A photoelectric sensor comprises a thin plate-shaped optical body, a circuit block and a tubular casing for incorporating therein the optical body and the circuit block. A signal light incoming through a transparent protector of the casing is reflected upon a first reflective mirror of the optical body. Following this first reflection, a second reflection is made upon a second reflective mirror thereof. The light having passed through a light collecting lens is received by a photoelectric converting element and converted into electric signals for output. Since the optical body comprises a plurality of reflective surfaces, an optical path up to the photoelectric converting element can be incorporated compactly within the thin plate-shaped optical body.

An optical sensor comprising a light detector, a converging lens located between the light detector and a light source, the light rays from which are detected by the light detector, and a light shielding means provided on a part of the converging lens, the shielding means shielding light rays emitted from the light source, thereby reducing dependence of the amount of light rays to be detected upon angles of incidence with which the light rays enter into the converging lens of the optical sensor.

An optical element includes an ogival reflective surface which diverges an incident radiant energy beam substantially uniformly into a 2.pi. solid angle. The precise shape of the reflective surface is defined mathematically in relation to the desired solid angle of divergence and energy distribution of the incident beam. To facilitate mounting and alignment of the reflective surface in an optical system, it is formed internally of a body of optically transmissive material. The external surface of the transmissive body is shaped so as to provide normal incidence for rays reflected from the reflective surface. The element may be used as a beam diverger, or alternatively in reverse, to converge and collimate radiation.

A scanning device for an optical recording and/or reproducing apparatus comprises a mirror objective in the form of a transparent body one surface of which forms an internally reflecting concave reflector and the opposite surface of which forms an internally reflecting convex reflector. A beam of radiation from a radiation source enters a window in the concave reflector, is divergently reflected by the convex reflector back to the concave reflector, and is consequently reflected thereby through a second window in the transparent body so as to form a scanning spot focussed on an information plane to be scanned by the device. The mirror objective includes a phase grating by which radiation reflected from the information surface is separated from the scanning beam and is directed to a radiation-detection system. A very compact write and/or read scanning device is thereby obtained, which may be given a very robust and stable from by fixing the radiation source and the radiation detection system to the surface of the transparent body.

An optical scanning device having a mirror-objective arranged between a radiation source and an object to be scanned, which mirror objective is a radiation-transparent body having a radiation window and concentric reflector on each of two opposite surfaces of such body. By providing at least one of the windows with an aspherical shape, the mirror objective is well corrected for spherical aberrations without imposing stringent requirements on the accuracy of such aspherical shape.