Emissivity

• Definition: A measurement of a material’s effectiveness at emitting thermal radiation relative to a perfectly-emitting object, known as a blackbody. Which is a surface that absorbs all radiant energy falling on it. Emissivity is expressed as a value between zero and one.  

• Benefits (Why Bother?): Since infrared radiation is a known function of temperature, if we can measure the radiation coming from a surface we can infer its temperature—as long as we also know the emissivity of the surface, so that we know how much radiation we should expect at a given temperature, relative to a “blackbody”.  An infrared thermometer does exactly this, by measuring incident infrared radiation and accounting for emissivity.

Generally, the closer a material’s emissivity rating is to 1.00, the more that material tends to absorb reflected or ambient infrared energy and emit only its own infrared radiation. Most organic materials, including the by-products of plants and animals, have an emissivity rating of 0.95. These are ideal surfaces for accurate temperature readings.

Substances with very low emissivity ratings, like highly polished metals, tend to be very reflective of ambient infrared energy and less effective at emitting their own electromagnetic waves. If you were to point an infrared thermometer with fixed emissivity at the side of a stainless steel pot filled with boiling water, for example, you might get a reading closer to 100°F (38°C) than 212°F (100°C). That’s because the shiny metal is better at reflecting the ambient radiation of the room than it is at emitting its own infrared radiation.

• Method (How to Do It): Most of our IR thermometers have an adjustable emissivity setting, although for some (such as IRFS) this setting is fixed.  If the setting is adjustable, make sure it is set to the correct value for the surface to be measured (for most organic materials, an emissivity of 0.95–0.97 is appropriate, but consult an emissivity table if necessary).  Then aim the IR thermometer at the surface and get the reading. The thermometer will automatically adjust the value on the display to compensate for the adjusted emissivity value. 

• Related Notes and Links: Table of common emissivity values, blackbody radiation wiki, https://www.thermoworks.com/infrared-tips-what-is-emissivity/, https://www.thermoworks.com/learning-center/, https://blog.thermoworks.com/thermometer/how-to-use-an-infrared-thermometer/

• Recommended Tools: Any infrared thermometer. One can also use a Blackbody Target, infrared calibrator to test or calibrate infrared thermometers.