Different methods for measuring intensity of IR wavelengths I am proposing a teaching experiment at my university for undergraduates to learn about black body radiation and part of it will involve measuring intensities of different near - mid IR wavelengths. This is where the problem lies.
As far as I am aware there are two methods I could propose to do this:


*

*Using a spectrometer that can measure in the required range. This equipment is however very expensive and so is probably a no go.

*Using IR filters and a suitable photodiode to measure intensities. The problem here however is that I do not know of any filters that will only allow small IR bandwidths through. I can only find filters that block out optical wavelengths and allow all IR to pass through.


Are there any other methods I could use or am I going to have to bite the bullet and propose buying a £20,000 for this lab?
 A: You could make your own low-resolution spectrometer with a diffraction grating, if you have a few different light sources with known wavelengths with which to calibrate it. How well this works for you depends on how many such sources you have and what resolution you're looking for.
A: Building your own spectrometer as suggested by @ostrichCamel is not a bad idea to save cash because a grating spectrometer is rather simple, although it would take some effort to get right.  Another option is these filters, which are still a little pricy, but I think they might be just what you need. 
Another issue is that you’ll need either a hot lamp source or have to shell out for a cooled detector (TEC or LN$_2$).  This is necessary because the detector is constantly emitting black body radiation at its own temp. Thus, a 300 K detector will detect no net light from a 300 K black body source. So you need to either heat up the source or cool the detector.  If you plan to use an incandescent lamp, probably the glass will block the emission in the mid-IR range, so that’s another issue.
You could probably do this decently for 1000-2000 $/£.  But if your department will agree to get you a spectrometer, I think it could pay dividends in many other ways as well, since it is such a versatile tool.
Good luck!
