The meter has a block (1) of a fluorescent substance formed of a bundle of scintillator fibres which have a radiation absorption characteristic similar to that of the tissues of a human body. The block is arranged so that the direction in which the fibres run is aligned orthogonally to the direction of incidence of the ray (4-1). An image measuring device (6) measures the distribution of the intensity of the fluorescence from the fluorescent substance on the end surfaces of the block fibres. The image measuring device can have a number of photodetectors each located on an appropriate end surface of the block fibres.
