2.1.2. Line scan sensor
The stability of the measured reflected laser intensities was crucial to ensuring low false alarm rates as well as reliable and repeatable system performance. Accordingly, responsivity and switching speed were critical parameters in the choice of line scan sensor. A Hamamatsu S9227-03 CMOS linear image sensor, comprising a single line of 512 photodiodes, was used in the PDU shown in Fig. 1. In comparison to previously used sensors (Paap, 2014), the S9227-03 exhibited much lower variation in pixel response, Fig. 3, which yielded robust and repeatable measurements of spectral properties.
System tests revealed that the outdoor performance of the line scan sensor was noticeably degraded by ambient light, which, if bright enough, caused the sensor to saturate, negating any form of reliable spectral measurement. To address this problem and to remove much of the broadband solar noise, a dual band-pass optical filter (Fig. 4) with pass-bands specified at 648 ± 43 nm and 783 ± 29 nm for the red (635 nm, 685 nm) and NIR (785 nm) wavelengths respectively, was designed and fitted to the front of the sensor (Askraba et al., 2013; Paap, 2014).
Experimental outdoor tests with and without the solar filter, show an average reduction of ambient light in the order of 65–75%, as shown in Fig. 5.