In situ optical sensing of diesel exhaust particulates using a polychromatic LED source

A novel optical probe for in situ monitoring of diesel exhaust particulates has been developed and is demonstrated. The probe uses a transmissive configuration to interrogate a particulate stream using polychromatic (white) light from an electroluminescent LED, the probe being mounted transversely to the exhaust flow in the tailpipe. Consequently the effects of scattering and absorption are measured on the transmission spectrum. The optical signal is relayed via an optical fibre bundle to a remote detection and data logging unit. Photodetector signals representing red, green and blue spectral information are monitored using detectors having overlapping spectral responsivities. Data collection is by red-green-blue (RGB) tristimulus detection and spectral analysis uses the hue-lightness-saturation (HLS) algorithms. It is shown that changes in spectral width may be used to represent particulate bursts (scattering) via the saturation parameter with a compensation algorithm being used to compensate for shifts in the operating point due to contamination of the optics. As polychromatic light is used it is demonstrated that variation in the mean particle size may be represented in terms of the ratios of polydisperse extinction for adjacent spectral bands. The potential advantages of chromatic sensing for signal processing in terms of reduced susceptibility to vapour content and optical contamination is demonstrated by comparison with an intensity based sensor.