ARRANGEMENTS

This invention relates to optical position sensing arrangements especially suitable for the remote measurement of the relative position or linear displacement of moving bodies. One known optical position sensing arrangement for remotely measuring the relative position or displacement of a body comprises. a holographic zone plate which may be attached to the aforesaid body and a pair of optical fibres one of which carries from a remote light source white light for illuminating the holographic zone plate and the other of which having its end co-terminous with that of the other fibre collects monochromatic light reflected from the zone plate and having a centre wavelength representative of the distance between the ends of the optical fibres and the zone plate. The collected light is conveyed by the optical fibre to a wavelength analyser which effectively provides an indication of the distance between the fibre ends and the zone plate. If this distance is varied then the wavelength of the collected light will vary and the change in distance or displacement of the plate relative to the fibre ends will be indicated by t-he wavelength analyser. This known arrangement suffers from the disadvantage that the light

emerging from the optical fibre connected to the source of white light takes the form of a cone the semi-angle of which is determined by the fixed numerical aperture of the input fibre. Consequently, unless the zone plate is made sufficiently large some of the light emerging from the. fibre will not impinge on the plate and will therefore be wasted leading to overall optical inefficiency. The position could be alleviated by providing higher power white light sources and/or more sophisticated wavelength analysers.

According to the present invention the above mentioned disadvantage is overcome by providing an optical position sensing arrangement comprising two optical fibres and a holographic zone plate, in which lens means is interposed between the co-terminous ends of said fibres, along one of which (i.e. light transmission fibre) white light is conveyed from a remote light source, and the zone plate, whereby all of the light emerging from the end of the light transmission fibre is focused by said lens means so that it all impinges on the zone plate and in which the co-terminous end of the other fibre (i.e. light collecting fibre) collects all the light from the zone plate and conveys it to a wavelength analyser which indicates the wavelength of the collected light and thus the distance between the co-terminous ends of the optical fibres and the zone plate.

By way of example the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a known optical position sensing arrangement; and,

Figure 2 shows an optical position sensing arrangement according to the present invention.

Referring to Figure 1, light derived from a white light source 1 is conveyed by an optical fibre 2 (i.e. light transmission fibre) to a remote location where it is utilised for the illumination of a holographic zone plate 3 in spaced relationship with the light emerging end of the optical fibre 2. The zone plate 3 may be attached to a linearly movable body (not shown) while the end of the light transmission fibre 2 may be fixed. Monochromatic light reflected by the zone plate 3 is arranged to be collected by the optical fibre 4 (i.e. light collecting fibre) which conveys h-he collected light to a wavelength analyser 5. The wavelength of the collected light which will be determined by the analyser 5 will be dependent upon the distance L between the co-terminous ends of the fibres 2 and 4 and the zone plate 3. Any variation in this distance L will cause a change in the wavelength of the collected monochromatic light which will be detected by

the analyser 5 and provide an indication of the position or displacement of the zone plate 3 relative to the fixed end of the optical fibre 2.

As can be appreciated from Figure 1, some of the cone of light emerging from the optical fibre 2 may fail to impinge on the zone plate 3, more especially as the distance L increases. Consequently, the overall optical efficiency of the sensing arrangement is reduced necessitating, more powerful white light sources and/or more sophisticated wavelength analysers.

In accordance with the invention, the efficiency of such position sensing arrangements is improved, as depicted in Figure 2, by providing a lens 6 which is interposed between the co-terminous ends of the light transmission and light collecting fibres 2 and 4 and the holographic zone plate 3. In view of the smaller angle of the cone of light emerging from the lens 6 all, or substantially all, of -the light emerging from the end of the light transmission fibre 2 effectively impinges- on the zone plate 3.