MEASURING DISPLACEMENT BETWEEN TWO RELATIVELY MOVABLE STRUCTURES

An apparatus for use in measuring relative movement between two structures is described. A support (7) having bores for receiving input and output optical fibres (2, 10) is mounted on a mounting element (24), secured to one of the structures, at a predetermined position using alignment means (50). A filter (8) is fixed relative to the other structure so that light passing down the input fibre (2) passes through the filter (8) to the output fibre (10). The filter comprises a split field filter so that when light of two wavelengths is transmitted via the input fibre (2), the intensity of light at each wavelength transmitted through the filter (8) depends on the position of the filter relative to the support. An analogue output proportional to the displacement is provided using the transmitted intensities. The invention is particularly applicable to measuring displacement of a railway rail with respect to an underlying structure, and a method and installation for this are described.