The instrument makes continuous measurements of ground reaching directly transmitted solar flux at one or more narrow wavelength bands from which the columnar optical depth of the atmosphere can be estimated. Aerosol optical depth can be deduced from this and by making such estimates at a number of wavelengths, inference can be made on the size distribution of aerosols. The instrument, quite simple to develop and operate, uses sun as the source and can be ruggedised for field use.

The device has two independent axes driven by stepper motor for positioning the detectorfilter assembly in the direction of the sun. The filter wheel, which is also driven by stepper motor, can hold up to 12 filters. Worm gear assembly of the filter wheel is made precise so that positioning of the filter with respect to detector is within close tolerance in continuous 24 hour duty operation. The system is constructed in aluminium alloy to reduce overall weight for portability.