Re: how do hand held IR detectors measure temp of cold objects
- From: Joseph Mack <jmack@xxxxxxxx>
- Date: Thu, 17 Dec 2009 02:15:26 +0000
Martin Brown wrote:
Joseph Mack wrote:More precisely, how does an IR detector detect a temp of -20C when it's at 25C inside a hand held gun.
here's one of these devices (for about $30) a Ryobi IR001 SKU 699814
Hi Martin,
If the thing is designed right then it can measure the amount of in band radiation coming from the source. Everything near the sensor needs to be a mirror finish at the active wavelength(s) and the sensor should see the source reflected in as many directions as possible. There will come a point where it cannot measure lower temperatures because its own emissions and signal to noise limitations blind it.
I don't know the bit about metal/mirror finish. So that's like the parabolic dish in a radio telescope then.
If you remember the first detection of the microwave background radiation at 4K it had been collected in and bounced off a huge metal microwave horn at room temperature before reaching the cooled detector. Indeed one of the initial working hypotheses about the noise was that it was dielectric material (notably pigeon droppings) in the horn so they had to give it a very good clean just to be sure.
yes know the story well. I've been to the horn.
Do these satellites have cooled IR detectors? If so how are they cooled?
Do they have a finite amount of liq N2 or liq He and when that runs out
the satellite has to be deorbited (to make room for the next one)?
I expect they use thermoelectric cooling if they need to use any active cooling at all. Cunning designs with heat pipes to shunt heat to the dark side of the spacecraft might be good enough.
so they have a refrigerator dumping out of a heat pipe?
I'm a ham radio operator and have done moonbounce and am familiar with
the concept of the front end FET having a noise temp of 10degK, while being at 300K, but I have to take it on trust. I can't derive the equations myself. However I'm less happy about accepting that an IR
detector doesn't detect its own temperature.
My device has arrived and I've been playing with it. It's spec'ed from -32 to 380C and gets expected temperatures for lights, hot coffee and
-20C for the back wall of the freezer compartment of the refrigerator.
I was quite surprised to point it at cloudless blue sky (the stuff that's at 9000degK for the visible spectrum personal cameras) to get a temp of -40C. (What that means when the device is spec'ed to -32C I don't know, but it must be cooler than -32C). The blue sky is from Raleigh scattering, not Stephan-Boltzmann black body and presumably has little 5-10mu IR in it. I expected the coldest part of the sky to be at 90deg from the sun (max polarization), but the coldest part of the sky was directly overhead (thinest atmosphere?) while the sky above the horizon was about 5 deg warmer (the murk we look through with a telescope at low elevation). At night the (clear) sky is only 5 deg cooler than during the day. I haven't looked at the base of clouds yet.
Pointing at the sun (30deg elevation), only got 50degC. The gun's viewed area is a 1:12 cone (ie tan(cone angle)~0.1) and with the sun's diam=0.5deg (tan(0.5deg)=0.009~0.01). Squaring the ratio shows the sun occupying about 1% of the field of view. With the sun at 5500K, then I guess I'm in the right ball park.
Walking around a lake just before sunset I got about 20C for the sun,
and 10C for the reflection of the sun in the lake. (the air temp was just above freezing). Still pointing at the lake, on either side of the sun, reflecting the blue sky, I got temps of about -10C, rising to about 5C when pointed at reflections of buildings or trees. So water is a good
reflector of 5-10mu IR at angles grazing through to about 30deg.
Surprising to me was the temperature of the unpainted side of metal road
signs (eg stop signs). Regular metal was seen to be its expected temp,
but the road signs (whether facing the sun or away from it in shade),
were -20C or below (possibly I got a reading at -37C but I forget). After some experimenting, I found I had IR detector pointed up and seeing the reflection of sky in the metal sign (although the metal was
brushed aluminium and I couldn't see the reflection of the sky in visible light). The signs were above my head and I had to point the IR gun upwards to get the temp of the sign. I found a sheet of brushed stainless steel (phone box) and by angling the IR detector
to see the reflection of the sky, I got -25C or so. When pointing
the IR gun straight at the brushed stainless so it would see the reflection of my clothing, the temp went back to 5C. So brushed metal is a good reflector of 5-10mu IR, even if it doesn't reflect well at visible wavelengths.
Presumably a nice polished mirror finish inside the detector
chamber wouldn't be seen at all.
The collimator has black baffles, presumably for the same reason
we have them in telescopes.
An interesting toy
Thanks Joe
--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!
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