I have one of these IR thermometers:

http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Test-Equipment/Thermometers/Miniature-infrared-thermometer/72890

It works quite well and I've used it quite a bit to help understand what happens with heat in my house. However, while doing this experiment:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2849&page=1#Item_29

I found a bit of a trap with using them.

The first time I tried the experiment I measured the wall temperature and it showed 9 °C. I removed the glazing and measured the temperature where it had been and it, I thought satisfactorily, showed a slightly lower reading. I then swapped over to the area which had not been covered and tried again and it showed a still lower reading and as I watched started counting down in temperature.

My first thought was that the batteries were being affected by the cold as I use rechargeables which are only 1.2 V whereas this device really prefers 1.5 V and only works of the rechargeables when they are pretty much full. However, it does have a battery level warning and that wasn't on and it was cradled in my hand so kept a bit warm - particularly the batteries which are on the bottom.

I tried it indoors and it seemed to work fine. I left it outside for a while then tried it on various things and it seemed to give sensible and stable readings.

My conclusion is that when the thermometer is being used to measure things much cooler than itself the radiation from either the lens or the shield around it is sufficient to mess up the reading. The moral of this story is, therefore, to cool the thermometer down as much as possible before use with cold objects.

At some point I might try wedging some foil round the shield to reduce its emissivity.

Most of these thermometers are calibrated for 95% surface emissivity. I tried my IR thermometer on a hot water copper pipe (clean & shiny with no corrosion). The actual water temperature was ~55C (and the pipe was too hot to hold with my bare hand) but the thermometer showed ~30C. Polished coper & polished aluminum have IR emissivity of <10%.

I thought they looked at the frequency of the radiation, not the intensity. Otherwise a bright cool thing would look "hotter" than a dim hot one.

A quick googling came up with this tip:

3. About Emissivity

This infrared "invisible heat" of an object is transferred in three ways: Reflected, Transmitted, and Emitted. Reflected energy is not particularly related to the temperature of the target object, and transmitted heat is related to the both the internal temperature of the target object and the temperature of the "transmitted to" object.

The only type of energy that can be used to tell that actual surface temperature of the object is Emitted energy.

When IR thermometers are used to measure surface temperature they can potentially sense all three kinds of energy, therefore all thermometers have to be adjusted to read Emitted energy only. Measuring errors are often caused by IR energy being reflected by light sources.

Some IR thermometers allow you to change the Emissivity in the unit. The value of Emissivity for various materials can be looked up in published Emissivity table.

Other units have a fixed, pre-set Emissivity of 0.95, which is the Emissivity value for most organic materials and painted or oxidized surfaces. If you are using a thermometer with a fixed Emissivity to measure the surface temperature of a shiny object you can compensate by covering the surface to be measured with masking tape or flat black paint. Allow time for the tape or paint to reach the same temperature as the material underneath. Measure the temperature of the taped or painted surface. That is the true temperature.

Accurate Temperature Measurement Tips

When measuring shiny surfaces, such as aluminum and stainless steel, the reflectivity of the surface will skew the reading of an IR thermometer unless you accommodate for it. Either coat surfaces such as griddles or cooking pans with a non-stick cooking spray before you take your reading, or place a small piece of masking tape on the metal surface and measure the taped area.

Paul in Montreal.

Posted By: ralphdThey look at only the intensity in the IR (~10 micron) spectrum. What you see as bright is in the visible spectrum (~550nm).
Intensity increases with T^4, but peak frequency changes little (+- a few microns micron) as you go from 250K to 500K (typical range for an inexpensive IR thermometer)

Hmm, I'm not convinced. Intensity is subject to the inverse square law so a distant object's intensity is much less than a close one. So if I have two objects at the same temperature, one that's 1m away will have 100 times the intensity of one that's 10m away. By "bright" above I did mean intensity in the IR spectrum, not visible since the original poster was concerned that the emissivity of metals was rather low (and hence the intensity would also be low).

Paul in Montreal.

Posted By: ralphdYour point about intensity vs. distance is completely true and completely irrelevant to measuring indoor temperatures.

Then how does it work in the case I described? If the thermometer only measures intensity, why isn't it affected by distance?

Last time I played with one was when we had our second ecoEnergy audit done to calculate the effective R-value of a wall that's triple-brick+8" stone construction. It wasn't obvious what to put in hot2000 for this ;) Since the temperature had been stable at around -11C outside for a couple of days, we just measured the surface temperature in a few places to allow an educated guesstimate of the thermal resistance.

Posted By: ralphdIf you are an astronomer it does matter.

If you're an astronomer then you're more interested in redshift rather than absolute intensity I think.

Paul in Montreal.

Posted By: Paul in Montreal

Posted By: ralphdYour point about intensity vs. distance is completely true and completely irrelevant to measuring indoor temperatures.

Then how does it work in the case I described? If the thermometer only measures intensity, why isn't it affected by distance?

It is. The thermometer measures the average intensity of the spot radius you point it at.
If you point it at a wall that is an even temperature, it doesn't matter how close or far you are.
If you point it at a light fixture it will read cooler and cooler temps the farther you go away, but never lower than the average temperature of the surrounding ceiling.

If the light fixture were suspended in outer space and you tried to read it from 10km away, you'd read the ~3K of the background temperature of space.