Focus detection by critical angle total reflection
1003102-65
[DEFINITION]
A method of displacement measurement in the optical axis direction
by
detecting the unbalance of light intensity on the sides of the optical axis in a prism using an
optical system consisting of an objective lens and a critical angle prism.
[DESCRIPTION]
In an optical system consisting of an objective lens and a critical angle
prism, the degree of parallelism of incident rays to the prism varies by
whether the specular reflector is on or out of the focus of the objective
lens. If the critical angle prism is adjusted so that it makes the exact
critical angle to parallel rays, that is, the angle at which the incident
rays are totally reflected, the amounts of light on either sides of the
optical axis in the prism are unbalanced in proportion to the distance
between objective lens and target surface. Detecting the unbalance with
a semicircular photo sensor makes accurate measurement of micro-displacement
in the direction of the optical axis. An application of this principle
has already been in use as a non-contact optical micro-probe, named the
High Precision Optical Surface Sensor (HIPOSS), which has a range of measurement
of about 2μm and sensitivity of about 1 nm.
[References]
(56)(58)
[Related Terms]
Displacement sensor, Astigmatic focus error detection