The "Spot Size" of the Measurement
The "spot size" of any given measurement is, in turn, controlled by two variables:
- the "distance to target ratio" of your particular infrared thermometer
- the distance between your infrared thermometer and the target
Typically listed on the thermometer itself, the "distance to target ratio" (or DTR) tells you the diameter of the "circle" of surface area an IR thermometer will measure at a given distance.
For example, an infrared thermometer with a 12:1 ratio will measure the temperature of a 1" diameter circle of surface area from 12" away, a 2" diameter circle of surface area from 24" away, and so on.*
Each time you take a measurement with your infrared thermometer, be sure you either measure or estimate your distance from the target and make a quick calculation in your head to estimate the diameter of the circle of surface area being measured. Some IR thermometers come with lasers to help you make that estimation. Be sure you are only measuring what you want to measure by standing close enough to your target to keep unintended background elements safely out of the "circle."
It is also important to make sure the surface you are measuring does not have large holes like a grate or a grill. An infrared thermometer WILL factor in the surface temp of whatever surfaces are visible through the holes when calculating a final temperature for your reading.
To accurately measure the temperature of a grate or grill, place a solid surface like an iron plate or pan on the grate or grill, let it come to temperature and measure the plate or pan (see Adjusting for Emissivity).
* If your infrared thermometer has a distance to target ration of 1:1 or less, it should be held as close to the target as possible.