Hi,
I'm curious how infrared thermometers work. Here is my hypothesis: There must be some simple photodetector -- I looked in electronics catalogs and didn't find it-- that can sense the range of IR light (8 to 12 micrometers?) that is radiated heat. This is surely placed at the end of a tube and only IR light that enters is from whatever object is being measured. This is then converted to a voltage and read with a D to A converter by a microcontroller and displayed.
More or less?
Thanks.
Something like that, and I am no expert. There are things to consider. When a blue laser hits a target, the color that best absorbs the blue is going to dissipate the most heat. I didn't understand this until i was talking to somebody at Coherent, while I was playing with a $15K laser here. I thought infra red lasers are inherently hotter, but thats not the case.
greg
D to A? Don't you mean A to D?
Anyway, it doesn't work by measuring the amplitude of the IR. If it did then the reading would be sensitive to the distance from the detector to the source (and it's not).
If you do some reading on "black body radiation" then you'll be able to figure it out.
Bob
snipped-for-privacy@comcast.net (G) wrote in news:fh2bfm$c83$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:
May be a microbolometer array or single microbolometer. (essentially a tiny antenna sized for the particular wavelength to be detected,etched in silicon) Texas Instruments makes them.
IR lasers simply emit a longer wavelength of light. "IR" begins at ~850nm.
I suspect that the laser simply tells the user where the narrow-beam optics for the bolometer are looking. the laser itself is not used in the IR measurement.
-- Jim Yanik jyanik at kua.net
More or less. The sensor is called Thermopile and basically is a bunch of micro thermo elements stacked together. The generated voltage is extremely small, in the microvolts.
Rene
-- Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
It _does_ measure the intensity of the IR. It's not sensitive to the distance from the detector because as you get farther away from the source each unit area of the source affects the bolometer as 1/distance^2, but the area that the bolometer "sees" goes up as distance^2, so it evens out.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Optical wavelength is measure in microns. A micrometer is a measuring tool. A micron is one micrometre
There are plenty of instruments around now, and before A to D was big, that are 100% analog.
It surely does read the amplitude.
Not necessarily.
That would depend on target size.
The standard, non imagery based IR thermometry device is the resistor bolometer. It measures IR amplitude, and most devices I have had experience with utilize a 2mm diameter target surface approx 2mm below the "window" on the package that the bolometer is in.
Sure, there are array configurations for tracking missiles and the like, but single element devices I have seen were configured this way.
"Looking at" a close up target (say 5 inches in diameter, ten inches away), the bolometer's image target surface would likely be flooded with the IR energy it will be giving a reading on.
The same target, ten feet away, might be small enough that the image projected onto the bolometer's image target surface by the instrument optics takes up less area than the entire surface. The reading, however, should still be accurate if the optics are of good design and focus is maintained. The bolometer measures absolute temperature, not bombardment energy. Pointing one at the sun is usually a sure fire way to burn up your instrument's primary transducer (the bolometer), especially if optics are involved...
Plank's Law governs the energy levels. Surface quality and medium type govern the emissivity levels.
We made two instruments that were to measure a 200C change in a one foot spot 1000' away with a 1 degree FOV. That's a ten foot spot.
We had no way to calibrate them at full field focal length. So we did it at 100 feet with a much smaller target.
They worked fine in the field, and as far as I know, are still pointed at the main launch pad at Kennedy SFC. They are in two small shacks
1000' from the pads and are pointed at what are referred to as "protected zones", and they are meant to sense a breach by hot thrust gasses during every launch.So, the energy is less, but the temp is the same.
How do you think we can tell the surface temps of distant stars, when there is very little energy delivered to us from them?
--a question that doesn't have a simple answer :-)
The only direct comment I'll give is to your wavelength range: 8 -- 12 micrometers is used for low temperatures at (usually) low accuracy.
(Yes, CoB, "micr Sheesh. Three different answers. All correct. All incorrect. Those calling the others wrong are at least as wrong, and the opposing views are at least as right.
All these temperature sensors measure the power of absorbed radiation incident upon the sensor itself. From this measurement of absorbed incident power, you can infer the temperature of the source, _provided_ you know, among other things:
--the emission properties of the emitter (what you're trying to measure)
--the transmitting properties of the propagating medium
--the absorption properties of the sensor
--the ratio of emitting source area to surrounding field of view
--the temperature of the surroundings
--etc., for characteristics having lower impact upon measured accuracy.
The bolometer is one such sensor. It absorbs a wide range of radiation, and integrates its measurements by converting the total absorbed power to a local temperature. If you know its absorption spectrum, and its specific heat, and the temperature of its surroundings (into which it's radiating its own energy due to the absorbed heat), you can infer the total radiation power incident upon it. If, then, you know the transmission characteristics of the propagating medium (for instance, air and the lens system), you can infer the emitted power per area of the emitting source you're hoping to measure. If, finally, you know the emitting spectral characteristics of the emitter, you can infer its temperature.
The various semiconductors absorb radiation and either convert it directly to electrical energy or change their own characteristics, for instance, conductance, in a predictable fashion. Then all the external considerations I listed above apply before you can infer a temperature.
The thermopile is a string of thermocouples that generate a voltage based on the temperature of the sensor (bolometer) they are embedded into. Then all the external considerations I listed above apply before you can infer a temperature.
The typical garden-variety IR thermometer makes a boatload of assumptions about all these characteristics and takes a more or less educated guess as to what the emitter's temperature is. For most common situations, you know the absorption characteristics of the sensor and the propagation medium (air and lenses) pretty accurately. Since different emitters typically are different in emission, and sometimes very, very different, the emitters are usually a problem. Thus, you'll find that many high-quality IR thermometers have an adjustment for emission characteristics.
And BobW was right about the total amplitude not being accurate -- at least in some sense. That's why the higher-quality radiation thermometers use the two-color method. This, too, has to deal with emission, transmission, and absorption spectral characteristics. But the real problem area, the emitter's characteristics, are much easier to sort out with the ratio of two appropriate wavelengths.
And, of course, if you can get close to a black-body source for your emitter, the two-color method gives you a precise, accurate fundamental temperature measurement -- after you have accounted for the field of view, transmission medium, and sensor absorption characteristics.
John perry
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