Are UV filters useless at blocking UV? Tune in to find out!

[ultrainfra]

I was bored tonight and playing around with my Lumenshooter S3 365nm UV flashlight, and wondered what would happen if I stuck a UV blocking filter in front of it.

 

To be clear, I don't bother using a UV blocking filter on a digital camera. I have a cheapo Polaroid branded one that came with a set, though. So, first of all I don't expect a cheap one to be very effective to begin with. Second of all, my goal here was to gauge the results visibly. So, I used my cell phone camera, rather than a full spectrum, since my only goal photographically was to make a record of what I saw.

 

My hypothesis: If this particular UV filter is at all actually useful for blocking UV, then shining a UV flashlight through it should nullify or reduce UV induced fluorescence.

 

Setup: I used a tabletop tripod, mounted my phone with an adapter, put the Lumenshooter 365nm flashlight on a small box so that I could put it right against the flashlight without the filter's rim casting a shadow, and a bottle of powerade about 3 feet away on a table. I set the ISO to 500, and aperture width to a manual value (I forget what, and since the phone saved the pics as a png I can't tell from the metadata. I didn't realize beforehand or I'd have made a point to remember). Nevertheless the point is all parameters on the phone's camera were held constant. I attached a diagram of the setup. I'm sure there are imperfections in the design and only so-much it can tell me, but again I just wanted to make a record of what effect I could see, not make a precise quantification of how much UV light is blocked. For that I'd need a spectroscope which is beyond my budget.

 

Also of importance is the fact that all lights were turned off. No visible light sources other than the fluorescence played a factor here. Importantly, the Lumenshooter has a filter (ZWB2 or ZWB3 glass I think) permanently attached over the LEDs, so very little visible light comes out of the flashlight. I dimmed my phone's screen to as low as it can go, and wore a black shirt so that no reflected light from my clothing played a role. As is evident from the control picture, no visible light is evident in the scene so all illumination photographed came from the UVIF.

 

 

The results... Well, they are subtle, but it is noticeable. For fun, I looked to see what happens if I stick a circular polarizer in front of the flashlight. Regardless of the rotation, the thing actually blocks all UV light. No fluorescence is visible, and the photo is identical to the no-flashlight control picture so I just attached this in lieu of wasting site bandwidth on two black images.

 

 

No Filter

 

Filter

 

Difference between No Filter and Filter images

 

Polarizer / No Light On

 

For those who own more expensive UV filters, or have equipment to do a better version of this, please feel free to share your results!

[Cadmium]

Yes, most "UV" filters, meaning the type that are intended to block UV, are not really very good at blocking UV, especially the upper UV range.

So those types of UV blocking filters are no good for cutting off reflected UV that might reflect off of a UVIVF target and enter the camera lens. Stray reflected UV, or 'SRUV', one might call that.

Two of my favorite filters to use for a UV barrier to prevent SRUV from entering the camera lens is the no longer made Schott KV-418 and the Zeiss T*. Both of those filters will cut off UV just above 400nm and block anything below.

Schott GG420 and GG435 (etc.) will also, but they can fluoresce if subjected to enough UV, what helps with those is to stack them with the Baader U UV/IR-Cut filter (which alone doesn't cut above 400nm), such a stack will reduce any such fluorescing.

Another filter that will work nicely is the Wratten 2E gel filter.

[ulf]

The Tiffen Haze-2A is also quite good at blocking and is sharply cutting just above 400nm without any fluorescing like the normal sharp cut glass filters have.

The 2E is cutting further up into the VIS range. It has a slight yellow tone. Tiffen also have a glass laminated 2E filter if you prefer that over thin gel-filters.

The kink in the green curve above is just due to that the measurements are taken in 10nm-steps. The real cutoff is more straight down continuing from the higher value slope.

[bvf]

I did some testing some time ago on a couple of Hoya UV filters, designated UV(C ) and UV(O). The UV(C ) did not seem to do much - I guess it only blocked UV at short wavelengths which wouldn't register on the camera anyway. The UV(O) was much more effective, though. I picked up this graph from somewhere which illustrates this:

 

 

I have always traditionally used UV filters in front of my lenses (for visible photography) just as a lens protector (that saved my Sigma 24-105mm once). Now, though, manufacturers like Hoya make filters designated specifically as lens protectors, slightly cheaper than UV block filters.

 

* Annoyingly, I can't prevent bracket-C-bracket from displaying as the copyright symbol!

