The shade ring correction for diffuse irradiance measurements

An experiment was carried out to make simultaneous measurements of the diffuse irradiance by using a ring (R) and a motor-driven occulting disc (D). Experimental results are analyzed to show the difference between the two diffuse measurements D and R and determine the correction necessary to equate the ring measurement to the disc measurement for various states of the sky as given by the ratio of the diffuse to global irradiation. The results are found to be in general agreement with the results of Drummond (1956) who found that his additional shade ring corrections were a function of solar declination but not of solar elevation.