About light bulbs.
1 the filament of a light bulb can be turned on to whatever desired 
level within almost arbitrarily fast time if one uses either a carefully 
profiled voltage pulse or an active photo feedback circuit. Of course, 
you can't turn it off fast this way.
2 It doesn't matter for the subject at hand because the wavelengths that 
most pir detectors use are blocked by the light bulb glass. So you need 
the glass envelope to heat up at "speed"
A loop of thin wire in air can be heated up rapidly and will cool fairly 
fast.
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Looking forward,
Al Shinn

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