What’s happen family? In today’s blog I am going to briefly talk about some basics of infrared emissivity and why it is important to the infrared practitioner using thermal technology. I love when people keep things simple, so I like to keep things as simple as possible when trying to explain concepts. With that being said, I found a very simple definition of emissivity at optotherm.com. They defined emissivity as: “…a term describing the efficiency with which a material radiates infrared energy.”
Blackbody and the Continuum of Emission and Absorbption
Ok, now we know that emissivity is simply the level of efficiency at which a material radiates infrared, so what is blackbody? When studying emissivity or the functionality of thermal technology, the term blackbody will be used eventually. When a person uses the word black body they are talking about an object with 100% emission and absorption of thermal energy.
Perfect blackbodies do not exist. However, there is organic matter such as the human body that is very close to perfect blackbody. The emissivity of the human body is between 0.98 and 0.99 depending on your source of information. This simply means that the human body both emits and absorbs 98 to 99 percent of thermal energy.
On the other end of the spectrum are materials that neither absorb or emit energy and they have an emissivity of zero or zero percent. This means the continuum of infrared emissivity is from 0-100%. An example of a material with low emissivty is silver. Silver has an emissivity of 5-10 percent. If we took silver or another polished metal with a low emissivity and applied heat to the material, what we see through an infrared camera would not be an accurate representation of the actual temperature of the metal.
Techniques to Adjust for Emissivity
Some techniques to compensate for materials with low emissivity such as the example with the silver stated above includes coating the surface of the object or adjusting the emissivity settings on the thermal device being used. Using paint on the surface of the silver or low emitting material will counteract the skewed apparent reading of the thermal device being used. Another material that can be used is black electric tape.
Tape used on surface of the silver (from this example) or low emissivity material will also provide a more accurate temperature reading because the tape possesses a higher emissivity than the silver thus allowing the thermal camera to read the infrared energy on the section of the surface the tape is located.
Another way is to simply adjust the emissivity of the infrared camera that you are using. Many infrared cameras on the market come from the manufacture pre-set at an emissivity of 0.95 or 95%. The assumption is that 0.95 of the thermal energy is coming from the object or material being observed.
For example, if there was a one ounce gold coin that was heated to a known temperature and a person knew in advance that the emissivity of gold is 10%. Then, the person can set the emissivity of their thermal camera to 0.1 to get an apparent temperature reading with their infrared that is much closer to the actual temperature of the heated one ounce gold coin.
Why is this Important?
Knowing the variable of emissivity is important because it will prevent the user of infrared technology from coming to incorrect conclusions. A person using this technology in a commercial capacity without the awareness of materials and their different levels of emission and absorption of thermal energy and its impact may incorrectly advise or unnecessarily alarm a client. In short, materials matter and can skew results, so it is sometimes up to the expert to correctly interpret these anomalies.
Those that we provide service to hire us for our expertise and experience. Knowing the equipment that we are working with and their limitations and capabilities help us to better consult, provide solutions, and advise clients.