220.127.116.11. Main thermographic camera types
In earlier times, the infrared (IR) radiation of an object to be measured was scanned by rotating prisms or mirrors and focussed onto a single detector (detector material for example InSb or HgCdTe). Nowadays, many detectors are arranged in focal plane arrays (FPAs). Data readout ('scanning') from the chip is done electronically. Spatial resolution of FPAs may be of 120 x 120, 320 x 240 (see Stabentheiner et al., 2012) or 640 x 480 pixels, or even reach 1280 x 1024 pixels in (rather expensive) top camera models.
There are two main types of thermographic cameras for the practical use in honey bee body temperature measurement. Cameras either use photon-counting detectors, like InSb, PtSi, HgCdTe, or QWIP (quantum well interference photodetectors), or microbolometer arrays. Cameras with photon-counting detectors measure infrared photons directly. To achieve their (usually) high sensitivity (<20 mK in some InSb cameras for example) they require detector cooling. While in earlier times detectors were cooled with liquid N2 or expanding Ar gas, cooling is nowadays done with miniaturized built-in compressors. InSb and QWIP chip cameras are usually fast in reaction (have a short exposure time), which make them especially convenient for the measurement of flying (or fast moving) honey bees.
Cameras with microbolometer arrays measure infrared radiation indirectly, via (small) temperature changes of the detector pixels. Detector materials in use are for example Vanadium oxide (VOX) or amorphous Silicon (aSi). A disadvantage of (earlier) bolometer cameras is that they may exhibit considerable drift due to internal temperature changes (Stabentheiner et al., 2012). This may require frequent internal and external recalibration. Modern cameras often have a better (though not complete) drift compensation. By now, the sensitivity of the top bolometer cameras is better than 0.03 K. Furthermore, small and relatively cheap handheld bolometer cameras have become available. Many of these allow to set the focus close enough to measure surface temperatures of thorax, abdomen, and head of honey bees (Fig. 6).
The main spectral ranges of thermography cameras in use are the 3-6 µm (short wave) and the 7-14 µm (long wave) wavebands. Short-wave cameras (with e.g. InSb or PtSi FPA) are fine for laboratory use. For field measurements, long wave cameras are recommended (e.g. microbolometer or QWIP FPA), as these are much less sensitive to reflected solar radiation (see next section).
Fig. 6. Thermogram of a honey bee sucking 0.5 M sucrose solution from an artificial flower. Body surface temperatures: head = 26.1°C, thorax = 38.6°C, abdomen = 23.3 °C. Ambient air temperature close to the bee: 16.2°C, ambient radiation: 18.1 W m-2, ambient humidity: 40.6 %. Thermographic camera: FLIR i60 (180x180 microbolometer detector). For instrument accuracy see Stabentheiner et al. (2012). Image by Anton Stabentheiner.