Micromirror-based sending and detection optical assembly for time-of-flight laser scanners

We design the optical unit for an imaging time-of-flight scanner camera based on partially steerable micro mirrors. This new class of 3D cameras enables video frame rates and-in conjunction with the accompanying user software-online real-time selection of regions of interest. The challenges for the optical design comprise (i) sufficient light collection from close-up objects, (ii) maximizing optical efficiency for objects at large distances, (iii) reduction of the dynamical range of signal returns and (iv) minimization of parasitic scattering. We present a solution based on coaxial beam guidance, where the emitted beam first passes a beam splitter, is then deflected by a dedicated emission mirror in the center of a point-symmetrical, synchronized arrangement of five micro mirrors and final ly passes a protective spherical glass cover. The mirror assembly is slightly displaced from the center of the dome in order to establish a secondary focus for parasitic reflections at the inside of the cover. The light scattered at the target surface which reaches the mirror array is directed towards an assembly of rhomboid prisms. These prisms reshape the distributed mirror array aperture such that a small lens with high numerical aperture suffices to focus the light onto a fast, small-area avalanche photo diode, thus maximizing the acceptance angle of the detector and permissible misalignments of the element mirrors.