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UltravioletPhotography

Germicidal fluorescent tube as UV source


enricosavazzi

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enricosavazzi

I made a few searches of the forum but did not find anything about this subject. This may not be new, but here I go.

 

Years ago I collected a handful of different types of compact fluorescent tubes. They did not prove very useful, but at that time I had poorer equipment. Today I took out of mothballs a UVC compact fluorescent tube designed for use in a water sterilizer/pond filter. This appears to be a low-pressure mercury emission tube without any phosphor, so I expected it to emit only the main Hg lines. For the purpose of UV photography, this means in practice 365.4 nm, and in the VIS mainly 404.7 nm (violet), 435.8 nm (blue) and some 546.1 nm (green). I assume this tube is not made of fused silica given its low price, so it should not transmit the 184.5 nm line. There are other Hg lines, but these are the ones usually seen in this type of lamp according to the literature. Most of the energy (40% of the used electricity) is supposed to be emitted at 253.7 nm, so the rest is "peanuts", unfortunately.

 

Before you try a tube of this type, remember that it emits mostly UV-C at one of the most dangerous wavelengths, and is supposed to be used entirely encased within a filter, with no radiation allowed to escape. I used a polycarbonate full-face mask, gloves and long sleeves, and I recommend you do the same.

 

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Baader U

 

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Astrodon UVenus (UG5 gave identical results)

 

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Baader U, illumination by MTE UV 301 torch (Nichia 3W 365 nm LED)

 

With Baader U, only the 365.4 nm line is transmitted. The false color is quite different with a 365 nm Nichia LED, perhaps because the LED has a somewhat broader emission spectrum. With UVenus, the 365.4, 404.7 and perhaps 435.8 nm lines are transmitted. The sensor is much more sensitive to the two VIS lines, so they swamp out the 365 nm "yellow" false color.

 

Exposure time was around 15 s at f/11 and ISO 200, lamp about 20 cm from subject. The MTE UV 301 torch emits a very narrow beam, and for this reason I had to hold it about 1 m from the subject, but still it allowed 1/8s exposure. UVenus and Nichia LED produces only the 356 nm "yellow", so I don't need to show the test picture.

 

Conclusions: Is it worth using a germicidal tube as a radiation source for UV photography? Not if you are after the 365 nm line and can tolerate the slightly wider emission band of the LEDs. It would certainly be a good idea if you need the 253.7 nm line, where LEDs are pitifully weak and very expensive. However, for now this wavelength is out of our reach with ordinary UV-enabled digital cameras and most lenses. It is also better to use a mercury emission lamp if you need a very narrow spectral line, e.g. to calibrate a spectrometer. Unless you need one of these specialized uses, better save up on a good UV torch with 365 nm Nichia LED. Mine emits about 600 mW of UV around 365 nm according to specifications, which is quite a bit more than the mercury lamp and is concentrated on a much smaller area.

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Thanks for this report, Enrico.

 

I've played briefly with a hand sanitizer wand about 4 years ago and reported on it in this thread on what was then Nikongear: http://www.fotozones...zer#entry192661

 

I will just excerpt my observation about the hand sanitizer wand from that long thread:

 

[excerpt]

Using the 254nm sanitizer wand, I did manage to make a photo with the Baader U. The exposure time was over a minute & the results were awful. Dim photo, muzzy details. Also I'm not sure just how "pure" this 254nm [sanitizer wand] is [given that some wavelengths] were getting through the Baader-U [whose cut-in wavelength is well above 250nm]. I bought [the wand] in a pharmacy.

 

The sanitizer wand does, however, produce a beautiful [visible] blue phosphorescence in my chunk of optical Calcite.

[/excerpt]

 

Maybe I will repeat the experiment someday using a narrowband filter down in the 250nm range. Currently I don't have such filters.

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