Astigmatic focus error detection1003102-11
A method of non-contact displacement measurements along the optical
axis by detecting the distortion of a spot image projected from an astigmatic optical system.
Astigmatism is an aberration produced by a lens that has different
focal length in two perpendicularly intersecting planes both of which including the optical axis.
An image of a spot projected by an optical system having astigmatism varies its shape from a tall
ellipse, to circle, and to wide ellipse depending on the position of observation along its axis.
By detecting this change of the shape of the image with a quadrant photo sensor, displacement in the
direction of the optical axis can be measured. A measurement system based on this principle attains
a sensitivity of about 1 nm in the measurement range of ±2μm.
Displacement sensor, Focus detection by critical angle total