How does a Hands Free Temperature Measurement Device Work?

Not that simple. It typically measures the ratio of intensity of two different wavelengths of IR light, and gets it from there.

At 'low' temperatures of 150F and if there is no convective contact and it appears to be an optical measurement, then it's likely this is a longwave IR measurement. A great many things can complicate useful measurements, though.

Yes, in the relative energy distribution over wavelength.

If it were measuring the apparent amplitude of a single wavelength, yes. Dual wavelength techniques (or still more than two) can measure the temperature by looking at the "slope" of the energy distribution instead of the simple amplitude and this type of measurement can work through uniform attenuation (so-called gray fog or interference that doesn't act selectively) and still read out reasonably well.

Blocking can be done optically, but I think these things will often read out what amounts to about the hottest target within optical view.

There isn't a single wavelength of light associated with one temperature. There is a distribution that often is taken to follow a "blackbody" distribution. You need to look up Planck's blackbody equation, Wein's and Rayleigh's equations (which preceded Planck's), and take a look at some of the displays of the curves at various temperatures.

Jon

wavelengths

How is this done? Are two different sensors used, which have different wavelength sensitivity? With this info. the ratio is calculated ? boat ranger

I'm not sure what "energy distribution over wavelength" means, can you elaberate? for instance, in the case a single wavelength, how would energy be distrubuted? Does florescence play a role?

boats_ranger

Energy distribution over wavelength means what it says and, nearly by definition, it does NOT mean "single wavelength." The communication doesn't get much clearer than that. I'd recommend looking up those blackbody radiation curves for a start to see what I intend to mean. The pictures may help a lot where my words have failed. You may also find some JAVA procedures that will provide nice plots for any temperature you want to provide. Another term to look up is "emissivity," but only after you get the main ideas down.

Fluoresence can be used to measure temperatures, as well. But it is an entirely different animal and it requires rare earths in contact with the target you are trying to measure.

Jon

heat generates infrared light.

I don't understand the "hands free" part. Do you mean non-contact? I've been messing with non-contact IR thermometers for a couple of years. There's the nasty little problem of emissivity. Seems that most everything I want to know the temperature of is shiny metal with emissivity near zero.

A PEV Thermal Imaging Camera fell into my lap at a swapmeet last week. Kinda cool to see the footprints left behind when you walk barefoot across the floor. Haven't discovered anything useful to do with it yet.

There's some interesting IR reading here:

mike

He means the amount of energy in each wavelength, as measured at different wavelengths. Take a small wire, put it in a glass container in which you replace the oxygen with an inert gas, and put a current through the wire. (Thomas Edison was famous for doing this) It heats up, and if you increase the current enough it starts to glow a dull red. That means it's not just generating heat, but visible light as well. Heat it more, it's bright red, then more and it's orange, then yellow. You probably know from experience that something glowing yellow (because the peak of its emission spectrum is yellow) is hotter than something glowing red (likewise with the peak being red), and this relationship holds at lower temperatures in the infrared range as well. The distribustion part means that when it's yellow it's also got some red mixed in, and also some green, but yellow is the brightest color at that temperature. If you match a distribution curve of color for a certain temperature to the observed levels of colors emitted by an object, the object is at that temperature.

Not in blackbody radiation. The name for this is incandescence.

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I read in sci.electronics.design that Ben Bradley wrote (in ) about 'How does a Hands Free Temperature Measurement Device Work?', on Wed, 23 Mar 2005:

If you find a lot of white powder on the floor, you've fitted a flourescent tube by mistake. They are very common.(;-)