The invention relates to a positioning apparatus (100) for positioning a light-guiding fiber (206) in a calibration port (208) of a medical apparatus (202) comprising at least one light source (204) for the light-guiding fiber (206), wherein the positioning apparatus (100) comprises an elongate body (102) with two end faces (110, 112) and at least one side face (116). A channel (104) for receiving the light-guiding fiber (206) is formed in the body (102), said channel extending along a longitudinal axis of the body (102) proceeding from a first end face (110). Here, according to the invention, provision is made for the body (102), at least in one portion, to consist of an opaque material in the region of the channel (104) and/or to be coated with an opaque material and for said body to have at least one cutout (113, 118), which extends from a side face (116) and/or the second end face (112) of the body (102) to the channel (104) such that radiation emitted by the light-guiding fiber (206) can only emerge from the positioning apparatus (100) in unimpeded fashion through the at least one cutout (113, 118).
