A flatfield is a localized flat spot on a spherical or aspheric optical element like a lens or curved mirror. These defects usually result from uneven polishing, mounting stress, or localized physical damage. Because they disrupt the designed curvature, flatfields cause severe wavefront distortion and localized aberrations in the reflected or transmitted beam.
The interferogram shows a massive flatfield near the edge, so the vendor must have clamped it too hard during polishing.
A flatfield often shows up on an interferometer as a sudden region of straight, parallel fringes interrupting a curved fringe pattern.
Related terms
Misfigure
A physical deviation in an optical component's surface shape from its intended design.
Spherical aberration
An optical defect where light rays passing through the edge of a lens focus at a different point than central rays.
Scratch-dig
The surface quality rating for optics, given as two numbers such as 20-10 for the allowed scratches and pits.