inclined in relation to the above mentioned detector surface the lens is disposed substantially at right angles to the reflecting surface of the mirror 22 furthermore the device 21a and the mirror 22 are arranged such that the imaginary extensions of these parts will make an acute angle in the plane of the drawing according to fig2 the different major directions 17 1 17 2 17 3 17 4 of the optical radiation each symbolize their incidence angle sector which is processed individually in the receiver this latter is provided with a device 24 which is operative to give a sharp edge of the reproduced target on the surface of the photodetector the radiation from the target in position 18 1 which is most proximal the unit 13 must be subjected to the extremest damping in relative terms the radiation 17 1 is therefore led in towards the diffuse mirror 22 before being reflected to the detector surface 19a the radiation 17 2 is led in via the gap 21 direct onto the detector surface the radiation from 18 3 is led in via the lens device 23 and the diffuse mirror 22 before impinging on the detector surface 19a finally the radiation 17 4 from the target position 18 4 is led in through the lens device 23 and directly down onto the detector surface 19a there will hereby be obtained a damping of the impinging optical radiation which varies depending upon the distance between target and unit the variation of.