Direct and diffuse solar irradiance are important components of environmental and climate research. Direct  solar irradiance is the solar radiation that passes directly through the atmosphere from the sun without being scattered or absorbed by the atmosphere. Diffuse solar irradiance reaches the ground and has been scattered by an atmospheric component such as air and water molecules, dust, or clouds. Measurement of direct and diffuse solar irradiance can provide information on the transformation of energy on the earth and in the atmosphere, analysis and distribution of the properties of atmospheric components and verification of satellite measurements of radiation.

Global (downwelling) solar irradiance is the total amount of solar irradiance on an upward-facing horizontal surface and is the sum of the direct and diffuse solar irradiance. The global solar irradiance provides information on the total radiance that affects the earth’s weather and climate system  and can be used in the study of climate change.

A SPECTRAL EVOLUTION field spectrometer such as the PSR+, RS-5400, RS-8800,  or RS-3500 can be use to measure global, direct and diffuse solar irradiance. Ground based measurement provides advantages such as high spectral resolution and the ability to adjust optical alignment of the measurement apparatus and target. The PSR+ with a cosine corrected diffuser measures global irradiance. A PSR+ with a Gershun tube measures direct irradiance and the diffuse irradiance is calculated from this measurement.

DARWin SP Data Acquisition software which comes with every SPECTRAL EVOLUTION instrument provides a tool accessed by a pull-down menu, for measuring energy. The pull-down utility allows users to calculate integrated energy levels as a function of wavelength interval (user adjustable).
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