# How does a Hands Free Temperature Measurement Device Work?

Discussion in 'Electronic Design' started by [email protected], Mar 11, 2005.

1. ### Guest

I was at the price club a few days ago and there was a device that
allow
a hands free temperature measurement. I believe it can measure down to
150f
(don't quote me).

In general how is the temperature measured? Does ambient light contain
temperature info, does it need to be compensated for? How does it
ignore the temperature of the objects surrounding it? If the
temperature is XXX degrees f, what is the corresponding wavelength
of light?

Boats_ranger

2. ### Ian StirlingGuest

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

3. ### Jonathan KirwanGuest

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

4. ### Guest

.Not that simple.
How is this done? Are two different sensors used, which have different
wavelength sensitivity? With this info. the ratio is calculated ?

boat ranger

5. ### Guest

Does ambient light contain temperature info?
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

6. ### Jonathan KirwanGuest

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

7. ### JamieGuest

heat generates infrared light.

8. ### Ben BradleyGuest

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.
[Fluorescence. I once worked for a company that makes a lot of
fluorescent light fixtures]

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

9. ### John WoodgateGuest

I read in sci.electronics.design that Ben Bradley
If you find a lot of white powder on the floor, you've fitted a
flourescent tube by mistake. They are very common.(;-)

10. ### mikeGuest

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:
http://www.x20.org/thermal/DemoIrCameras.htm

mike

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