An effect of the circumsolar sky radiation Ion the Ångstrom Pyrheliometric scale

According to the basic equation of the Ågström pyrheliometer, the heating power of the compensation electric current is directly proportional to the incident radiation intensity. However, as Anders àngström pointed out, this direct ratio needs two corrections; the “edge effect” correction and the correction for heat conduction in the black paint thickness. This work directs attention to the existence of a third correction arising from the non-uniform illumination of the strips. In cases of pyrheliometers having no edge effect, this inhomogeneity is due to circumsolar sky radiation. The value of this correction can be determined by applying àngström's theory using an irradiation term depending on distance measured from the midpoint of the strip. For simplicity a linear illumination distribution was used, even though the real distribution of the energy coming from the sun's aureole is not quite linear along the strip. The incident radiation flux can be exactly determined by applying Pastiel's theory. When calculating a reliable value for the correction in question, the sensitivity distribution along the strip should be taken into consideration. This can be determined on the basis of àngström's theory. For the pyrheliometer A₅₂₉, in mean Budapest circumsolar sky conditions, the correction due to non-uniform irradiation is 0.5%. This correction is nearly compensated by the correction for heat conduction in the black paint.