An image capturing apparatus is configured so that optimal one of usable position detecting sensors is selected. More specifically, the total reactor volume is held to a minimum such that minimum reactor volume is less than 0. The flanged surface is perpendicular to an axis of the inlet housing and defines a flange plane at an axial end of the inlet housing. 45 and the output beam of the first laser diode is confined to the central aperture zone. The apparatus comprises a horizontal assembly, a vertical assembly, and a table.