the invention relates to a measuring device 5 of the type which detects the interruption of a beam of radiation by an object in the beam . the device includes a light emitter 12 and a light detector 32 each enclosed in a housing 10 and 30 . in order to accommodate different spacings between the housings different caps 14 having varying size apertures 11 are fittable to the housings . this allows varying amounts of light to be issued and / or detected . other ways of adjusting the amount of light passing through the cap are disclosed .