[Filter Test] 340/10 #6 Progress was made, finally. UPDATE: Progress was then lost.

[Andrea B.]

Stefano, why do you think there is IR leak with the Edmund 340BP10 ???

[Stefano]

Because there is a difference between the two images, the one with the 1 mm thick U-340 looks more "washed-out". Both filters (1 and 4 mm thick) have similar UV transmissions, "same" visible blocking (the OD is four times higher in the 4 mm filter, but that doesn't make a noticeable difference because the 1 mm filter blocks visible light "completely" anyway), and the only thing that really changes is the IR bump, much flatter at 4 mm. This makes me think the filter may have a leak there. What happens if you add a UV blocker to the 340/10 + U-340 stack?

 

The leak may be a OD 4 bump, but at 340 nm it can be enough to be seen.

[Andrea B.]

Thanks for the input !!

I think I might see what you mean. It has been a bit difficult for me to determine how much is due to IR

because the frames are under-exposed.

 

Also I think that I have been so focused, pun intended,

on the flare issue that I have been totally ignoring any possible IR leakage. La! :grin:

 

Yes, I agree that there is probably some IR leakage in the Ed340BP10.

But we use the BaaderU, also OD 4-ish, quite successfully.

So I'm not sure how much of a role the IR leakage plays in the washout/flare problems.

 

The washout is uniform. The usual damage to UV files from IR leakage is most noticeable in what should be UV-dark areas. That is not a problem with this Ed 340/10. UV-dark remains UV-dark. Even the red channel from the photo in Post #11 above remains UV-dark. But sometimes we see what we want to see, heh-heh.

 

What happens if you add a UV blocker to the 340/10 + U-340 stack?

I have to postpone such an experiment because I ran out of natural light. Some storm clouds have moved in.

Tomorrow is another day, as they say. I'll try again then.

[Andrea B.]

Summary of results so far.

 

Indoors the Edmund 340BP10 works very well with the strong UV-light from a UV-flash and with possible stacking with an S8612 with AR coating. There was no uniform flare, washout or reflection rings. I must retest this and get some better comparisons with and without the S8612-AR.

 

Outdoors the best results so far came from a double 340 stack: Edmund 340BP10 + U-340 in either a 1.0 or 4.0 thickness. There was no uniform flare, washout or reflections rings.

Retests are needed that produce proper exposures because my initial tests were underexposed 'cause I couldn't see through that thick stack. (The 340/10 is 2-3 mm in thickness.)

Also, an IR test is needed to see how badly the 340/10 leaks IR. Is it better or worse than the BaaderU?

[Andy Perrin]

Andrea, the Baader is a wide band filter so it lets in a lot more UV light than a narrow bandpass filter. Even if the blocking is exactly the same for both filters, it will matter more for the latter because the ratio of UV to OOB light is much smaller in that one.

 

I see that effect a lot with my 980nm bandpass filter vs. using an 950nm long pass. The 980nm needs much more exposure time.

[JMC]

Good to hear it worked Andrea. Try holding the EO 340 filter up to a ceiling light and then twisting it. The blocking falls apart rapidly as you go away from 90 degrees to the filter. These filters aren't really designed for photography, and a lot of out of band light can get through when it comes in from the side. Adding the U-340 4mm has the effect of removing that visible and most of the IR light which would otherwise be a problem.

 

It would be interesting to see whether there is a big difference if the EO 340 is placed behind the lens (if that is feasible on your setup).

[Andrea B.]

I did try the Ed 340/10 mounted behind the lens. I still got flare/reflections/etc.

 

The Ed 340/10 is so hard-coated that it appears to be an actual mirror* on both sides. So when I hold it up to the sky or to a ceiling light, I cannot see a thing through it. And there are no obvious changes when tilting/twisting it.

 

Yep, I knew this was an industrial filter when I bought it. I just wanted to try it anyway.

 

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Note to self: be sure to add long-ish lens hood when using the Ed outdoors. This might help if oblique light is causing the reflection problem.

(I think I have tried a lens hood with the Ed in the past, but right now I can't remember for sure.)

 

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*I could carry this thing in my purse to use for lipstick application.

[dabateman]

That would most likely be the most expensive lipstick mirror.

[Andrea B.]

I'm beginning to think this particular mirrored filter is not good for much else.

