Low-coherence interferometer with surface power compensation

A method of measuring a curved test element with a low-coherence Fizeau interferometer comprising:

• a light source,
• a first beamsplitter configured to split light emitted from the light source into a first portion of light directed toward a test arm and a second portion of light directed toward a reference arm,
• the test arm comprising one or more optical elements adjustably positionable at a first distance from the first beamsplitter,
• the reference arm comprising a first collimating lens optically coupled to a flat mirror adjustably positionable at a second distance from the first collimating lens, and
• a second beamsplitter configured to direct the first portion of light focused by the test arm through a flat reference surface to reflect off the curved test element and direct the second portion of light focused by the reference arm through a second collimating lens to reflect off the flat reference surface, the method comprising:
• adjusting the first distance of the one or more optical elements of the test arm to focus the first portion of light at a first focal point and adjust a position of the first focal point, along an optical propagation path between the first beamsplitter and the second beamsplitter, such that a virtual image of the first focal point appears at a focal point of the curved test element;
• and
• adjusting the second distance of the flat mirror with respect to the first collimating lens such that the optical path lengths between the test arm and the reference arm match so that a reflection of the first portion of light off the curved-test element and a reflection of the second portion of light off the flat reference surface are coherent and interference occurs between the reflections, thereby forming an interference pattern.