Abstract: A relative position detector has a photodetector arrangement for providing light and responding to the intensity of the light when reflected thereto. A member such as a cylindrical pin having a surface reflective of the light, convex to the photodetector arrangement, and movable relative thereto reflectively sweeps the light across the photodetector arrangement for responding thereto to produce a continuously-curving response indicating with its peak the position detected. The continuously-curving response can be differentiated in a peak detector for even more precise position detection. A mount which may be used for the photodetector arrangement has a support member and a member having a lower coefficient of thermal expansion slidably supported thereon, except for one fixed point.