Radiation sources that have apertures and dimensions that are comparable to those of the receivers cannot be considered as point sources when they are close to the receivers. This paper presents a method of calculating the collection efficiency of a receiver for the case in which the receiver and source are closely spaced coaxial disks. The calculations consider sources with uniform and Gaussian brightness distribution; they also consider sources and receivers whose elements have both spherically symmetrical and Lambert's law response patterns.
D. A. BERKOWITZ and ENN TAMMARU, "Collection Efficiency of Receivers Placed Close to Sources," J. Opt. Soc. Am. 53, 1292-1292 (1963)