document: Historically, the interest in accurate measurement of DNI started decades ago. Early studies (e.g., Linke, 1931; Linke and Ulmitz, 1940) identified the difficulty of separating the measurement of DNI from that of the diffuse irradiance in the immediate vicinity of the sun, hereafter referred to as circumsolar irradiance. Pastiels (1959) conducted a detailed study of the geometry of pyrheliometers, and how that geometry interacted with circumsolar radiance, using simplified representations of the latter. Various communications were then presented at a WMO Task Group meeting held in Belgium in 1966 (WMO, 1967) to improve the accuracy of pyrheliometric measurements, including estimates of the circumsolar enhancement. Ångström (1961) and Ångström and Rohde (1966) later contributed to the same topic, followed years later by Major (1973, 1980). The whole issue of instrument geometry vs. circumsolar irradiance was complex and confusing at the time because different makes and models of instruments had differing geometries. This was considerably simplified after WMO issued guidelines about the recommended geometry of pyrheliometers, which led to a relatively “standard” geometry used in all recent instruments. The experimental issues related to the measurement of DNI are discussed in Section 3.2.
extractive_keyphrases: pyrheliometers separating the measurement of dni from that of the diffuse irradiance in the immediate vicinity of the sun
abstractive_keyphrases: improve the accuracy of pyrheliometric measurements, including estimates of the circumsolar enhancement study of the geometry of pyrheliometers, and how that geometry interacted with circumsolar radiance