Interrelations of Hourly and Daily Global Illuminance

It has been found that the values of ratio of hourly global illuminance to daily global illuminance, r_vt, are very close to the corresponding values for global solar radiation, r_t, examined from the measured data of six IDMP [1] (International Day light Measurement Programme) locations. This has been further confirmed by examining the values for r_vt and r_t as calculated from TMY2 [2] (Typical Metrological Year) data base for primary locations. Based on this, it has been proposed that the correlations available in literature to predict r_t can be employed to predict r_vt. Adequacy of the correlation due to Collares-Pereira and Rabl [3] available for r_t has been examined to predict r_vt as obtained from TMY2 data base for the 56 primary locations. It has been found that the values of r_vt obtained from measured illuminance data of six locations have been predicted within a rms difference of 2.71% and within a rms difference of 4.3% for the 56 primary locations of TMY2 data when the correlation due to Collares-Pereira and Rabl for r_t has been employed.