6.8.2. Optical activity detectors
Optical activity detectors, which use an infrared light source (LED) and a photodetector (photodiode, phototransistor, etc.) can monitor changes of the position of bees in the measurement chamber. The information on bee activity is reflected in fluctuations of the signal recorded by the photodetector. According to Lighton (2008), "the one challenge is to create an activity data channel that is easily interpreted." Among other possibilities, a possible approach is to sum the difference between absolute values (not considering sign) of successive data points of the activity signal. The resulting absolute difference sum (aADS; Lighton and Turner, 2004) shows a steep slope in phases of high activity. A clear breakpoint emanates if activity ceases. This method can be enhanced by calculating the aADS residuals (compare section 6.1.9.). It is considered as 'semi-quantitative'. A detector ready for use can be purchased at Sable Systems International.