Abstract: A device is disclosed for processing optical information comprising a system of optical elements (6, 7, 9, 10, 12, 13) arranged along an optical main axis (11) for guiding a light beam (4) to a radiation-sensitive detection system (8) at least one coma correction plate (13, 20, 21) is provided in the system of optical elements and comprises a substrate having a dielectric layer varying in thickness so as to at least partly compensate for axial coma due to differences in path length of the light rays.Also disclosed is a method of manufacturing such a coma correction plate in which during the vapor deposition of the dielectric layer, a plate-shaped substrate (31) is moved at a constant speed below a template (33) parallel to an axis (34) which is situated in the plane of the template. The template (33) is situated between the vapor deposition source (30) and the substrate (31) and has an aperture (34) which is divided by the axis.

Abstract: In a scanning electron microscope, a periodic structure in the object plane is used for the detection of the focus condition of a spot-focussed electron beam scanning an object in order to correct defocusing and astigmatism in the scanning electron beam spot. To achieve this the detector comprises a plurality of individual elements which can be pair-wise read and an electronic circuit for forming a control signal for controlling the excitation of a spot-forming lens and a stigmator, respectively, from signals representing movement of the electron interference pattern at the detector, relative to the object scan, due to an out of focus condition. The signals are derived from corresponding pairs of detector elements which are situated at a fixed distance from each other in order to correct the focus and compensate for the astigmatism in the electron beam respectively. In the case of astimatism, signals from at least two pairs of detector elements spaced in directions at right-angles to one another, are used.

Abstract: A record carrier is described which is provided with an optically readable structure of trackwise arranged areas alternating with intermediate areas. Areas of at least two different types are provided. As a result, a high information density can be achieved. The different types of areas may also be used for centering a read beam relative to a track and for focussing the beam onto the track.