A word of caution when using these cheap low cost hand held thermometers .
They are sold as a one shot measure everything device but are anything but.
Consult the specifications of the unit to determine the FOV ( field of view ) as this will determine the target size and distance from the point of measurement .
As you can see from above this has a huge effect on the accuracy of any measurement taken.
The target size MUST be greater than the FOV or you will be measuring anything else that is in the FOV , if it is a greater temp. then it introduces a positive error , if it is less then the opposite.
That's the simple bit .
The material and surface of the target is crucial to the accuracy of measurement , this is called emissivity .
Any Infra red measuring device must have an e adjustment to compensate for this .
Rubber , steel ( painted or raw) ,plastic , all types of alloys , water , oil ,SKIN , timber etc, ad nauseum all differ greatly , get the picture ?
I could spend a long long time explaining this .
Basically black is 0 and a shiny reflective surface is 10 but is not linear between the two. This has a HUGE effect on the accuracy and again is non linear across the measurement range of the device .
A crude , but reasonably effective , way around this is to either paint the surface black or wrap it in black tape and set the e value to 0.
Lastly the target "L" in the diagram above is circular . In essence the device takes a cross sectional average of the IR transmission in that area .
If you are not at 90 degrees to the target the circle becomes ellipsoidal and again greatly effects the measurement.
Most , if not all , of these low cost thermometers will have a minimum target size of 50mm + , I have a hand held with a MTS of 9mm and that cost over £3000 ( yep that's right ) ......
Just remember all this when the wee girl at the door of the supermarket you are about to enter points a £15 hand held thermometer at your forehead and says " Ah ,you're OK , on you go ...."
I hope this is of interest to you .