The present disclosure relates to a calibration insert for the adjustment, calibration, and/or implementation of a function test of an optical sensor that is designed to measure at least one measurand in a medium by means of light, the calibration insert including: an inlet cross-section through which light enters into the calibration insert; an outlet cross-section through which light exits from the calibration insert; and at least one blocking element that is arranged between the inlet cross-section and the outlet cross-section, wherein the blocking element does not entirely let through the light, independently of its wavelength, from the inlet cross-section to the outlet cross-section. Instead, the blocking element partially absorbs, reflects, or scatters the light, wherein the ratio of the intensity of the light at the outlet cross-section to the intensity of the light at the inlet cross-section corresponds to a value of the measurand.