Abstract: Infra-red from a field of view is periodically interrupted before being incident on a mechanically resonant detector. Resonation of this caused by the photo-acoustic effect is detected by an optical beam.

Abstract: A strain sensor includes a quartz crystal element 9 having a resonant frequency which is dependent on strain imposed on it. The crystal element 9 is caused to vibrate at its resonant frequency by pulses of light transmitted along an optical fibre 5 from a remote location indicated generally at 1. A signal produced by the vibration at the resonant frequency is taken from electrodes 10 and 12 and amplified at 13 and 14 before being used to switch on and off a switch 15 controlling a light source 16. The output from the light source 16, consisting of flashes of light of a frequency indicative of the strain, is transmitted along a second optical fibre 19 and is detected by a photocell 20. The frequency of the output of photocell 20 is measured at 21 to give the desired measurement of strain and to provide feedback to the drive unit 3 which determines the frequency of the flashes of light produced at 2.

Abstract: A code track on a rotable member (e.g., a disc) defines a pseudo-random sequence of binary digits consisting of ones and noughts. The pseudo-random sequence extends continuously around the rotatable member and includes a given number of n consecutive noughts. The binary digits of the sequence are arranged so that each group of n adjacent digits is different from each other group of n adjacent in the pseudo-random sequence. A reading mechanism is operatively positioned relative to the code track for reading n consecutive digits of the pseudo-random sequence, wherein each n consecutive digits of the pseudo-random sequence define a position of the rotatable member.