An infrared camera measures and images the emitted infrared radiation from an object. The fact that radiation is a function of object surface temperature makes it possible for the camera to calculate and display this temperature.
However, the radiation measured by the camera does not only depend on the temperature of the object but is also a function of the emissivity. Radiation also originates from the surroundings and is reflected in the object. The radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.
To measure temperature accurately, it is therefore necessary to compensate for the effects of a number of different radiation sources. This is done on-line automatically by the camera. The following object parameters must, however, be supplied for the camera:
- The emissivity of the object
- The reflected apparent temperature
- The distance between the object and the camera
- The relative humidity
- Temperature of the atmosphere
The most important object parameter to set correctly is the emissivity which, in short, is a measure of how much radiation is emitted from the object, compared to that from a perfect blackbody of the same temperature.
Normally, object materials and surface treatments exhibit emissivity ranging from approximately 0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while an oxidized or painted surface has a higher emissivity. Oil-based paint, regardless of color in the visible spectrum, has an emissivity over 0.9 in the infrared. Human skin exhibits an emissivity 0.97 to 0.98.
Non-oxidized metals represent an extreme case of perfect opacity and high reflexivity, which does not vary greatly with wavelength. Consequently, the emissivity of metals is low – only increasing with temperature. For non-metals, emissivity tends to be high, and decreases with temperature.
Finding the emissivity of a sample
Step 1: Determining reflected apparent temperature
Use the following method to determine reflected apparent temperature:
- Crumble up a large piece of aluminum foil.
- Uncrumble the aluminum foil and attach it to a piece of cardboard of the same size.
- Put the piece of cardboard in front of the object you want to measure. Make sure that the side with aluminum foil points to the camera and is at the same angle and orientation as your object.
- Set the emissivity to 1.0.
- Measure the apparent temperature of the aluminum foil and write it down.
Figure 1. Measuring the apparent temperature of the aluminum foil. (1 = Source of the Reflections)
Step 2: Determining the emissivity
- Select a place to put the sample.
- Determine and set reflected apparent temperature according to the previous procedure.
- Put a piece of electrical tape with known high emissivity on the sample.
- Heat the sample at least 20 C (36 F) above room temperature (the higher the better). Heating must be reasonably even.
- Focus and auto-adjust the camera, and freeze the image.
- Adjust Level and Span for best image brightness and contrast.
- Set emissivity to that of the tape (usually 0.95).
- Measure the temperature of the tape using one of the following measurement functions:
- Isotherm (helps you to determine both the temperature and how evenly you have heated the sample)
- Spot (simpler)
- Box Avg (good for surfaces with varying emissivity).
- Write down the temperature.
- Move your measurement function to the sample surface.
- Change the emissivity setting until you read the same temperature as your previous measurement.
- Write down the emissivity.
- Avoid forced convection
- Look for a thermally stable surrounding that will not generate spot reflections
- Use high quality tape that you know is not transparent, and has a high emissivity you are certain of
- This method assumes that the temperature of your tape and the sample surface are the same. If they are not, your emissivity measurement will be wrong.