IT is possible to test an optical system by means of white-light interference between two or more wave-fronts which have been scattered by equivalent obstacles out of the same collimated beam.

Scatter fringes of equal thickness

This thesis describes position-sensitive devices (PSDs) and optical sensor systems suitable for industrial tracking and displacement sensing applications. The main application areas of the proposed sensors include automatic pointing of a rangefinder beam and measuring the lateral displacement of an object. A conventional tracking sensor is composed of a laser illuminator, a misfocused quadrant detector (QD) receiver and a corner cube retroreflector (CCR) attached to the target. The angular displacement of a target from the receiver optical axis is detected by illuminating the target and determining the direction of the reflection using the QD receiver. The main contribution of the thesis is related to the modifications proposed for this conventional construction in order to make its performance sufficient for industrial applications that require a few millimetre to submillimetre accuracy. The work includes sensor optical construction modifications and the designing of new types of PSDs. The conventional QD-based sensor, although electrically very sensitive, is not considered optimal for industrial applications since its precision is severely hampered by atmospheric turbulence due to the misfocusing needed for its operation. Replacing the CCR with a sheet reflector is found to improve the precision of the conventional sensor construction in outdoor beam pointing applications, and is estimated to allow subcentimetre precision over distances of up to 100 m under most operating conditions. Submillimetre accuracy is achievable in close-range beam pointing applications using a small piece of sheet reflector, coaxial illumination and a focused QD receiver. Polarisation filtering is found to be effective in eliminating the main error contributor in close-range applications, which is low reflector background contrast, especially in cases when a sheet reflector has a specularly reflecting background. The tracking sensor construction is also proposed for measuring the aiming trajectory of a firearm in an outdoor environment. This time an order of magnitude improvement in precision is achieved by replacing the QD with a focused lateral effect photodiode (LEP). Use of this construction in cases of intermediate atmospheric turbulence allows a precision better than 1 cm to be achieved up to a distance of 300 m. A method based on averaging the positions of multiple reflectors is also proposed in order to improve the precision in turbulence-limited cases. Finally, various types of custom-designed PSDs utilising a photodetector array structure are presented for long-range displacement sensing applications. The goal was to be able to replace the noisy LEP with a low-noise PSD without compromising the low turbulence sensitivity achievable with the LEP. An order of magnitude improvement in incremental sensitivity is achievable with the proposed

/pdf/position-sensitive-devices-and-sensor-systems-for-optical-ytaszgw6pp.pdf

Position-sensitive devices and sensor systems for optical tracking and displacement sensing applications

A Fresnel zone plate of variable size can be formed as a moire pattern between a suitable pair of identical grids. The focusing power may be varied over a considerable range by relative translation of the grids if their line sbapejs are determined by a combination of cubic and linear functions. Elliptical and hyperbolic patterns are also obtainable. If both grids ae constructed in phase reversal form, the irradiance in the diffraction image due to the zone plate is enhanced substantially. The device may then be used with photoelectric detection to monitor the straightness of a continuous linear motion. A possible application to field widening in interference spectroscopy is also suggested.

Varifocal moiré zone plates for straightness measurement

The introduction of holographic techniques into optical measurements is reviewed with some detail of selected examples. Adoption of these techniques seems to have been slow, and many well understood advantages have not yet been adequately exploited. Prospects of more rapid development seem hopeful, however, especially if more convenient sources of coherent light soon become available. Attention is drawn to the importance of holography contributing to better presentation of the results of measurement in visible displays, and the vital effect such a development could have on the acceptance and wider exploitation of holography in measurement.

The influence of holography on measurement technology

Digital reduction of speckle interferometry data for the K2 III giant a Boo (Arcturus) has been done. Angular diameters of o': 019 i 0': 006 were obtained at 4200 A for a uniformly illuminated disk, and 0,027 i 0,010 for a highly limb-darkened disk. Arcturus is underresolved using the Kitt Peak National Observatory 4-meter telescope. The digital-reduction scheme described here proved ideal for studying such underresolved objects. A discussion of limitations of this technique is also provided. Key words: speckle interferometry - atmospheric seeing - resolution

Digital analysis of speckle photographs: the angular diameter of arcturus.

It is shown that afocal lens systems with unit magnification can be used to monitor displacement and tilt of an object moving in a straight line. The lens systems can be incorporated in double-ended optical alignment systems using visual or photo-electric read-out, and have potential applications in machine tool control.

Afocal lens systems for straightness measurement

A holographic technique is described whereby a 35 mm camera lens is used in conjunction with its own film to measure focusing errors and other symmetric aberrations. The method relies on photographing a scatter-plate whose fabrication is described. Results are presented showing how four different camera systems perform at various field angles.

J.M. Burch

Papers

Afocal lens systems for straightness measurement

A lens hologram technique for analysing symmetrical aberrations of a camera system