sensor to detect >3μm light

I hope this is the best section for this
so here's the problem: I need to detect some small objects (about 25cm^2, don't know the shape) which are heated up to <45°C and (due to temperature) they should emit light at about 3μm (and higher).
the first idea was to use some PIR motion detector, but these are usually sensitive to 5-14μm and react only to motion
the another idea was to use some IR thermometer like this one: DealExtreme: $20.05 1.2" LCD Non Contact Digital InfraRed Thermometer with Laser Sight (-50°C ~ 380°C), open it, extract the electronics and hack it (so it will give me logical or analog output)
any other ideas?

Not something that is cheap. You could use a HgCdTe sensor, which responds to 3-5um. No idea where you buy them directly. Maybe rent a thermographic camera.

According to the wikipedia blackbody radiation page, the peak wavelength is about inversely proportional to the temperature in Kelvins and a constant of the speed of light. So for 45°C, that should be 2898000/(45+273) = 9.1µm. There might be photons as high as 3µm, but there will be far more at 9.1µm.

If you think about it, 45°C isn't too far from nominal body temp.

To get most of the photons at 3µm, you'd need a temperature of 693°C :scared: