An apparatus for detecting a centroid of a spot produced by electromagnetic radiation, e.g., optic radiation, using an array of PIN photodiodes serving as photodetectors and being organized in columns and in rows. Vertical connections are used to interconnect the PIN photodiodes in the columns in accordance with a first pattern that interconnects two or more adjacent columns. Horizontal connections are used to interconnect PIN photodiodes in the rows in accordance with a second pattern that interconnects two or more adjacent rows. The first and second patterns of interconnections can include just two adjacent columns and two adjacent rows, respectively and form a checkerboard interconnect pattern. The interconnections are made such that there are no anode connections between the PIN photodiodes in the rows and columns. The apparatus has a processing circuitry for rapidly deriving an X-extent of the centroid from current signals obtained from the columns and a Y-extent of the centroid from current signals obtained from the rows.