Patent ID: 8896817

Claim:
A lithographic projection apparatus comprising: an illumination system arranged to illuminate a patterning device with an illumination beam of radiation having a defined illumination profile, the defined illumination profile comprising a range of illumination angles of incidence and the patterning device including a plurality of markers; a projection system arranged to project an image of the patterning device onto a substrate; a sensor selectively positionable in place of said substrate and arranged to measure a property of the image over a range of acceptance angles of incidence projected by the projection system, the range of acceptance angles of incidence being narrower than the range of illumination angles of incidence of the defined illumination profile; a plurality of diffuser modules, each one of said plurality of diffuser modules associated with a respective one of said plurality of markers, and each one of said plurality of diffuser modules comprising a respective diffuser plate and an associated actuator system; each diffuser plate configured to be selectively positioned with respect to the patterning device by its said associated actuator system into the path of said illumination beam and over said associated one of said plurality of markers on said patterning device and configured to be selectively removed with respect to the patterning device by said associated actuator system from the path of said illumination beam and from being over said associated one of said plurality of markers on said patterning device, and wherein, when each diffuser plate is positioned with respect to the patterning device into the path of said illumination beam and over said associated one of said plurality of markers, each diffuser plate is arranged to increase the range of angles at which radiation of the illumination beam is incident on the associated marker on the patterning device and detected by the sensor such that angular sensitivity of the sensor is mitigated.