 

EDIT by Birna: add a space after the 'C'

[JMC]

As Steve mentioned Zeiss T* is a very good one - blocks pretty much everything below 400nm. However make sure you get a good one, apparently there are plenty of forgeries out there. I've also had good experience with the Firecrest 400 ones from Formatt Hitech. Baader UV/IR cut is very good too, but you're limited with sizes there.

 

Steve, there is a UK company offering coated OG, GG and RG filters, specifically to reduce the fluorescence for forensics work. Don't know about price though, although anything forensics will not be cheap. Here's the link - https://www.fosterfreeman.com/images/stories/Antiglare.pdf

[Cadmium]

Jonathan, That is interesting. Have you tested them?

The 'dichroic coating' works basically the same as stacking GG filters with Baader UV/IR-Cut filter, or with the KV-418, or with the Zeiss T*, or such, the same idea.

The UV-Cut filter will reject the UV, alone even, and the GG filter will further delineate the longpass cutoff.

[OlDoinyo]

One issue with this test is that visible blue light can also excite fluorescence.

 

I agree with the assessment of so-called "UV" filters--many are little more than a piece of untreated glass, and some transmit UV better than most non-quartz UV lenses! It goes to show you that manufacturers have little incentive to make these filters actually block UV: most modern camera lenses transmit too little UV for it to be an issue anyway, and digital cameras are far less sensitive to UV than most films were. Most users merely want a protective pane in front of their lens, and do not really care about the filter's bandpass properties. Thus, manufacturers can get away with this.

 

The filters labeled "skylight," which are very pale pink, usually do block quite a bit of UV.

[ulf]

Jonathan, That is interesting. Have you tested them?

A transmission graph would be very interesting.

If this coating acts as a good cutoff would be one of the best alternatives.

Att the same time the products description say it has an AR-coating.

 

The Tiffen filters do not have any AR coating and I do not think the KV-418 do either.

[colinbm]

A transmission graph would be very interesting.

If this coating acts as a good cutoff would be one of the best alternatives.

Att the same time the products description say it has an AR-coating.

 

The Tiffen filters do not have any AR coating and I do not think the KV-418 do either.

 

From my reading of the PDF these are regular UV filters like the GG420 with a coating to reflect the UV fluorescing wavelength & therefore avoiding the the filter fluorescing, but still blocking the shorter wavelengths....?

[Cadmium]

The dichroic coating blocks the UV, at exactly what nm the dichroic coating cuts off the UV??

The UV light source still needs to be filtered and cut off at or below 400nm or the cut off point of the dichroic coating.

If the light filter and the dichroic filter coating of the longpass filter glass are paired correctly, then the longpass filter glass will simply be cutting off the range of visible fluorescence entering the lens, depending on what longpass glass is used.

This works the same as stacking any longpass filter with a Zeiss T* or Baader U, say.

Again, this only works if the light source is cut below the point of the UV blocking dichroic coating.

[Cadmium]

One thing about the Zeiss T*, it actually fluoresces ever so slightly, but you would need to shine the UV directly at it to see that.

[Stefano]

I think every filter has some fluorescence. Even my Hoya R72 fluoresces IR-yellow under strong UV, but I need to shine a UV torch directly on it.

[JMC]

No I've not tested the coated OG and GG filters. When I last spoke with the supplier as soon as I told them I wasn't a forensics person I got the distinct impression that it was a bit like 'oh well, bye then'.

 

I've tested about 30 of my filters for fluorescence and they all do to some extent - of course it depends on how strong your UV source is too. There is also a wavelength effect for the UV, so it depends to some extent on your sources spectral distribution. Even KV-418 I got to fluoresce, but at a very low level. Can't share it all at the moment unfortunately as it is written up for publication.

 

As mentioned above though Zeiss T* UV filters were excellent - cut off just above 400nm and low fluorescence.

[Cadmium]

You have any prices on those filters, Jonathan? I wrote them, but it will not be until next week before I expect to maybe hear back about prices and a few other questions.

Probably the least expensive and one of the least fluorescent is the Wratten 2E gel filter, already mentioned. Easy to find on eBay.

Those work great for blocking UV from entering the lens. Not much to think about with that filter.

[JMC]

No Steve, I don't have any details on them, sorry.

[otoien]

I find that a Nikon L-39c filter helps preventing UV fluorescence in the lens, as discussed before here:

https://www.ultravio...dpost__p__20896

and here:

https://www.ultravio...dpost__p__20897

 

But it is far from complete blocking.

