A test glass changer mountable in a vacuum coating plant on a movable substrate holder comprising - a carrier plate fastened to the substrate holder with a rotary spindle and a feedthrough for the rotary spindle having at least one measurement opening arranged eccentrically with respect to the rotary spindle, - a disc-like mount for at least one test glass element which is rigidly connected to the rotary spindle and rotatable relative to the substrate holder and which has at least two recesses offset eccentrically with respect to the rotary spindle and against each other at an angular distance, - wherein the mount is mountable in a space between the substrate holder and the carrier plate - with a control device configured to induce a rotational movement of the mount and to bring a test glass element arranged in one of the recesses into a first position having a smaller angular distance from the measuring position than the angular distance between the recesses, characterized in that a centering device is provided having at least one actuator element (13, 26) arranged at a fixed angle to the substrate holder and at least one bearing element (11, 35) which is arranged at a fixed angle to the rotary spindle and can be acted on by the actuator element with a spring force, wherein starting from the first position a torque can be applied by the centering device on the mount in order to bring the test glass element into the measuring position and a holding torque can be applied on the mount by engagement of the actuator element into the bearing element in order to hold the test glass element in the measuring position, wherein a) the bearing element is formed as a hub connected rigidly to the rotary spindle with a number of indentations arranged on the periphery of the hub and wherein at least one of the indentations corresponds to one of the recesses, and the actuator element is formed as a spring pin arranged at the carrier plate and configured to engage with one of the indentations via a free end with a spring force, or b) the bearing element is formed as a trough-like indentation arranged in the mount, the indentation corresponding to one of the recesses and the actuator element comprising a leaf spring fixed at one end to the carrier plate and a ball element arranged between the mount and the carrier plate, wherein the ball element has a stop opening formed in the carrier plate and can engage with the indentation with the spring force of the leaf spring. - a carrier plate fastened to the substrate holder with a rotary spindle and a feedthrough for the rotary spindle having at least one measurement opening arranged eccentrically with respect to the rotary spindle, - a disc-like mount for at least one test glass element which is rigidly connected to the rotary spindle and rotatable relative to the substrate holder and which has at least two recesses offset eccentrically with respect to the rotary spindle and against each other at an angular distance, - wherein the mount is mountable in a space between the substrate holder and the carrier plate - with a control device configured to induce a rotational movement of the mount and to bring a test glass element arranged in one of the recesses into a first position having a smaller angular distance from the measuring position than the angular distance between the recesses, a) the bearing element is formed as a hub connected rigidly to the rotary spindle with a number of indentations arranged on the periphery of the hub and wherein at least one of the indentations corresponds to one of the recesses, and the actuator element is formed as a spring pin arranged at the carrier plate and configured to engage with one of the indentations via a free end with a spring force, or b) the bearing element is formed as a trough-like indentation arranged in the mount, the indentation corresponding to one of the recesses and the actuator element comprising a leaf spring fixed at one end to the carrier plate and a ball element arranged between the mount and the carrier plate, wherein the ball element has a stop opening formed in the carrier plate and can engage with the indentation with the spring force of the leaf spring.