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UltravioletPhotography

Reminder: Flip that BaaderU


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Holy thread revival Batman.

EDITOR'S NOTE: I moved Jonathan excellent BaaderU experiment to its own topic so that it will be more searchable and also to remind everyone that it is useful to reverse your BaaderU so that the pink side faces the subject.


 

 

I know we've all been told the Baader U should be reversed for photography, so the pink side faces away from the camera, and I got to thinking how the two sides compared in what they reflect.

 

Slightly odd experiment, but the light source fiber and spectrometer fiber were at 45 degrees to each other. This is a specular reflection setup. However I don't have a specular reflection calibration standard. In place of that I used a 99% diffused reflectance one. This means that my reflectance values in this graph, are in relation to this diffuse reflectance standard. Obviously the Baader U is shiny, and has high specular reflectance, so the values go above 100%. It's fine for comparing one side vs the other, but that's it.

 

I measured both sides - pink and green/gold, on my version of the filter which is about 3 years old. I took the filter out of its holder, so it's the same distance from the fibers to the each side of the filter.

 

post-148-0-73628700-1589792679.jpg

 

The pink side is better at reflecting the IR so it definitely makes sense to have that side facing away from the lens.

 

Not sure whether this has been shown before, so thought I would share it.

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One side of mine looks green/gold, the other side looks pink Alaun. I've done one reflectance scan for each side.
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Useful info. about the pink surface of the Baader U facing the subject - I wasn't aware of that. But that's how my filter came, so I'm a bit confused about the statement that the filter has to be reversed to get to that state.
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One side of mine looks green/gold, the other side looks pink Alaun. I've done one reflectance scan for each side.

 

OK, the ones I have, the small 1.25" ones look green (and more green than green/gold) and the large 2" ones look gold (really gold, no green). The golden ones are far better with regard to IR subpression.

 

The pink sides are the same.

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Great test Jonathan.

Now I have to look at my Baader to see how I flipped it. I did that when I bought it in 2009.

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Useful info. about the pink surface of the Baader U facing the subject - I wasn't aware of that. But that's how my filter came, so I'm a bit confused about the statement that the filter has to be reversed to get to that state.

 

Depends on how the filter is attached,

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Bill De Jager
I recently had one remounted into standard 52mm threads. I asked for the pink side to face forward based on past advice here on UVP. It's good to see the value of that advice confirmed. I've only used the filter a couple of times since and with no flare problems. Thank you, Jonathan.
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EDITOR'S NOTE: I'm going to break this off into its own topic because it is useful to remind everyone (again !!!) about the BaaderU reversal so that the "pink" side is out.
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Jonathan,

You could also post your Baader in my "I will show you mine if you show me yours" thread I started somewhere here.

Seems to be difference between the years for the 2 inch ones. The color of mine doesn't look like the most recent one that Bernard has.

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Yep, the colors changed over the years. Remember I wrote that I had 4 different types of BaaderU. Seems to be no difference between the photos they make.
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Jonathan,

You could also post your Baader in my "I will show you mine if you show me yours" thread I started somewhere here.

Seems to be difference between the years for the 2 inch ones. The color of mine doesn't look like the most recent one that Bernard has.

 

David, happy to do a picture of mine, but it's not in the original ring (I mounted mine in a generic 49mm filter ring). So, not sure if it would still be useful to take a picture of it? Let me know if it's still useful.

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Thanks for the comments, and happy to share results like this as and when I can do them.

 

I've had it pointed out to me that ideally this test should be done with the incoming and reflected light at 90 degrees to the surface of the filter. This experiment was done with each being at 45 degrees to the surface.

 

Being dichroic coatings there is a large angular dependency on their spectral properties, so please keep this in mind when looking at the results.

 

Unfortunately my rig does not have attachment points for reflection other than the 45 degree ones, so I cannot easily do the other experiment. Also, ideally for that I would need a reflection probe, where the incoming and outgoing fibers are arranged together. This is not something I have, but if I ever get one in the future I can do more on this.

