The invention relates to a measuring device 5 of the type which detects the interruption of a beam of radiation (e.g. light 20) by an object (50 FIG. 1) 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.