Historical 365nm bandpass filter

[JMC]

I got this along with a recent order of microscope parts, and thought it might be interesting to share here.

 

It was advertised as a 3650A filter, made from a stack of three different glasses - UG11 2mm, BG12 2mm, and WG360 1mm. Size, 60mm by 26mm. It came with what looks to be a hand drawn transmission spectra, with a date of 1982 on it. The filter itself is beautifully made, with nicely chamfered edges. Here's how it looks.

 

 

 

 

 

Putting the filter details into the Schott filter program, gave the following transmittance curve.

 

The filter looked to be good for just over 30% transmission around 365nm, and a small OD3-OD4 leak in the 720nm region. To check this I did a scan on my USB spectrometer and got the following.

 

 

Measured max transmission at 365nm was down a bit on the theoretical one, and the IR leakage was a bit higher than expected. However the overall match was pretty good, and it should be noted that the surface of the UG11 on the filter was slightly degraded (not surprising as it is nearly 40 years old). Also I suppose the glue, whatever it is, could be impacting transmission at 365nm as well.

 

What was this made for? My guess is that it is a microscope filter for giving a pure 365nm line from a mercury xenon lamp for fluorescence imaging. Not sure what I'll be using it for, just thought it was a nice historical piece really.

[Andy Perrin]

Interesting— I have some kind of historical UV filter with unknown material and transmission that I inherited from my grandma. It lets through enough visible that I can see faintly through it. I haven’t tried using it yet.

[Stefano]

Andy, that's even more interesting. UV photography wasn't very known back then I think.

[Andy Perrin]

Stefano, the quality of the glass is such that I think it was probably used on a flash or a light of some kind to produce fluorescence. UVIVF has been well known for a very long time.

[Cadmium]

Typo up top: WG260 = WG360?

WG260 is not available, unless you were to find old stock, not in production.

Stack would perform better (better red/ir suppression) if S8612 were used in place of the BG12 which is also unavailable, not in production.

[JMC]

Yes Steve, you're right it is WG360 1mm. A typo on my behalf, and I've corrected it now.

[Cadmium]

To build a similar 360nm bandpass stack, I would do this, and also stack with Hoya UV34 or UV36 (keep in mind those Hoya graphs are for 2.5mm thick, so maybe UV36 1mm would be very close).

I don't have Hoya UV34 or UV36 data in my Schott program yet, or else I would plot that addition.

Keep in mind you cant get the Schott WG filters, the closest you can get would be the N-WG320, but unless it was very think then...? I think the Hoya UV## filters look like a good alternative for the low end cut-off.

Using this idea, you would gain more UV peak amplitude, and also suppress Far-Red/IR much more adequately, and essentially retain the same 360nm centered bandpass.

Just an idea if such would be needed. Also keep in mind that so many lenses will by default cut the lower end just like the W## and UV## filters do, so with many lenses those don't need to be added unless the lens UV transmission reaches down lower than most do.

 

https://hoyaoptics.c...ters-uv-series/

 

[dabateman]

Thats an unusually large filter for a microscope. Most of the UV ones fit into rotating cubes to look at various wavelengths. Or were 50mm round to fit the Leitz holders.

 

My guess is this was custom made to be placed below a microscope slide, which are typically 75mm x 25mm. Since 15mm can be used for labeling, it would fit perfectly under a specimen area.

So may have been used for fluorescence, if a uv cut off was placed over the eye piece.

That name Dr. Hammel sounds familiar. Did you buy this from Canada?

 

[JMC]

David, yeah it's an odd shape, but came from a microscope dealer in Germany, not Canada. Not sure where he got it from.

[dabateman]

Wow you just opened my eyes to my BG12 old microscope filters.

I have 1.5mm, 3mm and 5mm BG12. 5mm BG12 plus u340 knocks off all IR and gives you a lowish but 365nm peak.

I may have to see what that looks like. I have a 25mm round 1mm ug11, but only 2mm thick in larger filter sizes. Of course a U360 might better with S8612. But the BG12 doesn't oxidize and mine is very old.