Understanding How Infrared Thermometers Work

[cragar]

You know those hand held laser gun thermometers , I was reading how they work and it says that the material you point it at is emitting infrared photons , ok so does the gun shoot infrared photons at the material and then they bounce back and it measures the change in energy or does it just receive the infrared photons from the material .

 

[Stonebridge]

No it just detects the i.r. radiation emitted (not reflected) from the object you are measuring the temperature of. The laser beam is to help you aim it.

 

[cragar]

ok so it just receives the infrared photons from the material. how does it not get interference from the air molecules or other surrounding materials.

 

[mgb_phys]

Air is reasonably trasnparent to these infrared bands, especially since you are only using it a fraction of a meter from the target.
Since the air doesn't absorb nmuch IR it also isn't emmitting much IR.

The surroundings are also usually colder than the object you are checking.
If you are trying to measure body heat from a distance on a hot humid day in the tropics then the thermomenter isn't very accurate.

 

[Yeti08]

how does it not get interference from the air molecules

By being sensitive in what are known as "atmospheric windows" where the atmosphere is mostly transparent, the device can usually dismiss any air effects. The common windows are 3-5 micron and 8-12 micron, give or take (handheld bolometers are usually sensitive in 8-12 um). Imaging IR detectors will actually take into account the atmosphere by using distance and humidity as parameters.

or other surrounding materials.

You do need to be careful about field of view - having other objects in the FOV will skew the temperature. Another important factor is the emissivity of the object in question; if it has a very low emissivity, and therefore high reflectivity, you will get very inacurate readings, often the temperature of yourself or other hot/cold objects in the room. Many handhelds assume the emissivity is 0.95, which is rarely correct.

 

[cragar]

ok thanks for your answers guys . it helped a lot

 

[cragar]

ok one last ? so no matter what the material , let's say that we have steel and water both at 99c , both the water and the steel will be emitting the same amount of energy in the form of infrared photons . It seems like different materials would emit different amounts of photons even though they are at the same temperature.

 

[mgb_phys]

The emissivity of different materials (a black surface vs a shiny one) will emit different number of photons at the same temperature.
An accurate thermometer could use 2 (or more)different wavelengths to get a fit for the black body peak, but generally they just assume they are used for some fairly high absorption surface (skin or metal)

 

[Yeti08]

To expand a little on what mgb_phys said, water and steel at the same temperature will not be radiating the same amount of energy because energy emitted is proportional to the emissivity, and water will typically have much higher emissivity than steel. To make matters more complicated, the emissivity isn't actually constant in real life (treating it as constant emissivity is said to be grey body, an emissivity of one is a blackbody), but rather it is dependent on temperature of the object, wavelength, and angles. This may be more than you need to know to use an IR thermometer, but it is important to know how to take measurements correctly. For example, on a metal box that had a true temperature of about 30C I measured temperatures from 1000C (a reflection of the sun) to -30C (reflection of the sky) at the same time.