Here are some UV sensitive beads that show color on UV exposure (in this case sunlight), left Nikon NC filter, right Nikon L-39c:

 

 

Sun exposure only:

 

 

 

(After keeping in the dark or indoor light for some time they turn completely white.)

[Cadmium]

Those beads... What are those made out of?

[Stefano]

I don't know for sure, but here you can find a list of some photochromic chemicals: https://www.google.com/url?sa=t&source=web&rct=j&url=https://en.m.wikipedia.org/wiki/Photochromism%23Classes_of_photochromic_materials&ved=2ahUKEwjkibir0ODsAhVGDOwKHSSeArcQ0gIoAzAOegQIFRAL&usg=AOvVaw2NLf_vnZtB5s9lDKHGtR9C&cshid=1604209257377

[enricosavazzi]

The Astronomik L1, L2 and L3 filters ( https://www.astronomik.com/en/photographic-filters/luminance-filter-l-1-l-2-l-3.html ) have very sharp transmission shoulders at or near the UV-VIS border, and can be used to cut NUV. They also cut NIR equally sharply, so they cannot be used if you want to record both VIS and NIR. The L3 is the one you may choose if you need to cut all UV and also a bit of violet VIS, for example to cut most of the VIS emission tail of 365 nm LEDs.

 

"Protector" filters are only meant to provide a physical barrier in front of the lens, so they are not meant for cutting out UV.

[otoien]

On 10/31/2020 at 8:37 PM, Cadmium said:

Those beads... What are those made out of?

 

I really do not know. It was a handout at a the AAAS meeting a number of years ago as part of a UV demonstration meant to teach children about UV exposure safety and to be worn as a wrist band. They respond slowly over up to several minutes. Unfortunately I might have lost my sample - it is a long time since I have been able to locate it. I seem to recall that Andrea found a supplier and purchased some when I initially posted about them in the old Nikongear forum.

 

[dabateman]

Do you have a high resolution image. I can't read the company off the badge or from the page. But that would help track down this. Even the person Name might help.

 

I found this:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.amazon.com/JPSOR-Beads-Changing-Reactive-Plastic/dp/B01LX5HMJL&ved=2ahUKEwjGlO27leTsAhUtoXIEHU8HDXcQFjAAegQIJRAC&usg=AOvVaw1WpaJvX10aqVBcMw1a9Eii

 

 

[Stefano]

For who has those beads, try this experiment, I am curious: Do they return to normal faster when illuminated with red or infrared light?

 

Phosphorescent materials discharge faster this way, since these low-energy photons can still kick-off electrons from the metastable state down to the ground state, discharging the material. Those beads work differently, the molecules change shape and become colored (the new version of the molecule absorbs certain wavelengths), and then they turn back to their "default" shape.

 

Also, are these beads colored in reflected UV and IR?

[otoien]

Do you have a high resolution image. I can't read the company off the badge or from the page. But that would help track down this. Even the person Name might help.

 

I found this:

https://www.google.c...10aqVBcMw1a9Eii

 

Her badge reads University of California Irvine. This was an outreach effort, so no company name to extract. Your link looks promising though (or rather a link within it, https://www.amazon.com/JPSOR-Scientific-Changing-Reactive-Plastic/dp/B01LZFI7AF/ ). A search for "UV sensitive beads at Amazon came up with a number of other choices (https://www.amazon.com/s?k=UV+sensitive+beads&i=arts-crafts&ref=nb_sb_noss_2 ).

 

May be I have to do another look for my beads...

 

IR modulation of the response could perhaps just be a temperature effect?

[dabateman]

Do you have a high resolution image. I can't read the company off the badge or from the page. But that would help track down this. Even the person Name might help.

 

I found this:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.amazon.com/JPSOR-Beads-Changing-Reactive-Plastic/dp/B01LX5HMJL&ved=2ahUKEwjGlO27leTsAhUtoXIEHU8HDXcQFjAAegQIJRAC&usg=AOvVaw1WpaJvX10aqVBcMw1a9Eii

 

Ok I got these beads. There are 7 different colors, but I think I only see 5.

When I hit the same bead with 365nm or 388nm or 400nm light the color is different on the same bead.

Haven't tested sun light yet.

 

Ir flashlight doe not cause it to more rapidly loose its color. Nor does a hallogen flashlight.

 

They are quite fun and glow in the dark.

[Stefano]

So they even glow in the dark? You get two in one?