An apparatus for detecting a beam of radiation, such as a collimated laser beam, and determining the angle-of-arrival of that beam with a high angular resolution, the apparatus comprising a linear array of radiation detectors arranged in a plane and a digital Gray code shadow mask, located at a predetermined height above and parallel to the plane. The Gray code shadow mask is provided with parallel rows containing slot(s) with each of those rows being centrally positioned directly above one of the radiation detectors and arranged so that the rows are perpendicular to the linear array. The slot(s) in each row are located at predetermined distances from the center of a row with those distances differing for each row. A well collimated radiation beam, such as from a laser, directed towards the shadow mask will project images of the slots onto the plane with the positions of the images being dependent on both the positions of the slots and the angle-of-arrival of the beam. This will result, according to the digital Gray code, in some of the images being located on some of the radiation detectors, each of which will then generate an output signal, while some of the radiation detectors will be located in between the images of the slots and not generate any signal. Since the location of the images of the slots is dependent on the angle-of-arrival of the beam, that angle-of-arrival can then be determined from the outputs of all the radiation detectors.