[Andy Perrin]

Honestly, I don’t think it’s useless, it’s just that we always have to work within the limitations of our equipment, which means finding out where it works and where it doesn’t. You know what’s required to make the filter work properly now, so if you want to take a 340nm photo, you can do it.

[Andrea B.]

Yeah, that's what I'm always telling people -- "work within the limitations of our equipment", etc. It's good advice. But sometimes some gear is just some kinda pain in the butt. This Ed 340/10 filter being a good example. I hate the darned thing.

Oh well.

[JMC]

The type of narrow pass dichroic filters definitely have a place in UV imaging, but they can be difficult to work with, especially when using broadband light sources such as flash or sun. For general UV imaging there are better (and simpler) options.

[Andrea B.]

To be clear, for future reference, my displeasure with the Edmund 340/10 (an "industrial" filter) is that its hard-coating is extremely mirrored.

 

I can deal with the narrowness and the needed illumination and the bit of IR "leakage".

 

Jonathan's point is well-made though. Start with a broadband UV-pass filter when you are just learning how all this works.

[Cadmium]

Andrea, Aren't all narrow band dichroic filters 'mirrored'? Isn't the Baader U mirrored also, just as mirrored as the narrow band filters are?

What is makes a mirrored filter a displeasure?

[ulf]

Yes, that is the principle of their function and they have the absolute optimal rejection for only one incidence angle, for parallel beams.

Outside that angle rejection drops.

 

Baader U is made by U-glass substrate that limits the effects of internal reflections inside the filter, by absorbing light.

Not all dichroic filters are made that way.

I would assume the Edmund filter has a very transparent substrate.

That would limit the power losses in the filter substrate if exposed to high power light within the passband.

[dabateman]

Cadmium,

What I think Ulf is trying to explain is that if the dichoic filter is made from just fused silica. Then the interfering reflected wavelengths (what you don't want) can bounce around and cause more problems. Especially at an angle to the filter.

Whereas if some of those unwanted wavelengths are absorbed by the filter, using Ug11 glass as a substrate like the Baader, than the off angle wavelengths will not nearly be as many.

 

There are advantages and disadvantages both ways. Using a pure substrate like fused silica will not absorb any out of band light and will not get hot. Also should allow for maximum transmission of the desired, designed pass through wavelength. But will be more crazy when not used perfectly telecentically. Light that hits at an angle will not be filtered out correctly and cause a crazy rainbow flare.

[Cadmium]

I was not talking about anything Ulf said. I was talking about what Andrea said.

[dabateman]

I was not talking about anything Ulf said. I was talking about what Andrea said.

 

Yes I understand that. I just read Ulf's explanation and it didn't seem clear. So I thought I would also try to explain it.

But I don't sleep much. So maybe just me being fuzzy.

[Andrea B.]

Andrea, Aren't all narrow band dichroic filters 'mirrored'? Isn't the Baader U mirrored also, just as mirrored as the narrow band filters are?

 

Yes, the BaaderU and the SEU are both mirrored also. They are greenish and pink (BaaderU) or dark red (SEU). But the Edmund 340BP10 is SILVER. It is like a real mirror. It is the most incredibly mirrored filter I've ever seen. It is like somehow more mirrored than the other two. I know that sounds goofy, but if you had one in hand you would see what I mean.

 

With the BaaderU or the SEU I have only *rarely* encountered reflection or flare problems, both easily fixed by realignment of the shooting angle or adjustment of any artificial illumination together with some help from a lens hood. With the Edmund 340BP10 I have found no way to cure the mirroring problem except the most recent effort where I used SB140 UV flash by hand (I direct the flash onto the subject) and stacked the ED filter with an AR-coated BG block or with some U340 filtration.

 

What is makes a mirrored filter a displeasure?

 

It is not "a"/any mirrored filter. It is only "the"/specific Edmund 340BP10.

 

At the beginning of this topic you will see links to 5 previous topics showing the difficulties I've had with this filter.

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Reminder: I was told by my mentors (Birna, Klaus, Vivek) from the get-go over 10 years ago to always use a lens hood with the BaaderU. A long lens hood can cure a multitude of ills caused by off-axis light rays entering a lens at an angle to the main axis. UVR Optics supplies nice hoods for its dichroic filters. The BaaderU is sold for telescope work so does not arrive with a hood. That should not prevent anyone from obtaining and using a hood for any dichroic filter. I've even made filter hoods out of cardboard tubing.