Direct Normal Irradiance (DNI) may be measured directly via an absolute cavity radiometer. Absolute cavity radiometers are considered the most accurate method of solar radiation measurement and form the basis of the World Radiometric Reference (WRR). However, absolute cavity radiometers are not designed for continuous, unattended, outdoor use.
Thus, the primary method of measurement of DNI is with an instrument called a pyrheliometer. Pyrheliometers typically employ thermopile sensors at the base of a light-collimating tube and glass window face, although they may also be constructed with another photosensitive element in place of the thermopile. The light-collimating tube allows the pyrheliometer a 5° field of view and limits the pyrheliometer to measurement of the direct beam and circumsolar radiation. Furthermore, this small field of view also forces pyrheliometers to be tracked normal to the sun.
If direct measurements of DNI are not available, DNI may be calculated via co-planar measurements of the diffuse and total radiation by devices with a 180° field of view (the incident angle between the collection plane and sun must also be known).
If co-planar diffuse and total radiation measurements are unavailable, models have been developed to estimate DNI from the global horizontal irradiance (GHI) and other environmental factors. Two popular models are the DISC and DIRINT models developed by E. Maxwell and R. Perez et. al. respectively.