and being tested.

Link to comment

Spectralon targets are also quite porous, and get dirty very easily and need to be cleaned and wet sanded. They soak up water when wet sanded.

 

There is a difference between the 'membrane' filters and the thicker PTFE depth filters, I think the ticker ones are made of only PTFE.

To begin with, it doesn't make much sense to make a white balance target out of something you can partially see through, what is behind the target will slightly mix with the target...

like the membrane target shown and being tested.

 

I fully agree with that!

 

The thin membrane types might be usable with suitable, black paint on the rear side, to create a cheap target with lower reflectivity, at least if ≈ 50% is low enough.

 

The porosity of Spectralon is by design to make the reflection pattern direction-wise suitable for measurements.

That, I think is not needed for WB, even if a matted surface is nice to decrease specular reflections.

Link to comment

Dmitry, Yes, I just didn't copy and paste the Russian letters like I usually do.

NC looks close enough for me, but thanks for pointing that out for others.

 

Can't find an ALT ANSI/ASCII code for those.

Link to comment

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
×
×
  • Create New...