An instrument is designed to measure the intensity distribution of light scattered from optical surfaces. It uses two photodiodes, one to collect specularly reflected light and the other to record scattered light. This configuration enables us to measure both the reflected and scattered light intensities within a dynamic range over seven decades without introducing neutral density filters. The signal-to-noise ratio of the instrument is also improved in comparison with the single photodetector configuration. The instrument is capable of measuring scattering as close as 0.05 deg to the specularly reflected beam.
Orestes Nicholas Stavroudis,
"Design of a light scatterometer for the measurement of small angle scattering," Optical Engineering 35(4), (1 April 1996). https://doi.org/10.1117/1.600711