Abstract: A light beam scanning apparatus projects a light beam swept by a light scanner to a spherical mirror and scans by a light beam spot reflected and focused by the spherical mirror a surface of an object arranged to oppose to the spherical mirror at a position spaced from the spherical mirror by a distance equal to a focal distance f of the spherical mirror. An optical path length between the light scanner and the spherical mirror is set to 0.78 f-0.84 f to minimize a curvature of field (circle of confusion).

Abstract: A track displacement detecting and measuring system in which track displacement data are obtained as usual for a unit chord length of the track according to the conventional three-point measuring method by sensors mounted on three predetermined parts of a track inspection car, and such measured data are processed by an arithmetic or processing device of relatively simple structure to detect the amount of track displacement occurring over a track distance which is two or four times the unit chord length. In the system, an improved gyro device free from the adverse effect of rolling of the car is employed so as to permit precise detection of vertical track displacement irrespective of the track distance.