Astigmatic focus error detection


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.


[Related Terms]
Displacement sensor, Focus detection by critical angle total reflection, ,