Patent ID: 7483120
Filing Date: 2009-01-27
Classification: G03B,G03F

Abstract:
1. A displacement measurement system configured to measure a displacement of a component relative to a reference component, comprising: first, second, third and fourth targets, each mounted to the reference component and arranged such that a target surface of each target is substantially parallel to a reference plane; and first, second, third and fourth displacement sensors, each arranged to measure the displacement of a respective part of the component relative to the target surface of a respective target; wherein said first and third displacement sensors are configured to measure the displacement of first and third parts of the component relative to the target surfaces of said first and third targets, respectively, along a direction substantially parallel to a first direction that lies within said reference plane, wherein said second and fourth displacement sensors are configured to measure the displacement of second and fourth parts of the component relative to the target surfaces of said second and fourth targets, respectively, along a direction substantially parallel to a second direction that lies within said reference plane and is substantially orthogonal to said first direction, and wherein at least one of said first, second, third and fourth displacement sensors is further configured to measure a displacement of the respective part of the component relative to the respective target surface along a direction that is substantially parallel to a third direction that is substantially orthogonal to said first and second directions.