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UltravioletPhotography

Bowens Monolight (Used) @B&H


Bill De Jager

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Bill De Jager

One way to photograph in UV is to use a dedicated UV source. Another is to use a broad-spectrum light that emits some UV, as discussed here http://www.savazzi.n...ens1500pro.html by Enrico Savazzi. The Bowens monolights Enrico discusses are rather expensive but are high quality.

 

The Bowens Gemini 750 Pro is currently available used in very good condition from B&H at a cost of $600 US (vs. $1049 new): http://www.bhphotovi...6747+4291580458 . This is in the pro series which takes a wide range of international voltages, and which Enrico found emits more UV in comparison to lesser models than would be expected based on just power rating. I'm not sure how many of these used units are left but I already purchased two.

 

Notes:

-The one that arrived so far was missing the knob which fastens the rear bracket to a support. I need to inquire at Bowens about a replacement.

-A new clear (UV transmitting) tube costs $300 US. Mine had a working tube included.

-The used Bowens Gemini 500R unit also on sale at B&H is for European voltage, which may or may not be what you are looking for. This is not in the Pro series and probably emits less UV proportionately, aside from the lower power rating.

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enricosavazzi

If you happen to get a Bowens unit with a coated tube, it costs nothing to test it in UV before ordering an uncoated tube. At high power, even a coated tube may emit enough UVA (especially above 350 nm) to allow UV photography. However, my tests indicate a few stops of difference (at least 2) with respect to uncoated tubes.

 

Another factor to consider is that the Pro series uses a different tube than the R series. At least some Pro series models with uncoated tube seems to consistently emit a more UV (probably 1 stop or more ) than some R series models with uncoated tube, at the same power settings.

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Bill De Jager
Enrico, given that these units emit light at the same color temperature as the sun [Edit: well, in the general ballpark], I'd think there would be a risk of UV-C exposure since there is no protective ozone layer to intercede between the source and the target (intended or not) of the radiation. A simple pane of window glass should stop any UV-C directed at the subject, but then reflections still need to be minimized. I'm thinking of using a snoot with a piece of glass at the opening, but that still would require some means of providing adequate ventilation.
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It is true that an uncoated Xenon flash emits significant shorter wavelengths extending well below 300nm. I have some experience in measuring Xe-flash with pulse modulated (continuously rapidly flashing) sources. Single pulse flashes are more difficult to quantitatively measure but I am working on building a calibration method. Obviously the spectrum of an uncoated Xe-flash is not like sunlight and I have been thinking about filtering one to better match the solar spectrum. Filtering a Xe-flash somewhat to remove short wavelengths will also enhance photobiological safety and should not negatively impact usability for reflected UV photography since we aren't imaging these shorter wavelengths anyway. I will present spectral data and filtering solutions in an independent posting once the basic measurement technique is validated.
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Bill De Jager

JC, I'm looking forward to seeing your results.

 

I forget that the glass of the tube should filter out the UV-C and even most of the UV-B. Am I missing something here or did I answer my original question?

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Bill De Jager
I did a little more searching and found that the flashtube is probably made of fused silica which transmits UV down to much shorter wavelengths. Common graphs for this material show transmission down below 200 nm, but there's no telling what special formula a given manufacturer may use for its fused silica. There are graphs at http://medicalphotog...icle_01/04.html, but they are quite poor with no indication of whether the intensity scale is linear or log.
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Yes, my impression is that quartz flash tubes are not uncommon, some are coated but the half dozed types I have opened up weren't. Some models of big studio flashes may still be had uncoated as well.

 

Graphs from suppliers are notoriously bad, those you linked are somewhat artful....

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