US Patent 10107761 Method and device for focusing in an inspection system

An inspection apparatus including: a substrate holder configured to hold a substrate; an aperture device; and an optical system configured to direct a first measurement beam of radiation onto the substrate, the first measurement beam having a first intensity distribution, and configured to direct a second focusing beam of radiation onto the substrate at a same time as the first measurement beam is directed on the substrate, the second focusing beam having a second intensity distribution, wherein at least part of the second intensity distribution is spatially separated from the first intensity distribution at least at the substrate and/or the aperture device.