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Re: [Phys-l] IR thermometer buy

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


Brian - I suppose you are one of the several who don't read my posts, or could be memory lapse?

Normally I'd reply to you individually, but, as I find you are not the only one, I didn't.
Here's the relevant (plus some more) portion of my post of 2007/01/29 12:17

----------------------


I woke up from either a dream or hypnagogic state a few days ago w/ an answer. One of which is in the Vernier instructions and directly addresses LK's point. i.e. the subject temperature area must fill (better 1.5 => twice) the view field as defined by the LASER's pattern. LK's point refers to point source or at least far field. One must use near field wherein the energy received is independent of the distance. How it interprets the received energy is the other question. My quick calc. * of the emitted energy from a source at zero deg. convinced me the 0.6. => 1.4 micron window could not be correct. I suspect synchronous detection would be necessary! Following JG's implied suggestion I found the majority of Omega's (and other's) detectors use an ~ 8 to 14 Micron band. A vis./near IR one is used at high temps e.g. ~ > 2k deg. My other question related to how the thermometer would compensate for the deviation from T^4 that would come from detecting only a small portion of the total bandwidth. I finally found the below which also answered by question on the detector. [My old source suggests only doped Ge, TC, and thermisters would be suitable.]

http://archives.sensorsmag.com/articles/1099/80/main.shtml

* Off scale on my GE slide rule. Lazily I used:

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.html#c1

bc

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Not incidentally the Vernier is superior to all [ear thermometers, etc. excepted, as not applicable] the IR thermometers I've seen in that it defines the required area w/ its LASER pattern. Again not incidentally they all give the "field of view".

A BTW: I've noticed the majority of low temp. (as opposed to steel production, etc.) thermometers use ~ the same spectrum as the common IR camera. which would, obviously lead one to believe the only difference is the thermometers are one pixel while the cameras have many.

I've been playing w/ my Sony "nightshot" camera and find it detects quite well hot objects that are invisible in a dark room even tho its detector is supposed to fail above 1.2 micrometer. I can use it day time outdoors using a Kodachrome filter (pass ~ 0.8 => ? microm)
bc, sometimes thinks "Why bother?"

p.s. Often I point one at the sky including the night I purchased one from Radio shack during my walk home. If the night is clear it's off scale below zero. Today about 1100 I pointed it at the sky away from the Sun and was a bit surprised it read somewhat below zero. Unusual, but eh sky was unusually "clean; a recent rain (cleared of "dust"), but probably very dry.


Brian Whatcott wrote:

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!

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