ur sun photometer instrument, CIMEL Sun Photometer CE318, is #237 in the NASA AERONET (AErosol RObotic NETwork.) This microprocessor controlled, stepper motor positioned robot has a two component optical head containing the sun collimator without lens and the sky collimator with lenses and filter wheel. Sun tracking is controlled with a 4-quadrant detector. Data is temporarily stored in memory and once every hour uploaded to NASA via a GOES-E (geosyncronous) satellite uplink.

The instrument directly measures the incoming solar energy (sky radiance) at selected wavelengths of 340, 380, 440, 670, 870, 1020 nm (aerosols), and 936 nm (water vapor). Light is absorbed and scattered by atmospheric gasses, water vapor and aerosols. The concentration of different atmospheric components can be determined by the attenuation at wavelengths which are strongly absorbed or scattered. Total Optical Depth (TOD) is the sum of the Rayleigh Optical Depth (from atmospheric gasses, e.g. Nitrogen, Oxygen, Argon), the Ozone Optical Depth, the Mixed Gas Optical Depth (e.g. Carbon dioxide, oxides of Nitrogen), the Water Vapor Optical Depth and the Aerosol Optical Depth (AOD). AOD is of the most interest because it cannot be directly measured by other methods, and is determined by subtraction of the other quantities. Aerosol phase functions and size distribution can also be inferred. The sun photometer measurements are also important as standards for satellite-derived values of AOD.