5.2.2.4. Attenuation of infrared transmissive films

If honey bee body temperature has to be measured inside a hive it is often necessary to do this through infrared transmissive films to prevent the bees from leaving the colony (Stabentheiner et al., 2003a, 2010). Good choices are polypropylene or cellophane films used by florists for wrapping of flowers. They provide good stability and transparency. Polyethylene would be even more transparent to infrared radiation, especially in the 'long-wave' (7–14 µm) spectral band. However, when testing commercially available films, these were mostly of rather poor 'optical' quality, i.e. their surface was not sufficiently smooth. The attenuation of the infrared radiation by the transmissive film can be compensated for by covering part of the reference source with a stripe of the same film (Stabentheiner et al., 2012; and section 6 of this paper). By doing so, camera calibration with the reference radiator compensates for the attenuation of the film, and errors resulting from ambient reflections via the film surface can be minimized.

The attenuation of infrared transmissive films may also, with less precision, be corrected by determination of a 'special atmospheric transmission' coefficient (which usually compensates for infrared absorption of the atmosphere at greater measurement distances). This can be done by comparing the direct radiation of a black body radiator (Fig. 7) with radiation passing through the film. An iterative change of the atmospheric transmission coefficient for the film measurement, in the camera or in the evaluation software, until the temperature reading equals the direct measurement provides the correction coefficient.

Special care has to be taken to avoid different reflection of ambient radiation by room walls, IR camera, operators, etc. from different parts of the film surface. In outdoor measurements, reflection of the sky via the film may produce considerable measurement errors. For more details see Stabentheiner et al. (2012). In general, however, it is worthy of note that thermographic measurement through plastic films should be avoided whenever possible because it inevitably adds additional sources of error.

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