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Author Topic: Cheap UV filters  (Read 6329 times)

bvukich

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Cheap UV filters
« on: October 22, 2011, 12:25:03 AM »
So I was playing with a black light, which to be honest, I do more often than a grown man should; and decided to do a little experiment.

Guess which one is a Tiffen UV Protector, and which one is a Hoya PRO1 DMC UV:

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Cheap UV filters
« on: October 22, 2011, 12:25:03 AM »

recon photography

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Re: Cheap UV filters
« Reply #1 on: October 22, 2011, 03:56:11 AM »
hoya on right  :o 8) ??? ::) :P :-[ :-X :-\ ;)

epsiloneri

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Re: Cheap UV filters
« Reply #2 on: October 22, 2011, 07:59:22 AM »
Interesting. The sensors in dSLRs are not supposed to be sensitive to UV light, so for a dSLR "clear glass" filters or "protector" filters are as good as UV filters. More important is the anti-reflection properties of the coating. Since you have the UV lamp, perhaps you could also test the claim of dSLR UV insensitivity by taking pictures of the actual UV lamp through the two different UV filters with identical manual settings, and also without filter. The difference should be negligible.

neuroanatomist

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Re: Cheap UV filters
« Reply #3 on: October 22, 2011, 10:02:14 AM »
It's hard to conclude much from this. A 'blacklight' emits a substantial portion of visible light in addition to long UV. If it were truly UV, your eyes would see no light from it.

So, is the sensor picking up UV, or is the filter on the right attenuating some of the visible light?  Question: did your eyes see the same thing the picture shows? If so, that means the filter on the right is not transmitting some of the visible spectrum (in an area to which your eyes are less sensitive, which is why they both look clear under normal illumination.

I suspect the conclusion you want us to draw is that the filter on the right is better because it's blocking UV and the one on the left is not.  But as I said, if you can see the same difference with your eyes, the filter on the right is blocking some visible light - personally, I'd call that worse.
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epsiloneri

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Re: Cheap UV filters
« Reply #4 on: October 22, 2011, 10:10:34 AM »
So, is the sensor picking up UV, or is the filter on the right attenuating some of the visible light?

The sensor probably does not pick up any UV light, I think what is happening is that the UV light is converted to visible light as it scatters off the white paper. As the right filter blocks the UV, none reaches the paper (explaining why it is dark). With the filter above the paper and the paper illuminated from the side (not through the filter), I predict both filters would show an equally bright paper...

bvukich

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Re: Cheap UV filters
« Reply #5 on: October 22, 2011, 01:16:48 PM »
Attached is my control shot for the previous picture.

Looking at the paper through either, with standard fluorescent lighting, looks the same.  The Hoya filter casts a slightly yellow shadow on objects that fluoresce under UV like the paper (i.e. blacklight reactive).  My suspicion is the high amount of UV given off by a standard fluorescent bulb causes the paper to fluoresce much like the UV bulb would, you just don't notice it because there is significantly more visible light to start with, and nothing to contrast it against.

The sensor probably does not pick up any UV light, I think what is happening is that the UV light is converted to visible light as it scatters off the white paper. As the right filter blocks the UV, none reaches the paper (explaining why it is dark). With the filter above the paper and the paper illuminated from the side (not through the filter), I predict both filters would show an equally bright paper...

Correct, and I would have the same prediction (I'll take the shots tonight to test).

While there may be some UV contamination on sensor, I would think it would manifest as an imperceptible sharpness loss.  The UV portion of light striking the sensor would be out of focus on almost any lens, much like IR would be, but that portion should be attenuated enough by the reduced sensitivity of the sensor to that frequency band, that there shouldn't be any visible effect.

Also, any UV contamination should only show on the blue channel, which is mainly used for chroma information anyhow.  Our eyes (and sensors by design, to emulate the eye) are much more sensitive to green, and that is where most of the contrast comes from.

Interesting. The sensors in dSLRs are not supposed to be sensitive to UV light, so for a dSLR "clear glass" filters or "protector" filters are as good as UV filters. More important is the anti-reflection properties of the coating. Since you have the UV lamp, perhaps you could also test the claim of dSLR UV insensitivity by taking pictures of the actual UV lamp through the two different UV filters with identical manual settings, and also without filter. The difference should be negligible.

I will also test this tonight.  But as neuroanatomist correctly points out, there is still a strong visible component to a UV bulb, which may mask any noticeable result.

neuroanatomist

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Re: Cheap UV filters
« Reply #6 on: January 04, 2012, 01:46:53 PM »
I think what is happening is that the UV light is converted to visible light as it scatters off the white paper.


Actually, the white paper is fluorescing.  Modern paper is treated with whiteners to make it appear brighter.  That's the same reason your white clothes glow under a blacklight - the whitening agents in laundry detergents remain in the fibers, and they are fluorescent (new, unwashed cotton doesn't fluoresce). 

Interesting. The sensors in dSLRs are not supposed to be sensitive to UV light, so for a dSLR "clear glass" filters or "protector" filters are as good as UV filters. ... Since you have the UV lamp, perhaps you could also test the claim of dSLR UV insensitivity by taking pictures of the actual UV lamp through the two different UV filters with identical manual settings, and also without filter. The difference should be negligible.


I did a little test of my own, with a UV transilluminator (used to view gels stained with ethidium bromide, a flourescent dye used to label DNA).  The bulbs are essentially the same as blacklight bulbs - substantial output in the UV range, and a smaller output in the visible range.  Humans are nominally sensitive to 400-700 nm light, and wavelengths below 400 nm are UV, while above 700 nm is IR.  A typical blacklight bulb has a main peak at 370 nm with a 20 nm width, and a secondary peak from a mercury line at 404 nm - it's the 404 nm light that we can see. 

