Re: radiometers (toy) as IR meter

[brian whatcott <inet@INTELLISYS.NET>]

At 23:13 11/27/98 -0800, Leonardo wrote:
> Greetings,
>
> I'm working on a lab/science-project the goal of which is, for common
> transparent materials, to compare their %visible transmission with
> their %infrared transmission ....
> Finally, I set up a radiometer--the toy type we often use in
> class--and a strobe and found that the rpm of the spinning vanes is
> indeed both easily measureable and is inversely proportional to the
> number of layers of plexiglass between it and the infrared lamp.  Has
> anyone done this?  Comments?
>
> Any suggestions for an artificial source suitable for the project?
...
> Thanks, LEon Leonardo


I salute the Physics teacher who is evidently doing experimental physics.
 Possibly the most stable sources for Leon's purposes would be two LEDs.
(20 mA at 2 or 3 volts is a high power feed: a 12 volt supply through a 500
ohm resistor would suffice)
 The visible spectrum is easily handled with a variety of red or yellow
LEDs which can these days be specified at up to 10 candela from your local
friendly RadioShack. (Near) IR LEDs are not much harder to obtain from
electronics distributors. The emissions of these devices may be too low to
drive Crookes's radiometer, however.
  As a higher-power alternative, an incandescent filament lamp (a halogen
headlamp capsule, possibly?) would provide more copious amounts of
broadband IR
if under run to provide a just invisible filament viewed in darkness.

 At least one list contributor will certainly berate me if I make the
mistake of associating "special" heating properties to IR radiation. He
would remind me that the energy associated with em radiation is well, the
energy associated with it in the ordinary way: Planck, Einstein and all
that....

 Poor Dr. Crookes must turn in his grave to think his immortality rests on
the status of his radiometer as a scientific toy. He understood that the
basis of his radiometer was thermal, so it seems to me that this is a
sensor that easily appeals to the youngest observer.   I wonder about pivot
friction affecting the very lowest radiation levels detectable - one thinks
of the virtues of a pair of silicon or germanium devices, one shaded from
the radiation, and connected in opposition to an op amp.


brian whatcott <inet@intellisys.net>
Altus OK