As solar radiation passes through the earth's atmosphere, some of it is absorbed or scattered by air molecules, water vapor, aerosols, and clouds. The solar radiation that passes through directly to the earth's surface is called Direct Solar Radiation. The radiation that has been scattered out of the direct beam is called Diffuse Solar Radiation. The direct component of sunlight and the diffuse component of skylight falling together on a horizontal surface make up Global Solar Radiation.
Global radiation is measured by a pyranometer. The modern pyranometer manufactured by the Eppley Laboratory, using wirewound plated thermopiles, can be one of two types: the Precision Spectral Pyranometer (Model PSP) and the Black & White Pyranometer (Model 8-48). The PSP has a black sensor protected by two precision ground, polished hemispheres. The 8-48 has a black and white "star" sensor that is protected by a single polished hemisphere.
Over the years, there have been efforts to increase the linearity of response, durability, the adherence to the Lambert cosine response law and independence from ambient temperature effects. The precision pyranometer operates without deviations at various inclinations and is, therefore, often used to test solar panels, where it is mounted in the plane of the collector.
Direct radiation is best measured by use of a pyrheliometer, which measures radiation at normal incidence. The Normal Incidence Pyrheliometer (Model NIP) consists of a wirewound thermopile at the base of a tube, the aperture of which bears a ratio to its length of 1 to 10, subtending an angle of 5°43'30". This limits the radiation that the thermopile receives to direct solar radiation only.
The pyrheliometer is mounted on a Solar Tracker (Models ST-1 and ST-3) or an Automatic Solar Tracker (Model SMT-3) for continuous readings.
Diffuse radiation can either be derived from the direct radiation and the global radiation or measured by shading a pyranometer from the direct radiation so that the thermopile is only receiving the diffuse radiation. Eppley has developed Shade Disk Adaption Kit (Model SDK) that mounts on the SMT-3 which allows you to measure the diffuse and direct at the same time. We also manufacture the Shadow Band Stand (Model SBS).