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Re: [Phys-l] IR thermometer buy



At 04:51 PM 1/24/2007, Ludwik, you wrote:

On Jan 24, 2007, at 3:31 PM, Bernard Cleyet wrote:
> I doubt a $10 IR thermometer. is that sophisticated *, even in this
> 21st. cent. They likely make use of the T^4 relation,

What is proportional to T^4 is the total intercepted intensity. But
that intensity is also proportional to 1/r^2, where r is the distance
from the source. Therefore, the reading, for a given T, would change
with r. Does the indicated T change when r is changed?

The only things that do not change with r are the location of the
maximum and ratios of partial intensities at two different wavelengths.
My guess is that T is determined from a ratio of two partial
intensities. Some kind of a filter, and two detectors, would make this
possible, at least in principle.
_______________________________________________________
Ludwik Kowalski


Ludwik's question, "Does the indicated T change when r is changed?"
has been hanging in the air for some time.

I noticed yesterday that when I pointed the $10 Cen-Tech
IR thermometer at the sky, covered at that time with thin stratus,
the indication was 1.8 degF , and when pointed at the blacktop
under that sky, the temperature indication was 48 degF

This model has a limited range, but I found it a useful reminder
about the temperature of the dark or even blue sky.

But to answer the question more reasonably: I found that doubling
the distance to a diffuse reflecting wall, the temperature
indication was, as they say, "sensibly constant".



Brian Whatcott Altus OK Eureka!