Abstract: The invention relates to a fibre-optic apparatus for determining in a material a phenomenon affected by light back-scattered by surface and/or volume refraction. The apparatus comprises a light source; an illuminating system connecting the light source to a sensor head; a light detection system connected to the sensor head; and within the sensor head an optical illuminating fibre connected to the illuminating system, and a juxtaposed optical detection fibre connected to the light detection system. The optical fibres are mounted in a mutally fixed position. In the sensor head according to the invention at least one solid optical illuminating fibre and at least one juxtaposed optical detection fibre are disposed with their optical axes parallel to each other through an axial length from the end of the optical fibres arranged to face the material to be examined, and the optical illuminating fibre is adapted to be also used as an optical detection fibre in addition to the optical detection fibre first mentioned.