Abstract: An optical information recording and reproducing disc information has a substrate in the surface of which are formed signal recording grooves and cue signal recording pits. A light-absorbing thin film is formed on the surface of the substrate, the optical characteristics of the light-absorbing film being changeable thermally through irradiation by a light beam. The pitches of the grooves and pits, as well as the widths of the grooves and pits, are determined to meet specific conditions in relation to the diameter of the laser beam which is used for recording and reproduction of the signals. In addition, the depths of the grooves and pits are determined suitably in relation to the wavelength of the laser beam. With this arrangement, it is possible to attain a high quality of reproduction of information from the disc.

Abstract: An apparatus utilizing a flat plate type rotary transformer which has a rotary member with a collar fixed thereon, a stationary member provided with a bearing, the rotary member being mounted on the stationary member through the collar and the bearing, a first magnet core which is securely held on the rotary member and a second magnet core which is securely held on the stationary member. The magnet cores respectively of the rotary member and the stationary member are securely set by using the surface between the rotary member and the stationary member at which the members touch as the reference level in securing the desired gap between them.