Abstract: Two optical windows serve for the passage of a light beam through a treatment chamber in which a product is disposed whose dimensions are to be detected. Between the treatment chamber and each one of the optical windows there is disposed a rotary valve, which permits the separation of the optical window from the treatment chamber. Thus it is possible to prevent a medium that adversely influences the optical windows in the treatment chamber from gaining access to the interior face of the windows if protection of the windows has priority over a measurement. It is thus possible to considerably extend the service life of the windows.