Abstract: Method for determination of a straight line intersecting the axis of an optical radiation beam of a photoelectric levelling apparatus where the optical radiation beam transmits power from a stable radiation transmitter to a stable radiation receiver by screening the optical radiation beam by a screen and by evaluation of the signal detected by the radiation receiver by means of an evaluation circuit wherein the optical radiation beam is screened by screening means from at least two different sides of the optical radiation beam subsequently in mutually symmetrical positions of the screening means at a simultaneous indication from which side the optical radiation beam is screened, the evaluation circuit is evaluating the magnitude of the signal detected by the radiation receiver in case of screening from different sides of the optical radiation beam and by shifting the screening means transverse to the optical radiation beam the position is found.

Abstract: Arrangement for measuring, checking and control of the position of chosen points of objects with respect to an optical aiming line, which optical aiming line is determined as a connecting line between the center of an optical system of a transmitter of a light measuring beam and of the center of an optical system of a receiver, selectively sensitive to the wavelength and modulation of the light measuring beam. The position of the body with respect to this optical aiming line is determined by the screening of the light measuring beam by means of circumference of this body or by means of the edge of a screen connected with this body. A substantial feature of this invention is the application of directional optical slot systems both for the transmitter and for the receiver selection of proper sizes of outlet slots of these optical slot systems.