Making xenon quartz flash tubes (relatively) safe

[enricosavazzi]

I am finally getting some free time for experimenting, after one year during which I was largely forced to postpone it (both my wife and I changed jobs, first we spent one year in a commuting apartment in Uppsala to cut down on commuting time while selling our house, culling a mountain of accumulated junk, then bought an apartment in Stockholm and had the customary painting and renovating to attend to). I don't have as much space as before, but I am almost at the point of using again my remaining studio UV imaging stuff.

 

In one of my boxes I found a couple of quartz xenon flash tubes (one just about right for a speedlight, the other for a studio strobe perhaps up to 100-200 Ws) that I intended to use for flash modifications (and then had to pack away without trying). Now my question is: what is an effective but relatively cheap UV-C cut filter large enough to mount on the flash window? It must transmit UV-A and preferably some UV-B down to 300-320 nm, and should preferably transmit most of VIS and NIR to avoid overheating. It will not be mounted on a camera lens. Its only purpose will be to remove UV-C and make the emitted radiation relatively safe, so that I will only need to wear my usual face mask and gloves instead of needing a spacesuit or suit-of-armor.

[rfcurry]

Enrico,

 

A COC (Cyclic Olefin Copolymer) or a COP (Cyclic Olefin Polymer) plastic should do the trick. Here are some transmission numbers:

 

[Cadmium]

It would depend on the size, but Schott makes several NWG type longpass filters for that range (see graph below), comes in 165mm x 165mm x 4mm thick sheets, can be made thinner and smaller.

Also here is some plastic that transmits bellow 300nm, the UVT type might work for you (see the graph at this link below).

http://www.polymerpl...ents_uvta.shtml

[enricosavazzi]

Thanks for the information. UVT acrylic would seem to be the most promising material for this application. It is available in large sizes (used e.g. in tanning beds), reasonably priced, and is also easy to cut and file by hand to make a replacement flash window, and to sand the surface to make a UV diffuser. It will also be useful as a protective UV-transparent window in case a flash tube explodes. It sounds like it has plenty of uses in UV illumination.

 

PS - It seems one can order custom-cut sheets (2, 3 and 5 mm thick) in the EU from http://www.plasticstockist.com/Sunbed-Grade-Acrylic-Sheet/Clear-Sunbed-Grade-Uv-Perspex-Acrylic-Sheet-fabrication.aspx

[rfcurry]

Thanks, Enrico, that's good to know. We should have a sticky for such things.

[JCDowdy]

Enrico,

 

Try a clear CD case, it might have sufficient UV-A transmittance.

[Cadmium]

I just now tested the 'clear CD case' transmission. Using my Sparticle and a clear CD case, and a spiral CF black light, the CD case plastic transmits the full range of my PB filters, all the way down to the 325BP10.

So it doesn't seem to attenuate the light much, just an ever so slight overall bluer cast to the clear CD case plastic filter, but overall the colors of the individual BP filters remain the same across the range.

I have no way of testing if the CD case plastic cuts off UV below 325nm, so it may or may not be good for 'UV-B/C safety protection'.

I will also try this with the flash later.

[Cadmium]

Here is a 199A flash test using the Sparticle + a clear CD plastic case, half and half, to show the difference.

CD plastic shows some small amount of bluish cast to the 15mm thick PTFE background.

Test is shot in absolute darkness, other than rear flash illumination. 15mm thick PTFE is rear illuminated by the flash, same as the bandpass filters.

The CD plastic may be a good safety feature to add to the flash when unfiltered, however, I have no way to test if the CD plastic cuts UV-B/C, or at what amount.

[Cadmium]

Here is a comparison, CD case, UVT, and N-WG280. The N-WG280 is pretty hard to see, it is only two circles laid over a few of the upper and second row.

 

Here is a closeup of the N-WG280, do you see the outline of the two circles now?

[Andrea B.]

Enrico, it is nice to hear that you are returning to some UV projects. Welcome back!

 

Do let us know how the UVT works out.

[Cadmium]

Here is how my Canon 199a flash looks like with a replacement UVT lens installed:

http://www.ultravioletphotography.com/content/index.php/topic/2006-first-flash/page__view__findpost__p__14255

[Cadmium]

More plastic and flash lens tests.

First set is rear illuminated with 199A flash (no flash lens).

 

Second set compares rear illumination with UVT flash lens added.

 

UVT plastic info:

http://www.polymerpl...ents_uvta.shtml

[enricosavazzi]

Enrico, it is nice to hear that you are returning to some UV projects. Welcome back! Do let us know how the UVT works out.

I received my order from Bay Plastics LTD. Here is a first test, with Asahi Spectra XRR 0340 (which is probably based on a U340 substrate with multiple interference coatings) and Bowens 500 R with uncoated tube. I added a few other diffusing and lensing materials I had at hand (some acrylic, others I don't know). They all look completely transparent in NUV, so just to make sure there are no unexpected factors I included also a Baader U and B+W 486 UVR. The B+W 486 evidently transmits some UVA, because you can see some reflections from the underlying microlens sheet.

 

Edit: the transmission graphs for the B+W 486 show a roughly 10% transmission at 390 nm, which does fit these observations.

 

One of them is a wide angle diffuser labeled V-285 that came with a Vivitar 285 flash (newer series), and looks just as good in UVA transmission as the other materials (but of course this test does not tell us about shorter wavelengths than roughly 350-360 nm).

 

The microlens sheet at the bottom right is an odd material. The pattern of ovals is visible also in VIS and appears to be located about 1 cm below the surface, or above the surface when seen from the opposite side (the sheet material itself is less than 0.5 mm thick). The ovals visible in the picture are not individual lenses and the sheet is flat, not embossed with ovals. The individual microlenses are perhaps 300 microns across.

 

All materials were placed directly atop a white paper sheet. One of the microlens sheets is overexposed because it scatters radiation in a very broad angle (anisotropically, as the individual microlenses are not circular), and it was simply not possible to avoid this.

 

[Andrea B.]

This is a good reference for the various materials. Thanks, Enrico.