Interferometric apparatus for precision measurement of altitude to a surface

Interferometric apparatus for directly measuring changes in altitude between points along a datum line on an object and corresponding points along a datum line on a displaced reference body, said apparatus comprising:

• means for defining an object mounted for nominally plane translation with respect to said reference body in at least two orthogonal directions while experiencing relatively smaller changes in altitude in a direction nominally normal to said at least two orthogonal directions;
• elongated reflector means mounted with respect to said reference body;
• a source for generating an input beam that is initially parallel to said object plane of motion;
• and
• at least one interferometer system mounted at least in part on said object for movement therewith, said interferometer system comprising at least one multifaceted reflecting assembly structured to be scanned by said input beam along a collection of points representing an object datum line and provide a measurement beam that travels along an optical path part of which is folded to and from said elongated reflector means and the remaining part of which is parallel to said object plane of motion, scanning said elongated reflector means along a collection of points corresponding to those on said object datum line to define a reference body datum line associated with said reference body, said interferometer system providing a signal containing information that varies directly in accordance with only the altitude between said object and reference body as said object moves, said interferometer system being configured so that said signal is insensitive to variations in angular rotation of said object at least about one of said two orthogonal directions and such that said signal variation is decoupled from movement along at least one of said orthogonal directions.

Interferometric apparatus for measuring changes in altitude between an object and a datum line, said apparatus comprising:

• a dimensionally stable metrology frame having a reference surface containing a datum line;
• at least one object mounted for nominally plane translation with respect to said metrology frame in at least two orthogonal directions while experiencing relatively smaller changes in altitude in a direction nominally normal to said at least two orthogonal directions;
• elongated reflector means fixedly mounted with respect to said object for movement therewith, said elongated reflector means lying in a plane substantially parallel to the plane in which said object translates;
• and
• at least one interferometer system mounted at least in part on said dimensionally stable metrology frame, said interferometer system comprising a source for generating reference and measurement beams, said interferometer system having a reference path along which said reference beam travels to and from said reference surface and a measurement path along which said measurement beam travels to said elongated reflector means, striking it at normal incidence and returning therefrom to be combined with said reference beam to provide a signal containing information that varies in accordance with said changes in altitude between said object and said datum line as said object moves, said interferometer system being configured so that said signal is insensitive to variations in angular rotation of said object at least about one of said two orthogonal directions.

Interferometric apparatus for measuring changes in altitude between an object and a displaced reference body, said apparatus comprising:

• means for defining an object mounted for nominally plane translation with respect to said displaced reference body in at least two orthogonal directions while experiencing relatively smaller changes in altitude in a direction nominally normal to said at least two orthogonal directions;
• elongated reflector means fixedly mounted with respect to said object for movement therewith;
• a source for generating an input beam that is parallel to said object plane of translation;
• and
• at least one interferometer system mounted at least in part on said displaced reference body, said interferometer system having a reference path and a measurement path which includes said elongated reflector means, said reference path being structured to be scanned by said input beam along a collection of points representing a reference datum line to provide a reference beam and said measurement path being adapted to provide a measurement beam that travels along said measurement path, to and from said elongated reflector means, scanning said elongated reflector means along a collection of points corresponding to those on said object datum line to define an object datum line associated with said object, said interferometer system being operative to provide a signal containing information that varies directly in accordance with only the distance between said object and said displaced reference body as said object moves, the distance between corresponding points of said datum lines at any location of said object with respect to said reference body corresponding directly to the altitude, said interferometer system being configured so that said signal is insensitive to variations in angular rotation of said object at least about one of said two orthogonal directions and such that said signal variation is decoupled from movement along at least one of said orthogonal directions.