Study of the corrective factor involved when measuring the diffuse solar radiation by use of the ring method

By comparing the diffuse solar radiation on a horizontal plane as continuously measured in Geneva over a full year by using both a fixed ring and a moving disk, we conclude that the ring corrective factor can be evaluated on the basis of simple models, but with a daily uncertainty of the order of 5 per cent. In this study, the isotropy of the diffuse radiation can be used as a reasonable approximation, even if this assumption is not fully verified. More precise measurements would require the use of a disk.