5.2.2.2. Honey bee cuticle infrared emissivity

Thermographic measurement of the surface temperature of an object requires knowledge of its infrared surface emissivity. This quantity is a quotient (ranging from 0-1) describing how much radiation an object emits at a certain temperature in comparison to an ideal black body radiator, which emits the theoretical maximum. The emission and absorption of infrared radiation are directly coupled according to Kirchhoff's law, which means that an opaque surface with an emissivity of 0.97 also has an absorptance of 0.97 (i.e. 97 % of incoming infrared radiation is absorbed and 3 % are reflected). Therefore, the emissivity value can be used to compensate for ambient infrared radiation reflected into the camera via the object, which adds to the radiation emitted by the object and therefore would produce a measurement error if left uncorrected.

The honey bee cuticle has an infrared emissivity of ~0.97 (Stabentheiner and Schmaranzer, 1987), so approximately 3 % of the ambient infrared radiation is reflected via the cuticle into the infrared camera. With standard evaluation software provided with thermography cameras, this can be compensated by considering ambient ‘black body’ temperature, and this is usually done by measuring ambient temperature.

During measurements in sunshine, however, there is an additional error caused by solar radiation reflected by the cuticle, which is not corrected for by the above mentioned correction of reflected ambient infrared radiation. This effect can be measured by thermographing insects at different intensities of solar radiation with a real-time infrared camera (frame rate > 30 Hz). The cuticular surface temperature is first measured in sunshine and then the insect is shaded and its temperature measured again within a few milliseconds. The resulting measurement error turns out to be small in the 7-14 µm waveband (long wave cameras). The applied correction factor for reflected solar radiation amounted to 0.2183 °C kW-2 m-2 (Stabentheiner et al., 2012). However, with short wave cameras (3−6 µm waveband, which is closer to the solar radiation maximum in the visible range) the measurement error may be considerably higher (several degrees). Therefore, long-wave cameras should be used for field measurements in sunshine.

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