The invention relates to a test glass changing system (10) for selectively coating and optically measuring a test glass (24, 24″) in a coating chamber (1) of a vacuum coating installation (3). In the coating chamber, a movable turntable (2) is used to guide substrates (7) on a path through a stream of a coating material. The test glass changing system (10) comprises a test glass holder (8, 8″) with a test glass plate (26) for holding the test glass (24, 24″), and a cover (28, 28″) for selectively covering the test glass plate (26). The test glass changing system (10) also comprises a rotary apparatus (34) for rotating the test glass plate (26) about an axis (51) which is oriented approximately parallel to the axis of rotation (5) of the turntable (2). The test glass holder (8, 8″) can be positioned on the turntable (2) and removed from the coating chamber (1) in the form of a unit.