Infrared Thermometers

You need to check for:

  • Incorrect emissivity
  • Dirty lens or window
  • The instrument is not focused correctly
  • The line of sight is blocked with an obstruction.

You need to check for: A. Incorrect emissivity B. Is the instrument seeing reflections from a hot surrounding source C. Is the instrument being affected by electrical noise entering into the cables or other wiring.

For instruments that operate from the visible to 2.6 microns the color will usually change the emissivity. For wavelengths longer than 3 microns the color will not affect the emissivity. However, color does affect heating. Dark objects will get hotter than light colored objects.

As the instrument is placed further from the target, the spot size resolved by the instrument becomes bigger, therefore the target has to be large enough for the instrument to view it.

Targets as small as 0.017 inches can be measured. The limitation is the temperature and emissivity. As the temperature goes lower or the emissivity is lower, then the instrument cannot measure as the spot size becomes too small.

All infrared thermometers have a specific target size they need to see in order to measure the temperature. The target should be 2 times the spot size in order to indicate the correct temperature. In case the target is smaller than the spot the instrument will measure anything that is filling the remainder with the target. This is not true of a two-colour instrument.

No, the infrared thermometers cannot see thru these interferences. However, in most industrial applications the dust and smoke are rising from the hot object and, if your eye was as fast as the IR thermometer you can see openings where the instrument has a clear line of sight. With the aid of a function called a peak picker the instrument can indicate the target temperature and ignore the cold readings caused by the dust and smoke.

Yes, the window has to be transparent for the wavelength of instrument you are using. For 1 to 2.6 microns we suggest quartz, for 3.4 to 5.4 microns we suggest Calcium Fluoride.

Yes, but you have to consider the problem of reflections. Usually the oven is hotter than the target. The target has some reflectivity so the thermometer measures the emitted energy as well as the reflected energy and indicates too high. This can be eliminated by using a sight tube, or possibly measuring the target at the exit of the oven. For glass, plastic films, and paper applications selection of the right thermometer can eliminate the reflection problem without a sight tube or looking at the exit of the oven.

If the scale is tightly bonded to the steel it will usually have no effect because it is the same temperature as the hot target. However, if the scale breaks loose it will cool and cause a cold spot on the target. If the instrument is a single wavelength instrument it will indicate a low temperature. If the instrument is a two color and only a portion of the spot is filled with the cold scale it usually will not affect the temperature indication.