A process interface of a process gas analyzer operating by a transmitted light method includes a purging tube, which extends between an optoelectronic element and an interior of a plant part carrying a process gas, wherein the purging tube is closed off, at its end opposite from the optoelectronic element by a window, in the vicinity of which a purging gas feed enters the purging tube, where an annular part is arranged in the interior of the purging tube opposite the entrance of the purging gas feed and is coaxial in relation to the purging tube, and the part has a convex outer side, the vertex line of which divides the entrance of the purging gas feed into a smaller region, open toward the window, and a larger region, open toward the interior of the plant part.