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Oznog said:I'm not aware that the cadmium sulphide photocell has any ability to detect thermal infrareds, just visible light.
Thermal infrareds are lower energy than the infrareds of say a TV remote, and consequently harder to detect. It's not a matter of noise discrimination either- the photoelectric effect that all photodetectors are based on have an inherent lower threshhold. If a photon doesn't have the energy (its wavelength is too long) to trigger the effect, nothing happens no matter how many photons hit.
Thermal infrareds are also difficult to interpret. Everything above absolute zero, including the body of the sensor and lenses, emits their own infrareds. And the infrared of the environment often varies by orders of mangintude.
The PIR detectors do detect thermal infrareds, and deal with noise by only triggering when a sudden change in infrared occurs. Additionally, the funky milk-white lens in front creates "zones", the purpose being that a person walking in front will cause multiple rise/falls in the detection level rather than one when he enters the field of view and one when he leaves.
There's also thermal imaging, which has a video output and gives an entire picture. Much more complicated and expensive, but not nearly as expensive as it used to be.