I shot in M mode, with and without a B+W MRC 010 UV Haze filter (5DII, 135mm f/2L, 0.5 s, f/2, ISO 100).  Identical exposure settings, the only difference was the presence or absence of the filter on the lens.  As you can see from the image pair below, there's not a lot of difference with the filter.  There are two possible explanations for that:

1)  The sensor is sensitive to UV, and the filter is not blocking any UV light.
2)  The sensor is not sensitive to UV light.

I'm going to go ahead and trust that when B+W sells UV filter, it actually blocks UV light.  Given that assumption, it's safe to say that dSLR sensors are relatively insensitive to UV light (which I was hoping was the case, since I've posted that tidbit plenty of times!)

Actually, if you look closely at the images, you will see that the lower one (with the filter) is slightly dimmer than the upper one.  Looking at the luminosity values, the filter is attenuating about 5% of the light.  However, if you look at the transmission curve for the B+W 010 UV filter (here), you see that ~5% attenuation is exactly what you'd expect at 404 nm. 

So, I conclude that dSLR sensors are insensitive to UV.

...there is still a strong visible component to a UV bulb, which may mask any noticeable result.


Certainly true...and it does confound the issue a bit.  The problem is that there aren't typical bulbs available that emit only in the UV range.  Of course, I could definitively answer the question - I've got a 100 mW laser with a 355 nm beamline, and being a laser it emits only at 355 nm.  But I'm not about to fire a laser at one of my dSLR's sensors...   :P
« Last Edit: January 04, 2012, 02:10:15 PM by neuroanatomist »
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Re: Cheap UV filters
« Reply #6 on: January 04, 2012, 01:46:53 PM »

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Re: Cheap UV filters
« Reply #7 on: January 04, 2012, 03:29:13 PM »
If you ask me though, the first thing that I thought of on the right item is that its more of a polarizer than a UV filter...

D.Sim

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Re: Cheap UV filters
« Reply #8 on: January 11, 2012, 04:55:48 AM »
Would you get a better read if you held up the two filters side by side in front of a UV light and took a picture of that?

And eliminate the paper flourescence, sort of like the other example posted, but in one image...

pp77

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Re: Cheap UV filters
« Reply #9 on: January 11, 2012, 09:33:48 AM »

I did a little test of my own, with a UV transilluminator (used to view gels stained with ethidium bromide, a flourescent dye used to label DNA).  The bulbs are essentially the same as blacklight bulbs - substantial output in the UV range, and a smaller output in the visible range.  Humans are nominally sensitive to 400-700 nm light, and wavelengths below 400 nm are UV, while above 700 nm is IR.  A typical blacklight bulb has a main peak at 370 nm with a 20 nm width, and a secondary peak from a mercury line at 404 nm - it's the 404 nm light that we can see. 


Wouldn't it be pretty easy to test the UV filters on a transilluminator by just placing the filter on the transilluminator, and a standard UV cuvette with a cheap fluorescent dye like ethidium bromide on the filter. If we see fluorescence, the filter does not block the light? Assuming that you do not care to play around with EtBr on your camera equipment...
Otherwise fluorescein might be a non-toxic alternative, but then a different source of UV light would be necessary.

neuroanatomist

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Re: Cheap UV filters
« Reply #10 on: January 11, 2012, 11:31:41 AM »
Wouldn't it be pretty easy to test the UV filters on a transilluminator by just placing the filter on the transilluminator, and a standard UV cuvette with a cheap fluorescent dye like ethidium bromide on the filter.

Great point! In fact, I did this a few days ago, just to confirm.  I used an EtBr-stained gel (one of the eGels which are sealed in plastic).  With the filter placed on transilluminator, setting the gel on top of the filter eliminated the fluorescence of the DNA - so, the filter blocks UV very efficiently, as advertised.

BTW, just plain EtBr in a cuvette/tube wouldn't work well, as it's fluorescence in the absence of DNA is very weak; also, fluorescein absorbs in the visible range.
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bigblue1ca

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Re: Cheap UV filters
« Reply #11 on: January 11, 2012, 03:30:18 PM »
Wow this topic has quickly gone over my head.  :o  Now, since I don't feel like going back and getting a B.Sc.; out of curiosity would one have a transilluminator at work, school, or home, and what is its commercial purpose? Forensics/medical research?

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Re: Cheap UV filters
« Reply #12 on: January 11, 2012, 03:47:39 PM »
Here is a link to UV filter tests with a different approach.  As noted, UV is not a concern for digital, since there is a UV filter over the sensor.  For film, however, it could be important.

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Re: Cheap UV filters
« Reply #12 on: January 11, 2012, 03:47:39 PM »

neuroanatomist

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Re: Cheap UV filters
« Reply #13 on: January 11, 2012, 03:51:05 PM »
Wow this topic has quickly gone over my head.  :o  Now, since I don't feel like going back and getting a B.Sc.; out of curiosity would one have a transilluminator at work, school, or home, and what is its commercial purpose? Forensics/medical research?

A UV transilluminator is used in scientific/biomedical research, but 'UV transilluminator' is just a fancy name for a box with UV bulbs inside, and those same bulbs are used by medical examiners at crime scenes, used for effect lighting in nightclubs, and a different but related type of bulb is used in bug zappers.
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bigblue1ca

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Re: Cheap UV filters
« Reply #14 on: January 11, 2012, 10:53:52 PM »
Thanks Neuro.

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Re: Cheap UV filters
« Reply #14 on: January 11, 2012, 10:53:52 PM »