Abstract: A radiation detector, including, a photodiode, has improved uniformity of sensitivity over the field of view because the receiving surface is provided by an optical diffusing layer, and the field of view is defined by a baffle arrangement, each internal surface of whichy is reflective. The baffle arrangement also defines a volume and the diffusing layer conforms to a part of the volume adjacent to the apex thereof. The diffusing layer may be provided by material, initially in liquid form, solidified in situ within the baffle arrangement. The receiving surface may be provided within a receiving head of the detector, spaced from the photodiode. Radiation may be transmitted from the receiving surface, whether part of a separate receiving head or not, to the photodiode by an optical fibre. An assembly of four consituent detectors, each having three orthogonally arranged mirrors, may together define a hemispherically shaped volume.