The invention relates to a carrier with a binding surface at which target components that comprise label particles, for example magnetic particles, can collect and optionally bind to specific capture elements. An input light beam (L1) is transmitted into the carrier and totally internally reflected at the binding surface. The amount of light in the output light beam (L2) and optionally also of fluorescence light emitted by target components at the binding surface is then detected by a light detector. Evanescent light generated during the total internal reflection is affected (absorbed, scattered) by target components and/or label particles at the binding surface and will therefore be missing in the output light beam (L2). This can be used to determine the amount of target components at the binding surface from the amount of light in the output light beam (L2, L2a, L2b). A magnetic field generator is optionally used to generate a magnetic field (B) at the binding surface by which magnetic label particles can be manipulated, for example attracted or repelled.