Optical element for device for determining relative brightness of plurality of hair fibres

An optical element (10) for a determination device for determining the relative brightness of a plurality of hair fibres or more particularly a lock of hair wherein the optical element (10) comprises a body (2) made of an isotropic material and defines a measurement region (36) which is arranged outside the body (2) and forms a measurement surface of the optical element (10). The optical element (10) comprises in particular: a first incident optical surface (20) of the body (2), through which a measurement light beam (30) emitted by the light-emitting unit (12) can pass into the body (2) of the optical element (10); and a first exit optical surface (24) of the main body (2), through which the measurement light beam (30) from the first entrance optical surface (20) can exit the main body (2) of the optical element (10), the first exit optical surface (24) being configured such that the measurement light beam (30) exits in the direction of the measurement region (36), the measuring region ...