The Model TSP-400 Total Solar Pyranometer is a precision meteorological instrument that measures global (direct and diffuse) total solar radiation. Like the other members of the YES Total Solar Pyranometer family, it is ruggedly designed for long-term field use. The Model TSP-400V adds a ventilation system for increased accuracy under all ambient temperature conditions. When properly characterized, the Model TSP-400V meets or exceeds the requirements for a World Meteorological Organization high class pyranometer (formerly called a secondary standard).