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Light that is backscattered is collected with a 0.5 m telescope and monitored as a function of time to determine a vertical distribution of the backscattered energy. This results in the measurement of the aerosol and molecular backscatter cross-sections, aerosol and molecular depolarization, and particle size.
The signal collected by the telescope is directed to a series of detectors. The aerosol and molecular return is measured with a pair of photomultiplier tubes. A 0.3 nm bandwidth interference filter is used to reduce background signal in both channels. A Fabry-Perot etálon is placed in the optical path for additional background filtering during daytime operation. A 1.8 pm bandwidth iodine absorption filter is placed in the path of one channel to spectrally remove the aerosol scattered component (refer to Figure 6). The other channel observes the combined molecular and aerosol return. The seed laser is temperature tuned to determine the spectral signature of the iodine absorption cell. This allows deconvolution of the two-channel data into individual molecular and aerosol components. Depolarization for each channel is determined by monitoring the ratio of the return signal of successive pulses, which are transmitted with orthogonal polarization.
Data collected by a variable aperture detector, following an 1 nm bandpass interference filter to reduce background signal, is utilized to measure multiple scattering events (Eloranta and Piironen, 1992). Cycling through a series of aperture sizes (0.21 through 4.0 mrad, for evening operation) adjusts the effective detector FOV, where an increased FOV allows a greater reception of multiply-scattered photons. This feature is limited during daytime operation with a maximum aperture size of 0.5 mrad.