A device for directing a defined light beam onto a photosensitive area, for testing the performance of a photovoltaic module including said photosensitive area, comprising a support structure (1) which can be turned about a vertical central axis (4) in accordance with the actual azimuthal sun position. Housing (7) receives the photovoltaic module with the photosensitive area (6) of said module in a horizontal position, the housing being located in the lower portion of the support structure. A first mirror (2) which can be tilted about a horizontal axis (3) in order adapt its position continuously to the sun elevation position and to optimize the sun light impact thereon, and a second mirror (5) is fixed to the support structure (1) vertically above the photosensitive area (6) of the module and receives the light beam from the first mirror (2) for reflexion towards said area (6). At least one of said mirrors (2, 5) being associated to a mechanism (8) which allows one to vary the convexity of the mirror in accordance with sun intensity detectors.