This International Standard establishes a classifkation and specification of instruments for the measurement of hemispherical solar and direct solar radiation integrated over the spectral range from 0,3 pm to 3 u,m. 3.3 direct solar radiation: Radiation received from a small solid angle centred on the sun’s disc, on a given plane. Instruments for the measurement of hemispherical solar radiation and direct solar radiation are classified according to the results obtained from indoor or outdoor performance tests. Primary standards, which are direct solar radiation instruments, are classified on the basis of their design and specification of measuring reproducibility under outdoor test conditions verified by periodic pyrheliometric intercomparisons