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Which germicidal lamps generate the purest 254 nm? (or 254 + 185 )

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klugesmith

Mon Aug 01 2011, 06:32PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715

I want to reproduce an experiment familiar on youtube, to visualize plumes of mercury-vapor-rich air.
A fluorescent screen is excited by UV light at a wavelength strongly absorbed by free Hg atoms, and you can see the shadows cast by a vapor plume. For example:

Last night I tried using a green light source made for optical flatness testing, like

It's a box with mercury lamp and a filter to select the 546.1 nm line. Label says "10.7 Millionths", which is the half-wavelength in inches. It's easy to see fringes with a pair of microscope slides, but I couldn't see any shadows when I squeezed an uncapped plastic bottle of mercury.

Which brings us to germicidal lamps. These all emit primarily 253.65 nm, which I think is what we want for the atomic absorption , but ordinary ones also emit plenty of visible blue and violet. Some makers of UV water and air sterilizers tout their lamp spectra as more pure -- with or without 185 nm for ozone generation.
I should just find the germicidal lamp already on hand, and try the vapor experiment before shopping.

Can any of you offer advice about finding a germicidal lamp that is extra-low in visible emissions?
Or a handy filter that transmits 254 and blocks visible light?
I think "Black Light" bulbs are not the answer (but worth a try).
I think a hollow cathode lamp of the Hg persuasion would be overkill, with relatively low radiant flux and narrow beam.

Myke

Mon Aug 01 2011, 06:51PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

You could use a Wood's glass filter to remove some of the visible light. I'm guessing they used a UV source illuminating a fluorescent background but I'm not completely sure.

klugesmith

Tue Aug 02 2011, 02:52AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715

Thanks. I had considered Wood's Glass (as used in BLB lamps), but online references

suggest that it blocks shortwave UV as well as most visible light.

Will try two T5 hot-cathode "fluorescent" tubes on hand: a BLB and a clear germicidal lamp. I have long thought of them as "the longwave and shortwave UV lamps", but the shortwave puts out a lot more visible light.

The caption under youtube video in OP includes "... The method used to visualize the Hg is very sensitive & exactly how much Hg is vaporized in the demonstration is not stated."

radiotech

Tue Aug 02 2011, 03:24AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The ones I have experience with are the 30 watt T8 clear glass 254 nM
germicidal.

They were used in a process, operated on DC, and each machine had about 20 of them.
The automatically pole shifted every 8 hours.

They were very pricey. They had a metal reflecting label on each end, put on by the
machine maker, after they were aged until they stabilized wrt light output.

Once we tried just buying the stock lamps from GE to save money. The machine
went out of calibration fast so we bought the stabilized ones again.

I often wondered if part of the trick was that metal label on the glass at the ends.
It was about 2 inches long from the base.

Since we typically changed out the whole set of lamps at regular intervals, I got to
take home cartons of the perfectly good used ones.

We used them for a high speed sheet hole scanner/recorder that had to operate
under normal light, and the detectors were 931 PMs detecting fluorescence caused when
the pulse of UV got through a thick violet glass filter and landed on the crystal layer.

haxor5354

Tue Aug 02 2011, 03:27AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

Mercry smoke, don't breath that.

klugesmith

Tue Aug 02 2011, 11:24PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715

Thanks for the story, radiotech!
Last night I tried my T5-size BL and germicidal lamps, without seeing any vapor shadows above about 1 gram of mercury in a pill bottle cap.

Three x-ray cassette screens didn't appear to fluoresce significantly with either illuminant, except for a little patch where a white coating had peeled off & exposed a yellow-green-glowing layer underneath. (? !).
A normally greenish-yellow screen with an opaque backing, from "plasmatron", glowed fine with both lamps.
But I think the best-case contrast will be poor with my germicidal lamp, due to the large amount of visible light coming off.

Am thinking of getting a substantial sheet of diffraction-grating film (say, 6" or 15 cm square) and making a crude monochromator with that, to separate the linear lamp's 254 nm from longer wavelengths . Any hints?

Also did order an hollow cathode lamp for Hg. It's nice to be frugal but not too poor.

testtest

Fri Aug 05 2011, 09:40PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Interesting video. Wonder if you could see that with a broken CFL mandated in all our homes now.
After several trials with UV sources I ended up with an Oriel high pressure discharge lamp in front of a monochomator. Can attach photo of setup if you want. Only way I managed high intensity UV light of a known wavelength without the junk from the rest of the spectrum emitted by typical discharge lamps.

Proud Mary

Fri Aug 05 2011, 10:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I would have suggested a diffraction filter followed by two or three monochromators in series.

testtest

Sat Aug 06 2011, 03:15AM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Proud Mary wrote ...

I would have suggested a diffraction filter followed by two or three monochromators in series.

I did not find the need to add more than a single monochomator to get decent filtering off a mercury or xenon arc lamp. Of course more is better but you loose light at each stage.

On one pix is the Oriel 7340 with a deuterium lamp and a high pressure mercury arc lamp, fan and the focusing optics. The second smaller box is the 77250 monochromator with a 1mm entrance slit. It is grating based and the output slit is adjustable. Excellent throughput. great for curing epoxy, looking at scintillators or erasing eeproms too.

The second pix is the ISA spectrometer with a cooled EG&G CCD detector. Spectro has a fiber optic input so it can reside under the table. The green jar on top has an aquarium pump to circulate the water for the Peltier cooler. Works like a charm. This is how I can guage the amount of background making it through the monochromator when setting up a fluorescence measurement.

1312600515 3271 FT121472 P8055534 Medium

1312600515 3271 FT121472 P8055535 Medium

I'll see if I can tune it to the mercury line and see the vapours as shadows on a phosphor screen. Should be neat.

klugesmith

Sun Aug 07 2011, 05:25AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715

I'm eager to see your results, Rich.

Haven't yet found any quantitative data about atomic-absorption sensitivity in measurement of trace concentrations of pollutants or, uh, tracers.

Those visible vapor plumes can't have much more than about 2 ppm Hg by volume, or 15 ppm by weight (18 mg/m^3).
I think the youtube narrator mistakenly said the Hg vapors would go down if people weren't walking around in the lab. Well, the increase in air density from being saturated with Hg would be more than offset by a 0.01 degree increase in temperature.

Am wondering if one needs a fractional-nm linewidth, and if the broadening of emission line due to high plasma temperature in an arc lamp might significantly reduce the contrast, since the Hg atoms in vapor plume are at room temperature.

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