Integrating Sphere-Based Nephelometer for UAS Applications

Description

With DOE SBIR Phase I & II funding, we have developed and characterized prototypes of a novel, multi-wavelength integrating nephelometer, for use on small aerial platforms such as UAS. The main goal was to reduce the size of a research grade integrating nephelometer while maintaining its detection limit through technology innovation. Modern commercial nephelometers measure aerosol scattering coefficients by illuminating a sample volume from all scattering angles and quantifying the optical power scattered into a sight path that is viewed by a sensitive detector against a dark background. Available research grade nephelometers are quite large and can weigh up to 40 lb. Detection limits of <0.3 Mm-1 or better for a 1-min measurement are achieved. In such a nephelometer, the generated illumination transverses the sample volume one time before being discarded. In contrast, our novel nephelometer concept uses an integrated cavity for homogeneous illumination of the sample volume, combining a very low truncation angle with multiplication of the light intensity using multiple reflections of the cavity. We have developed nephelometer prototypes (Fig. 1) and tested them for ground-based measurements under various environmental conditions. Aerosol scattering coefficients are measured simultaneously at three wavelengths (red, green, blue) with each color LED power-modulated at a different modulation frequency. Therefore, the signal from a single detector can be demodulated through Fourier transform to separate the red, green, and blue scattering signals in frequency space. Testing of our prototype nephelometers yielded a sensitivity of 1 Mm-1 for a 1s measurement, much better than larger research grade nephelometers, making it ideal for mobile applications. We have also demonstrated that, at different seasons and geographical locations, ambient multi-wavelength integrating nephelometer data correlate very well with Federal Reference Method PM2.5 data, while delivering exceptional short-term precision measurements. In the proposed Phase IIA extension of our work, we will perform extended testing of our technology to ensure proper operation for intended airborne platforms and climate conditions, including flight tests on helicopters and UAS. Furthermore, we propose to build additional prototypes for (1) long-term tests comparing prototype performance to that of typical research grade integrating nephelometers and (2) tests of performance in highly polluted environment.