Potential attenuator lens for our cameras - ZnSe and Ge available Tr 23% and 30%

[Fraser]

I was just visiting my favourite ZnSe lens suppliers eBay store and note that they stock a product that may be of use to some of us.

I have often been asked how the upper temperature limit of a thermal camera may be extened. My common answer is to use attenuators in front of the lens. Calibration becomes an issue, but I leave that for another thread.

Laserlands sell semi mirrored ZnSe and Fermanium windows for use with CO2 lasers at 10.6um. These can act as good quality, predictable attenuators for us if placed in front of our cameras objective. The transmission figures provided are 23% and 30%. Not ideal for mental arithmetic on measurements but beggars cannot be choosers. The price is certainly reasonable.

I have not tested these yet but they do look very promising. In theory, you could triple or quadruple the maximum temperature that may be viewed with a FLIR One, E4 etc.

https://www.ebay.co.uk/itm/ZnSe-Ge-Output-CO2-Laser-Coupler-Partial-Reflector-Mirror-for-Engraver-20mm-25mm/372198075697?hash=item56a8bbed31:m:mbiobKSZvI7_ouOt0guDWHw

A 20mm diameter ZnSe semi reflective window with a transmission figure of 30% is $30 
Germanium is very. Ice but can be more expensive and remember, Germanium changes its transmission characteristics with temperature. The materials transmission decreases as its temperature increases. Not much of an effect for lenses and windows at normal ambient temperatures,  but significant if the lens or window temperature rises above around 50C. Of importance if viewing very high temperature radiators like a furnace as the actual lens housing can heat up and conduct heat into the lens or window.

Fraser

[Fraser]

I just ordered a ZnSe semi reflective window of 20mm diameter, with transmission spec of 23%. That gives me a theoretical increase in camera maximum temperature of approximately X4. So in theory my E4 could at least view a 1000C emitter or flame.

It will take about a month to arrive but for £21 ($30) it is worth taking a look at IMHO.

I chose the 20mm diameter because I already have nice lens holders for that diameter element. It looks like there is quite a wide unmirrored area around the edge of the window, but the active area should be plenty large enough for the E4 and I can mask the unmirrored area to create a themally dead zone.

Fraser

[Fraser]

On the same topic, I read of a researcher who was viewing Plasma with a thermal camera. He used a Germanium Mirror that is gold coated as an improvised attenuator. The mirror had a transmission figure in the LW spectrum of around 5% if I recall correctly. It worked perfectly for the researchers application.

Fraser

[mrflibble]

On the same topic, I read of a researcher who was viewing Plasma with a thermal camera. He used a Germanium Mirror that is gold coated as an improvised attenuator. The mirror had a transmission figure in the LW spectrum of around 5% if I recall correctly. It worked perfectly for the researchers application.

Do you happen to have a link to that paper? Also, was it high speed or a slow poke camera? There's quite a few nifty application areas. But I suspect the above researcher was well aware of that. It'd be interesting to see what kind of attenuation characteristics he got.

[Fraser]

I may be wrong, but a I thought it was Andrew Seltzman at RTF Technologies. He 'plays' with Fusors 

http://www.fusor.net/board/viewtopic.php?f=6&t=8734

http://www.rtftechnologies.org/

I will check and see if I can find the relevant detail for you.

Fraser

[Fraser]

I found the picture of the 'mirror'

It is a 99.5% reflective coupler and not the 95% that my defective memory remembered  He gets 1/200 attenuation of the laser beam before it reaches the thermal camera lens.

http://www.rtftechnologies.org/physics/fusor-mark3-microbolometer.htm

I recall he covers its use in his write up on the design but I have not found that yet

http://www.rtftechnologies.org/physics/fusor-mark3-microbolometer.htm

Fraser

[Fraser]

OK, I found Andrews paper on his Fusor experiment. This is pretty hard core experimentation !

http://www.fusor.net/board/download/file.php?id=8627

He only says he uses the 0.5% transmissive Germanium coupler window as a "Power Attenuator"

In his picture page he states the following:


"Thermal imager (7/19/2014)

Synrad H48-1 with a 9x beam expander bench test. Shadow from soldering iron tip cast in beam. Imaged with a thermal eye 2500AS, 99.5% reflective rear coupler from laser cutter used as 200x attenuator, laser run at ~20% power and 1kHz square wave with 6% duty cycle."

I think you would need to contact Andrew for more details.

Fraser

[Fraser]

I found a different, and slightly cheaper, coupler window at Laserlands. It is 25mm diameter Germanium and provides 22% transmission.

https://www.ebay.co.uk/itm/T-22-25mm-10600nm-CO2-Laser-Ge-Output-Coupler-Partial-Reflector-Mirror-Engraver/381878754760?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have contacted the seller in the hope of changing my order from the ZnSe type to this Ge window. I may end up with both the ZnSe and Ge windows.

Fraser 

[Fraser]

Some information on CO2 laser coupler windows.

http://www.iiviinfrared.com/CO2-Laser-Optics/output-couplers.html

For those unaware, the coupler sits at the opposite end of the laser tube to the mirror. It is what passes the laser beam out of the laser tube. The quality of such a coupler window needed to be pretty good to do its intended job, so hopefully it will work with LW thermal imaging cameras as well.

Fraser

[IwuzBornanerd]

If you are only trying to reduce the transmission to something like 23% as opposed to the 0.5%, wouldn't it be simpler & cheaper to put a thin piece of plastic close in front of the lens with a small hole in it to reduce the aperture appropriately to achieve the desired reduction?  2 F-stops tighter would get you 25% transmission, right?

[Fraser]

Maybe, but is there not the issue of alignment and vignetting with a pinhole aperture ? I am no expert on optics. There is also the matter of creating a hole of the correct dimensions for a desired attenuation.

These partial reflector Laser Couplers appear a quick and easy way to increase the maximum temperature capability of a camera with some level of accuracy. Those using the ZnSe close up lens may like this simple window attenuator format.

My suggestion is just one way to achieve an objective and there are surely other ways to do it. I like the fact that these coupling windows have a stated transmission value and are available in the 20mm diameter that I tend to use. Thank you for offering an alternative potential solution though.

Fraser

[Bill W]

All it needs is an external iris, so a disc with a hole stuck on the front element will be just fine.  Just be aware that thin plastic could be transmissive in itself.

Attenuation is simply the ratio of the areas.  So:
Lens full diameter 10mm
Hole diameter 2.5mm
Attenuation 16x

If it is 15mm focal length then it used to be f/1.4 and is now f/5.6

It will also slightly improve the image optically, you are using the centre of the lens and also increasing depth of field.

[Fraser]

Thanks Bill and Iwuzbornanerd

I had not realised placing an iris in front of the objective would work that well. I may have to source a variable iris to experiment with   

Fraser

[Vipitis]

You may be able to 3d print one, as you only need some screws and rubber bands for some designs.

[IwuzBornanerd]

Yes, centering can be difficult with the "cheap & dirty" methods I use.  One has to poke, drill & cut carefully.  I have only done that with webcam lenses--in order to prevent washing out in bright sunlight on snow, etc., and like Bill W said, I just stuck the disc to the front of the lens. 

I have no experience with spacing from the lens but I expect the new aperture has to be rather close to the surface of the lens element, so if your lens assembly or iris assembly is such that the iris cannot be within a mm or so (??) of the lens surface the window might indeed be the more viable option.