A measuring apparatus for measuring a physical property of a substance using radiation has a source of radiation for irradiating the substance, a radiation detector which is disposed on the opposite side of the substance from the radiation source, a mask for allowing radiaton to enter the radiation detector only along n prescribed pathways, and a signal processing and calculating device for calculating the physical property of the substance based on the radiation which is incident upon the radiation detector. The mask has n different mask patterns each comprising a plurality of pattern elements which allow the passage of radiation and which can be positioned between the substance and the radiation detector in alignment with the radiation pathways.