Emissivity is the ratio of the energy emitted by an object at a given temperature to the energy emitted by a complete radiator (also known as a black body) at the same temperature. The emissivity of a black body is 1.0. All emissivity values are between 0.0 and 1.0. Most infrared thermometers can compensate for different emissivity values for different materials. In general, the higher the emissivity of an object, the easier it is to measure an accurate temperature via infrared. Objects with very low emissivity (below 0.2) are more difficult to apply this method. Some polished, glossy metal surfaces, such as aluminum, reflect a lot of infrared light, making accurate temperature measurements sometimes impossible.

 

Five ways to measure emissivity

To ensure accurate temperature measurements, there are five ways to determine the emissivity of a material:

Using a precision sensor, a sample of material is heated to a known temperature, and the temperature is measured by an infrared instrument. Then force the indicator to show the correct temperature by adjusting the emissivity value.

For relatively low temperatures (below 500°F), masking tape with an emissivity of 0.95 can be measured. Then force the indicator to show the correct material temperature by adjusting the emissivity value.

For high temperature measurements, drill holes (at least 6 times the diameter) in the test object. This hole is equivalent to a black body with an emissivity of 1.0. Measure the temperature in the hole, then adjust the emissivity to force the indicator to show the correct material temperature.

If the material or part of it can be painted, a layer of dull black paint with an emissivity close to 1.0 can be applied. Measure the temperature of the painted part, then adjust the emissivity to force the indicator to show the correct temperature.

Normalized emissivity values for most materials are available. These values can be entered into the instrument to estimate the emissivity value of the material.