At 05:30 PM 11/16/97 +1300, you wrote:
>> Does anyone know how non-contact thermometers work? They are now coming
>> down in price - there's even one for domestic use that stick in your ear.
>
>They use the same sensor as the garage door opener.
>
>Everything above absolute zero emits radiation that is dependant on its
>temperature. In the case of stars (which are really really hot), the light
>is visible. Blue stars being hotter than yellow stars, etc. At temperatures
>that are a bit more comfortable, that light is in the infrared range. The
>temperature of the object can be determined from the amount and wavelength
>of the emitted radiation.
>
>The sensor is a crystal (Lithium Tantalate is the most common) that will
>generate a voltage as it heats up from the IR radiation that falls on it.
>The voltage leaks away very quickly, so the output is as if it was
>capacitively coupled. The voltage is very small so the part usually has an
>integrated JFET amplifier.
>
>Because of the AC coupling aspect, the output is measurable if the IR
>radiation changes rapidly. In the case of the garage door opener, a
>multi-faceted lens is put in front of the sensor. As the subject moves
>across the field, the IR image will move into the field of view on one
>facet, then out of field, then into field of the next, and so on. The
>effect is an IR image that appears to the sensor as on-off-on-off, etc (ie
>a square wave). The derivative of that is a series of spikes that trigger
>the door opener.
>
>To measure temperature with the same system, the standard approach for
>years was to put a mechanical shutter in front of the sensing element.
>Something that would alternatively expose and hide the sensor quite fast
>compared to the droop rate of the voltage. By measuring the voltages in
>both the covered and uncovered state, you can determine the amount of heat
>reaching the sensor from external sources.
>
>It took me ages to find out how this was done without any moving parts, but
>it's pretty obvious now.
>You can put the sensing element into an oscillator, much like a quartz
>crystal except that the frequency will vary with the amount of IR
>radiation. Then you have something to measure. Simple really.
>
>Steve.

How do they measure temperature, though? It seems to me that the
temperature would govern the wavelength directly, not the intensity of the
IR radiation. The intensity is related to the quantity of radiant heat,
which would be a function of the surface that it was being radiated from,
etc. So how does one get temperature from measuring the magnitude of the
voltage generated on the sensor? It seems to me that this is like saying
that you can sense color by measuring the amount of voltage generated by a
solar cell.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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