A fast-response multi-pass transmissometer operating over variable wavelength ranges

An instrument capable of making accurate measurements of the extinction of visible and near infrared radiation through the thickest of clouds is presented. The transmissometer is multi-pass with a path length of 17.1 m and has a short integration time, operating at ∼ 6 Hz. The instrument is compact and portable and can be operated at any site of interest. "Clear-air" signals are obtained by physically isolating the sample volume from the ambient conditions; this ensures that these signals can be measured without waiting for cloud-free conditions. The current operation is in the wavelength range 575-1150 nm, set by the limits of the optical coatings and the monochromator. The spectral extinction can be calculated to within 10% over the wavelength range 600-1100 nm. Phase-sensitive detection at 650 Hz is used to reduce detector noise. The instrument has also been used to investigate the atmospheric transmission in the infrared region (1400 nm-2900 nm).