188.8.131.52. Thermography camera calibration with reference radiator
Even rather cheap microbolometer cameras have a sensitivity of <0.1 °C. This means that they can sense temperature differences between different areas within a picture with this thermal resolution. Measurement accuracy of most cameras, however, is only 2°C or 2 % (whatever is smaller). This is true for cameras with both photon counting and microbolometer detector chips. In special cases, for example if macro lenses are used, these specifications may not be valid some time after the camera was turned on or at certain ambient conditions (Stabentheiner et al., 2012). Only a few models offer an accuracy down to 1°C. It has to be kept in mind that this refers to the internal instrument accuracy. Errors resulting from wrong input of surface emissivity (see section 184.108.40.206.) or environmental data (ambient temperature, relative humidity, distance to the measured object) may add to these values. Therefore, calibration with an external reference radiator is indispensable if an accuracy of better than 2 °C has to be guaranteed.
A simple precision cavity black body reference radiator of high accuracy (<0.2 °C) can be constructed by immersing a hollow metal cylinder (e.g. brass) in a regulated laboratory water bath (Stabentheiner and Schmaranzer, 1987) or, for field use, in a thermal coffee pot filled with warm water (Fig. 7). Its inner surface has to be covered by matt 'black' lacquer (infrared emissivity should be at least 0.95; e.g. Nextel Velvet Coating 811-21). The cavity increases the apparent cavity emissivity to a value >0.995, which is close to an ideal black body radiator (emissivity = 1), if the ratio of cylinder depth to radius is at least 10:1 (Sparrow and Cess, 1970). The accuracy of such a black body depends primarily on the accuracy of the water bath temperature measurement. An electronic thermometer or a laboratory thermometer with 0.1 °C scaling should provide the desired accuracy. Reference temperature should be at least 5 °C above ambient temperature for utmost accuracy.
Since thermography cameras may display a drift over time, camera calibration should be performed as often as possible. Correction works best if the reference radiator is permanently visible in the infrared pictures (Stabentheiner et al., 2012).
Fig. 7. Infrared camera offset calibration with a cavity black body radiator. Dimensions of the brass cylinder: ratio of cylinder depth to radius ≥ 10:1. ε = infrared emissivity (see section 220.127.116.11.). Graph by Anton Stabentheiner