Infrared thermometers measure temperature usingInfrared radiation emittedfrom objects.
They areoften refered to as laser thermometers The laser is simply used to aim the thermometer and give you guidance as to where you are measuring the temperature. Depending on the "distance to spot ratio" the further you are away from an object the larger area the infrared thermometer will be looking at! See examples below:
Distance to spot ratio of 10:1 (assuming the optical lens is 16mm in diameter)If you were 1000mm away from the object you are measuring you measurement area woudl be 116mm in diameter. At 5000mm it woudl be 516 mm in diameter.
Distance to spot ratio of 30:1 (assuming the optical lens is 16mm in diameter)If you were 1000mm away from the object you are measuring you measurement area woudl be 36mm in diameter. At 2000mm it woudl be100 mm in diameter.
So you can see the bigger the distance to spot ratio the smaller the measurement area is at a distance.
Emissivity
The emissivity of a material (usually written or e) is the relative ability of its surface to emit energy by radiation.
Alot of Infrared thermometershave a fixed emissivity at 0.95 which is fine for the majority of applications however great errors can be introduced into a readign when the emissivity is set incorrectly for a particualarobject. for example cast iron oxodised cast iron has a emissivity of 0.95 however unoxodised has an emissivity of 0.21.
Determining an emissivity can easilly be doen using a ordinary thermometer to measure the actual temperature of the object when at the same time you adjust the emissivity of the infrared themperature gununtill they read the same.You have now determined the emissivityfor that object.