This is electronics related so please bare with me. I was pondering making a low resolution thermal imager out of SMD thermisors - i.e. make a grid of say 16 thermistors to give a 16 pixel image.

The question is what sort of lens do I need. I am assuming normal glass will not do for far infrared?

 

Google provided this answer:

http://www.naturfotograf.com/UV_IR_rev05.html

 

Normally IR lenses are made from germanium.

 

regular lenses (glass) work with infrared, just look at all the camcorders what work for infrared "nite shot" pictures. you would need an nfrared filter to block regular light though. I have 3 sony "handi cam" cameras that work fine for infra red, but are not sensative enough for thermal imaging, they need an infrared illuminator. are yo sure that thermistors are sensative enough?

 

Glass can be used in the near IR, such as where photography is often done, but glass is opaque in the far IR. For that, other materials such as germanimum are commonly. Here's some example lenses, but they're not cheap.

You may have to cool the focal plane/detectors with a TE cooler to get reasonable sensitivity.

 

I wonder if a front-surface metal-coated parabolic mirror, like those commonly used for reflective telescopes, would image far infrared? Any reason they wouldn't?

 

Thanks for the replies. Looks like a lot of stuff out there is for near infrared and for that glass works but I want far infrared and crutschow says it looks like special lenses are needed and they are not cheap. I have found some from a UK supplier for around £100 but still too expensive just for playing.

davebee - great idea! Reminded me that a while ago I took the kids to a science museum and they had a proper thermal camera set up for people to play with. One of the objects they had was an aluminum sheet, just standard brushed aluminum, but it looked like a mirror in the thermal camera, so I guess any first surface mirror may do it.

 

Any metal parabolic mirror should work. At the long wavelengths of far IR the finish does not need to be shiny, as the experiment with the piece of brushed aluminum showed.