A FAST-RESPONSE MULTIPASS TRANSMISSOMETER OPERATING OVER VARIABLE WAVELENGTH RANGES

An instrument capable of making accurate measurements of the extinctio n of visible and near infrared radiation through the thickest of cloud s is presented. The transmissometer is multi-pass with a path length o f 17.1 m and has a short integration time, operating at similar to 6 H z The instrument is compact and portable and can be operated at any si te of interest. ''Clear-air'' signals are obtained by physically isola ting the sample volume from the ambient conditions; this ensures that these signals can be measured without waiting for cloud-free condition s. The current operation is in the wavelength range 575-1150 nm, set b y the limits of the optical coatings and the monochromator. The spectr al extinction can be calculated to within 10% over the wavelength rang e 600-1100 nm. Phase-sensitive detection at 650 Hz is used to reduce d etector noise. The instrument has also been used to investigate the at mospheric transmission in the infrared region (1400 nm-2900 nm).