Abstract: There is provided herein a sensor for determining relative displacement between two surfaces which utilizes two optically conductive members that have been mounted in proximity to each other, whereby the amount of displacement between the two surfaces may be determined by measurement of the magnitude of the intensity of light transmitted through them. The instant invention preferably utilizes two planar light transmitting surfaces—an emitter and a receiver—which are free to move laterally with respect to each other and are positioned so that when light is introduced into the emitter some proportion of that light falls on the receiver. By electronically monitoring the intensity of light impinging on the receiver an estimate of the amount of overlap between the two optical surfaces and, thus, an estimate of the relative displacement between them, may be obtained.