The invention comprises a method and an apparatus for gauging a reflector for radiation during operation of said reflector, in which method, in order to determine the present reflection properties of the reflector in a number of at least one measurement point provided in the path of the radiation reflected by the reflector, the pattern of predetermined properties of the presently reflected radiation is measured and compared with a predetermined reference pattern, wherein a conclusion is drawn from the comparison on the present geometric properties of the reflector and, in the event of undesirable geometric properties, corresponding operational parameters of the reflector are altered. Preferably, this method is used in trough collectors for solar power plants in order to gauge flexible concentrators arranged in a pressure cell during operation of said concentrators.