US Patent 7349082 Particle detection device, lithographic apparatus and device manufacturing method

To enable differentiation between a particle and a ghost particle, a detector system is presented. The detector system is configured to output at least two detector signals corresponding to an intensity of radiation being incident on the detector system. If radiation is received from a ghost particle, not all of the at least two detector signals has a level above a predetermined threshold level, whereas radiation received from a contaminating particle results in all signals having a level above a threshold level. Thus, it may be determined with a high accuracy whether a particle or a ghost particle is redirecting radiation towards the detector system.