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I got one of those reflective probes. Came as a surprise with my DT100 lamp. It can see from UVC to IR.

 

When I get my spectrometer back, I will have to test my BaaderU filter. I am curious if I see the same thing.

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Interesting. I had always been told that for anything to have different transmissivity in different directions potentially violated the second law of thermodynamics--placed in the middle of a closed box with reflecting sides, it could trap all photons on one side, a sort of Maxwell's Demon. Clearly the picture is more subtle than I thought.
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Andy Perrin
Clark - yeah when you slide the filter into the box you are doing work (you are essentially compressing the photon gas into a smaller volume) so there is no second law violation, any more than there would be with the more common case of separating two gases with different size molecules with a Maxwell Demon. If you do work on the system you can decrease the entropy of the system (at the expense of increasing the entropy of the surroundings).
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  • 1 year later...

Back to another thread resurrection.

 

About 2 years ago I shared some reflectance data from both sides of my Baader U. The experiment did show differences between the two sides but was far from ideal. I had to have the incoming light and outgoing fiber at 45 degrees to normal (and 90 degrees to each other) and I didn't have a specular reflection standard. And I could only go up to 850nm. I now have some new data.

 

Yesterday I received my reflection probe from Ocean Insight (formerly Ocean Optics). This has the incoming and outgoing fibers mounted next to each other and can be positioned to get it 90 degrees to the surface being measured. I also now have a spectrometer to take me out to 1100nm.

 

I measured both sides (red/pink and green/gold) of the Baader U in three regions - UV, Vis and IR - with two Ocean Insight spectrometers (FX for the UV and Vis, and STS-NIR for the IR). Here's what they look like plotted together;

792098092_Fullrangebothsides.jpg.8bd75ae5c94057e38dbe0ec27ddb324c.jpg

 

It's a bit messy looking at everything on one graph. Here's the red/pink side;

2017781953_Fullrangeredpinkside.jpg.f7784decc03790d0980b0a205e0d97f4.jpg

 

And the green/gold side;

1434776195_Fullrangegreengoldside.jpg.2acffbbc2d4f4f0334b14a762a9ff24e.jpg

 

And finally, a comparison of the two sides with more of an emphasis on the UV region;

1888597800_UVbothsides.jpg.5dc55541839edaa87586342283205d81.jpg

 

The obvious differences between the two sides are still there, but the IR behavior is certainly striking. The red/pink side has good reflectance between 650 and 950nm, but then drops off. The green/gold side has good reflectance from 950nm upwards. The low reflectance in the 335nm to 410nm region is also more obvious now.

 

The probe seems to be very good, but I did not go with their specular reflectance standard. I couldn't justify the cost of it, so I went with two mirrors from Thorlabs - one UV enhanced Al, and the other a protected silver one. Those do not have 100% reflectance at all wavelengths, so all measurements are relative to the mirrors at a given wavelength. To be fair though, the Ocean Insight reflectance standard also varies in % reflectance as a function of wavelength, so I see this as a reasonable compromise.

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Red side facing the subject (once it has been flipped compared to the starting orientation).

 

Nate, how strange, I hadn't realised there was that big a difference in colour compared to mine. Mines about 4 years old now. I think stick with the original plan of 'flip the filter' in the absence of any further tests. I do have another Baader U on order (been on order for 6 weeks now), and it'll be interesting to look at that once it arrives, and just to see what colour the coatings are.

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Thank's JMC, hope the one you get is like mine to test. I was thinking, I might just screw on my 590nm filter and take a 20sec shot for both sides and see which one is the brighter, and not use that side.

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Nate, in theory the image should look the same irrespective of the side which faces the subject - after all, the light has to pass through both surfaces. However my thoughts on this is that there might be small differences say in contrast, so it is worth testing both ways around. Based on the base material for the Baader U, there is likely to be some transmission in the IR (without any coatings present). Therefore to me it makes most sense to have the side reflecting most of the IR between 700 and 1000nm facing the subject.

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