In our previous installment of this series we discussed some of the challenges of using infrared temperature sensors (pyrometers) when your target is behind a window. In this article we’ll describe a far more common problem – emissivity.
Emissivity describes an object's ability to emit thermal radiation or infrared energy. It’s measured on a scale from 0 to 1. Since emissivity is expressed as a ratio it can be explained through the following formula:
At the highest end of that spectrum is a perfect blackbody with emissivity of 1. Objects at the lower end of the spectrum, or near 0 emissivity are typically shiny mirrors and metals. As you may have guessed from the title of this article it can be quite challenging to get an accurate temperature measurement using infrared sensors on objects that have near zero or zero emissivity.
The Perfect Blackbody
Blackbody is an important term to understand if you want to have a basic understanding of Emissivity. It’s pretty much exactly what it sounds like, a black object that has emissivity of 1. The “perfect” blackbody, doesn’t really exist, but we can get pretty close. While it’s unlikely that you’ll be attempting to measure the temperature of a blackbody in your application, it’s very common to calibrate infrared sensors using a blackbody calibrator.
Other terms that you might hear as you explore emissivity are whitebody and graybody. A whitebody object is the inverse of a blackbody, it’s a perfectly reflective surface that has 0 emissivity. Since blackbody and whitebody are theoretical terms, we describe all other objects that are found in nature as graybody’s.
Emissivity of Objects
There are generally four factors that play into the emissivity of an object:
Material of an object
Metals tend to have lower emissivity than less reflective objects like rock or wood.
Surface of an object
Rough or oxidized surfaces will generally have higher emissivity than a polished surface.
Wavelength of the radiant energy emitted from an object
Some objects can have pretty drastic changes in emissivity at different wavelengths.
Temperature of an object
Planck's law describes how total energy emitted from an object increases as temperature increases.
Below is a list of objects and their emissivity at different wavelengths:
|1.0 µm||1.6 µm||5.1 µm||8-14 µm|
Alloy A 3003
|Iron Wrought Dull||0.9||0.9||0.9||0.9|
|Paper (any color)||NA||0.95||0.95||0.95|
Over 20 mils)
** It’s not recommended to attempt to measure radiant energy of materials with NA at a specific wavelength.
** For more information, refer to https://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/emissivity_table.pdf
Getting an accurate temperature reading of objects with low emissivity
As you can see from the table above, emissivity issues typically come into play when you’re attempting to measure radiant energy of metals. For most metals that have low emissivity, they often have slightly higher emissivity at smaller wavelengths, which will increase the accuracy of your readings. Not all IR sensors can read small wavelengths so you’ll have to have an instrument designed to handle that type of application. If you’re in the market for this type of instrument, IOThrifty has a few options:
- IR-CA-PUA2-151-LT: 2.0 - 2.6 μm
- IR-CA-PUA2-751-HT: 2.0 - 2.6 μm
- Lo E Version (metal): 0.1 - 5 μm
FibreMini Fiber Optic Pyrometer for Harsh Applications: 2.0 - 2.6 μm
Additionally, many instruments have an emissivity adjustment. Looking at the table above you can see how some materials will have a different emissivity at different wavelengths. You can use this information to set your sensor's emissivity, but you may want to check with the manufacturer of the material since different formulations of the material may result in a different emissivity.
For some materials, adjusting the instrument still won’t yield an accurate reading. In these cases, we’d recommend painting the object if possible. This solution won’t work for every application because most paints won’t handle extremely hot temperatures, but when you can paint the object you’re attempting to measure, it’s typically very effective.
As always, if you have any questions or require any assistance please reach out to IOThrifty’s team of experts.