Some nice finds in an eBay mystery box

[JMC]

I keep an eye out for various bits on eBay, and recently came across what was described as a 'box of microscope filters'. It had loads of stuff in it, and I thought it was worth a bid, and I was the only one that did. Turned out that while a lot of it was not that interesting, there was some funky stuff in there.

 

There were 3 filters described as Hg Mon 365, 405 and 546 which were quite big.

 

 

This is their transmission spectra.

 

I believe these are used for filtering the output from a light source rather than for imaging. The 405 one is really thick, and I think is probably two filters stacked, although haven't taken it apart to check.

 

There was a box of lenses, which had about 10 in it of different sizes. No individual wrapping for them, they were just chucked in there.

 

I assumed they were all glass, but checked the transmission spectra and to my surprise 2 of them transmitted down to 250nm so are quartz or fused silica!!

 

One was a plano convex lens 36mm across and with a 75mm focal length (although with some chipping around the edge).

 

The other was a biconvex 35mm across, and a focal length of 50mm.

 

Also in that box was a 305nm cut off filter, just over 25mm diameter (probably 1inch given the age of the stuff in the box) and 3mm thick, which I suspect is Schott WG-305.

 

 

Always nice to have some interesting finds. 3 of the glass lenses were given to a neighbour who is teaching his daughter about optics, and I suspect I'll donate the rest of the stuff to a microscope club I'm a member of, but these bits I'll keep and use.

[Stefano]

The 405 nm filter is interesting. It is basically a violet bandpass, and doesn't even leak IR. Do Schott or Hoya make their version of this filter?

[JMC]

Stefano, the 405 is really thick (much thicker than the other two), and I suspect is a stack. Could well be that one part blocks the IR. If I take it apart, I'll check.

[JMC]

Just dismantled the 405 filter, and it is actually a stack of 3 filters glued together. The front one is 4mm thick, and looking from the side is almost transparent. The middle one is 5mm thick and looks black. The one at the rear is 3mm thick and also looks black. I won't be separating them to see what they are though.

[Stefano]

12 mm total, a brick basically. I should search in the Schott filter program some filters that can make this graph. The hardest part is finding a 450 nm shortpass.

[bvf]

I envy you! I bought a job lot of random filters on ebay for a small price hoping to find something interesting, but they were all just far-red and NIR filters - no UV.

[Stefano]

It probably isn't the same stack you have, but I got something vaguely similar:

 

BG3 (4 mm) + BG36 (5 mm) + S8612 (3 mm). 12 mm total.

[dabateman]

BG12 was more common for microscope filters. But since its specific for 405nm, may be anything. The transmission looks good. I bought a specific 405nm filter and at best I think its 10%. But may need to look again to be sure. I was disappointed it wasn't as good as my 390 filter for transmission.

 

Looks like you got some fun stuff in your box. Those silica lenses maybe fun to play with on your 135 format cameras. I worry that all my 25mm lens and filters wouldn't cover the sensor size if I move up from m43rds. But now looks like I have moved down to 1/2.3. So should be ok for a bit.

[JMC]

Yes there are some nice finds in there. I've had my fair share of duff purchases in the past, so nice to actually get something useful.

 

The big Hg filters look brand new but I suspect that are around 50 years old if not more. Stefano, they may we be using older glass types such as BG12 as David mentioned, so may well be quite hard to model.

[Stefano]

Yes, probably it uses some old glass no longer manufactured. Yesterday I checked every filter in the Schott 2017 filter program, and the only one that worked is BG36.