Vasile Surducan gmail.com> writes: > The ideea is the IR wavelenght. Human IR radiation is different from > air. Water IR emission is different from human body wavelenght. ?! Next you will tell me colored people emit differently from whites ? It's blackbody radiation, and everything emits it and absorbs it. With the exception of some semiconductors and non-linear optical materials, most materials emit as nearly pure black bodies at the temperatures considered (~ 37 deg C +/- 10). That includes air. The emissivity is determined among other things by density and imaged area. Differential PIRs will sense a beam or jet of different temperature air (or any gas or liquid) when passing in front of them, as long as it forms an image on the sensor. Due to the low density of air, a constant background is needed, and the jet must be rather large by volume. Normal alarm PIRs filter such inputs out but if one connects a scope to the analog PIR preamp output one can see even one's own breath as it passes in front of the sensor (which must be shielded with something *transparent* at 3 to 10um to avoid direct air current influences, and with the differential half shielded off - the contrast of a bubble of warm air is not sufficient when both halves can 'see'). The signal is small (say 1/20 of normal output when say a hand passes in front of it). Cooling the sensor is needed for S/N, contrast and speed reasons, if both the target and the background are cooler than the sensor. In any case, it is enough to cool the sensor below the temperature of the hotter of the two (target or background). Afaik modern 'all aspect ratio' anti-aircraft missiles 'see' the jet exhaust plume (gases) behind the aircraft, and any hot parts thereon (like the engine cowling). Most seem to work without cryogenics now (or use single use chemical cartridge coolers or whatever), according to web data. Peter -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist