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