The invention relates to an apparatus for determining optical surface properties of workpieces, comprising a housing, in the interior of which there is provided a carrier on which the workpiece be arranged, and comprising a radiation device which directs radiation onto the workpiece in a predefined emission direction (E). According to the invention, the housing has in at least one wall an observation opening, through which a region of the workpiece illuminated by the radiation device can be observed in a predefined observation direction (B).