The ideea is the IR wavelenght. Human IR radiation is different from air. Water IR emission is different from human body wavelenght. Read please the physics laws . Usual PIR has a filter for human wavelenght. Your assumtion is correct unless you are not using a cold reference (a peltier). It's worthless to discuss more, non contact thermometers with PIR detectors have been manufactured since the PIR has been invented. The same principle you can see in the pictures has been used in rockets heads following any moving hot engines. The only difference was the sensor sensitivity which was Pb-Se and not PIR. Differential PIRs are not affected by hot air. On 4/23/09, Peter wrote: > Air does emit IR as anything else does. Its emissivity is low due to low > density, however. Iow, one needs a large amount of warm (not hot) air to make > the PIR see it. Too hot will not be seen, it needs to be at about 30-40 degrees, > and the best way to see it is against the cold night sky, but it *will* be seen > by the PIR. A similar situation can occur when a PIR faces outside in winter and > some hot air comes out of the house and passes in front of it. > > Peter > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist