Optical properties of fire and non-fire aerosols

Abstract

Optical detection principles play an important role in automatic smoke detection systems. Significant optical properties of an aerosol can be determined from angle- and polarization-dependent light-scattering experiments. A simple instrument was built to continuously monitor the polarized scattering cross-sections of aerosols. Two solid-state diode lasers with orthogonal polarization vectors serve as light sources. They emit light at a wavelength of 690 nm, and their light emissions are modulated at frequencies of 4 and 5 kHz, respectively. Fourteen photo detectors are arranged on one half of an annulus 30 cm in diameter. Each detector simultaneously measures the scattered-light intensities for both polarizations. After demodulation and separation of the two signals by a lock-in technique, the signals are filtered, amplified, and sampled with a time resolution of 1 s. The flat and open mechanical outline of the apparatus is well suited for direct measurements on the ceiling of a fire laboratory since it does not interfere with the natural aerosol convection. Comparisons between aerosols from test fires conforming to the European standard EN54 and aerosols not originating from combustion processes (water vapour, oil vapour) were carried out. Back calculations of the aerosol size distribution from measured data are demonstrated for latex test aerosols.