Method and arrangement for displacement

A method of displacing an image, where the image is being transferred using electromagnetic radiation of a reproduction beam having a beam direction, the image being displaced perpendicular to the beam direction by use of a rotatably arranged radiation-refracting body that is disposed in a course of the reproduction beam and that has a refraction index different from an ambient medium with respect to said radiation, wherein the radiation-refracting body comprises at least one of either a plane parallel medium or a glass plate, the method comprising:

• rotating the radiation-refracting body;
• directing the reproduction beam to an area on the radiation-refracting body;
• directing a measurement beam distinct from the reproduction beam from an interferometer to said area of the radiation-refracting body as the radiation-refracting body rotates, wherein said measurement beam impinges on said area at each of various rotational positions of the radiation-refracting body, wherein at each of said various rotational positions the measurement beam comprises a first radiation component and a corresponding second radiation component, wherein the first radiation component passes through the radiation-refracting body to a flat reflecting surface distinct from the radiation-refracting body where it is reflected back to and at least in part through the radiation-refracting body and onward to the interferometer to have a different wavelength than the measurement beam as directed to an incident surface of the radiation-refracting body, and wherein the corresponding second radiation component is reflected from the incident surface at said area to a measuring device distinct from the interferometer;
• measuring rotation of the radiation-refracting, body by determining an optical path length change in the measurement beam caused by the radiation-refracting body, wherein said determining said optical path length change is performed by said interferometer and comprises evaluating radiation of the first radiation component reflected back to and at least in part through the radiation-refracting body to the interferometer relative to radiation corresponding to the measurement beam as output from the interferometer and directed to the incident surface of the radiation-refracting body;
• deriving a position-dependent electrical signal by the measuring device based on the corresponding second radiation component to provide a reference point of origin for motion of the radiation-refracting body;
• supplying the position-dependent electrical signal and the measured rotation to a control unit for controlling rotation and offset of the radiation-refracting body relative to said reference point of origin;
• displacing said image of the radiation-refracting body in response to the reproduction beam;
• and
• controlling said rotation and offset of the radiation-refracting body with said controller to achieve a desired absolute displacement of the image relative to said reference point of origin, wherein said controlling is based on the measured rotation of the radiation-refracting body and said offset of the radiation-refracting body relative to said reference point of origin as determined from the measured rotation and the position-dependent electrical signal using said first component and corresponding second component as occurring during a rotational interval of the rotating body.