Monitoring distance variations

Abstract

An optical device for monitoring variations in the distance between an object and a datum is particularly useful for monitoring running clearances in the demanding environment of a gas turbine engine. The device depends upon the principle that an image focussed by an astigmatic lens system changes shape as it passes through the focus. To monitor variations, e.g., in the working clearance between a turbine blade tip and a turbine casing, light from a light source is projected through an astigmatic lens system, onto the blade tip. Reflected light from the astigmatic image on the blade tip is passed back through the lens system and re-projected onto an image shape monitor which includes photo-cells. The photo-cells may be mounted in the main optical assembly or probe, or they may be distant from the probe and fed by light guides. Signals from the cells are combined to produce a monitor signal, the magnitude of which is a measure of the shape of the image on the blade tip, and hence of the clearance.

Dependencies are inferred from the claim text.