Patent ID: 8553218

Claim:
A method of calibration of an optical inspection apparatus for inspecting a target, the optical inspection apparatus comprising at least one ingoing optical system for illuminating the target and at least one outgoing optical system for delivering radiation from the target to a detector, the method comprising: using the ingoing optical system and the outgoing optical system in a first arrangement to obtain a first measurement of radiation in an outgoing direction, the measured radiation in the outgoing direction in the first arrangement being a combination of radiation illuminating a target from a first plurality of ingoing directions; using the ingoing optical system and the outgoing optical system in a second arrangement to obtain a second measurement of radiation in the outgoing direction, the measured radiation in the outgoing direction in the second arrangement being a different combination of radiation illuminating a target from a second plurality of ingoing directions; and using the first and second measurements and modeling a difference between the first and second arrangements to determine separately properties of the ingoing and outgoing optical systems.