From a system perspective, the solar filter effectively increased the sensor dynamic range, which in turn improved the optical signal-to-noise ratio (OSNR) of the measured spectral reflectance signal levels. For captured unfiltered/filtered measurements, the difference in the noise bandwidths of the data prevents direct comparison of signal-to-noise ratio improvements. A relative measure of the improvement in dynamic range is possible by defining the figure-of-merit (q):S½i
q½i ¼ for i ¼ 1::512
S½i þ NAmbient½i
where S[i] is the raw 10-bit signal value at pixel i and NAmbient[i] is the raw 10-bit background noise value at pixel i. Defining qUnfiltered and qFiltered as the figures-of-merit for unfiltered and filtered spectral reflectance measurements respectively, a measure of the relative improvement in the dynamic range is given by:
qFiltered
DDR ¼
qUnfiltered
Experimental results illustrating the relative dynamic range improvement obtained on using the solar filter are presented in Figs. 6–8. The data was captured on an overcast day with a measured ambient light level of 4000 lux and an S9227-03 exposure time of 0.25 ms. The PDU, mounted on a static holding frame, captured reflectance levels from a uniform green test card placed 950 mm vertically beneath the PDU, with the card sequentially illuminated by the individual 635 nm, 685 nm and 785 nm lasers.