Charged particle beamlet lithography system for transferring a pattern to a surface of a target comprising a sensor for determining one or more characteristics of one or more charged particle beamlets. The sensor comprises a converter element for receiving charged particles and generating photons in response. The converter element comprises a surface for receiving one or more charged particle beamlets, the surface being provided with one or more cells for evaluating one or more individual beamlets. Each cell comprises a predetermined blocking pattern of one or more charged particle blocking structures forming multiple knife edges at transitions between blocking and non-blocking regions along a predetermined beamlet scan trajectory over the converter element surface. The converter element surface is covered with a coating layer substantially permeable for said charged particles and substantially impermeable for ambient light. An electrically conductive layer is located between the coating layer and the blocking structures.