A sensor arranged to determine a position of a charged particle beamlet in a multi-beamlet exposure apparatus, wherein the sensor comprises: a converter element for receiving the charged particle beamlet and arranged to generate light in response; and a light sensitive detector arranged to receive the light; wherein the converter element comprises a charged particle beamlet receiving surface provided with a two-dimensional blocking pattern design, wherein the two-dimensional blocking pattern design comprises: a plurality of blocking regions arranged to block the charged particle beamlet; and a plurality of non-blocking regions; and the charged particle beamlet receiving surface being divided into sub-cells, each sub-cell having a two-dimensional blocking pattern covering the charged particle beamlet receiving surface; characterized in that: the two dimensional blocking pattern of each sub-cell is unique relative to the two-dimensional blocking patterns of the other sub-cells, wherein the two-dimensional blocking pattern of a sub-cell has a variation of the two-dimensional blocking pattern in a first direction along a first dimension of the two-dimensional blocking pattern and a variation of the two-dimensional blocking pattern in a second direction along a second dimension of the two-dimensional blocking pattern, and the sensor is adapted to determine, from the variation of the two-dimensional blocking pattern within each sub-cell, which sub-cell the charged particle beamlet is occupying and the position of the charged particle beamlet within that sub-cell. a converter element for receiving the charged particle beamlet and arranged to generate light in response; and a light sensitive detector arranged to receive the light; wherein the converter element comprises a charged particle beamlet receiving surface provided with a two-dimensional blocking pattern design, wherein the two-dimensional blocking pattern design comprises: a plurality of blocking regions arranged to block the charged particle beamlet; and a plurality of non-blocking regions; and the charged particle beamlet receiving surface being divided into sub-cells, each sub-cell having a two-dimensional blocking pattern covering the charged particle beamlet receiving surface; a plurality of blocking regions arranged to block the charged particle beamlet; and a plurality of non-blocking regions; and the charged particle beamlet receiving surface being divided into sub-cells, each sub-cell having a two-dimensional blocking pattern covering the charged particle beamlet receiving surface; characterized in that: the two dimensional blocking pattern of each sub-cell is unique relative to the two-dimensional blocking patterns of the other sub-cells, wherein the two-dimensional blocking pattern of a sub-cell has a variation of the two-dimensional blocking pattern in a first direction along a first dimension of the two-dimensional blocking pattern and a variation of the two-dimensional blocking pattern in a second direction along a second dimension of the two-dimensional blocking pattern, and the sensor is adapted to determine, from the variation of the two-dimensional blocking pattern within each sub-cell, which sub-cell the charged particle beamlet is occupying and the position of the charged particle beamlet within that sub-cell.