In a known method of testing braking effectiveness, a sheet carrying a series of lines is attached to a moving part of a machine and these lines are sensed sequentially by a photo detector as the machine part moves. The output of the photo detector thus gives an indication of the movement of the machine part during the braking operation. In the invention the photo detector 21,22 is associated with an optical system 20 which is used to project light to the series of lines; and to focus light reflected from the lines, onto the photo detector. In this way the photo detector and associated equipment can easily be set up, e.g. on a tripod, at a position spaced from the machine whilst still enabling a reliable signal to be generated in the photo detector. An important preferred feature of the invention is the use of retroreflective material on the aforementioned sheet. This greatly enhances the strength of the signal generated by the photo detector.

The specification describes an interference optical measuring or sensing device, comprising a light source, a beam splitter, two component light paths and a radiation sensor for a multiple hole centrifuge rotor. When the rotor is in motion sample cells and the counter-weight move successively through the component light paths and at a particular position have both light paths extending through them. There is furthermore an arrangement for producing a position signal indicating that a selected hole is in a certain position in which it has both component light paths extending through it. A control arrangement ensures that the measuring device is activated briefly in the predetermined position of the selected hole. The light source continuously supplies light between the periods of activation of the measuring device. The component beam paths are respectively coupled with a control light sensor at positions, which in terms of the direction of light from the light source lie behind the multiple hole rotor and the control light sensor produces a component beam output signal when the respective component beam path is completed by a hole in the rotor. The position signal and the two component beam path signals are supplied to a coincidence circuit, which activates the measuring or sensing device on the simultaneous arrival of all three signals.