Emissivity tables may or may not contain real useful information concerning the actual emissivities of the objects you wish to measure.
There can be many variations within and among different emissivity tables. Here are some factors of concern:
- Total Normal (broadband, perpendicular) emissivity - This is the emissivity over a very wide waveband. It may or may not be close to the actual emissivity with respect to your infrared camera.
- Midwave emissivity - Some tables are listed as shortwave (now called midwave) or at a specific narrow short waveband. Even if the emissivity is specified to be within the same spectral waveband as your infrared camera, it still may not be as accurate as you suspect. This is due to differences in camera detector responses.
- Longwave emissivity - Longwave tables can also be somewhat unreliable for the same reasons described for the shortwave. Older cameras used different detectors that had different responses within the long wave band.
- Narrow waveband - A narrow band can be just as unreliable as a wide band. Some materials can have significant changes in emissivity over small wavebands.
- Temperature - Some tables take into account the temperature of the object when the emissivity was measured. If you consider the previously mentioned variables, this does not necessarily make the tables any more reliable.
- Conditional (rough, smooth, corroded, rusty) - Conditional parameters seem to offer useful information concerning emissivities, but it is sometimes quite difficult to ascertain the condition of a metal surface by looking at it. If you use an emissivity table to determine the emissivity of copper, you may find values ranging from 0.05 to 0.86, depending on the surface. Copper that appears to be very tarnished can still have an extremely low emissivity.
So, what is one to do regarding emissivity tables? First, realize their limitations. They can offer a ball park estimate. If you really would like to use emissivity tables, the best thing to do is to create your own based on measurements taken with your camera.
Click this link for instructions on how to measure emissivity.
