Method and apparatus for measuring opposite surfaces

A method of measuring the parallelism of two opposite surfaces, comprising:

• radiating parallel beam via reference surfaces to a pair of test surfaces, said test surfaces being opposed to each other, at least one of said test surfaces being rotary relative to the other about a rotational axis substantially along the opposing direction;
• and
• observing individually interference fringe images each obtained from interference between a light reflected at each of said test surfaces and a light reflected at the corresponding one of said reference surfaces to measure the parallelism of said pair of test surfaces, said method further comprising the steps of:
• measuring obliquities of said rotary test surface of said pair of test surfaces at a first position and at a second position rotated from said first position about said rotational axis by a predetermined angle;
• assuming a cone or cones described by normal vectors of said rotary test surface based on said obliquities of said rotary test surface measured at said first and second positions;
• and
• computing at least one of the largest and smallest angles between said pair of test surfaces from axes and vertical angles of said one or more assumed cones.

An apparatus for measuring the parallelism of two opposite surfaces, comprising:

• an interference optical system for leading parallel beam to a pair of opposed test surfaces and then leading, from said test surfaces via different optical paths, interference fringe images formed by radiation of said parallel beam to said test surfaces respectively;
• and
• means for imaging said interference fringe images led via said different optical paths, respectively,
• wherein at least one of said test surfaces is rotary relative to the other about a rotational axis substantially along the opposing direction, said apparatus further comprising an arithmetic unit for computing the parallelism of said test surfaces from said interference fringe images taken by said means for imaging, said arithmetic unit executing:
• measuring obliquities of said rotary test surface of said pair of test surfaces at a first position and at a second position rotated from said first position about said rotational axis by a predetermined angle;
• assuming a cone or cones described by normal vectors of said rotary test surface based on said obliquities of said rotary test surface measured at said first and second positions;
• and
• computing at least one of the largest and smallest angles between said pair of test surfaces from axis and vertical angles of said one or more assumed cones.