In the calculation of the interaction of radiation from a point source with a body (or conversely, the intensity of radiation from an emitting body at a point in space), a somewhat complex exponential integral must be evaluated. If the interaction in the body is small, however, the integral may be approximated byI=∫Vdv/R2, whereVis the volume of the body andRis the distance from the point to the volume elementdvin the body. This integral is discussed and applied to the case of a cylindrical body. Extensive tables (>12 000 entries) and formulas have been prepared which will enable the reader to determine the value of this integral for a cylinder of any size and any point in space. A discussion of non‐negligible absorption and multiple interactions is also included.