Abstract: A radiation sensor with particular application to detection of sunshine has at least two (but typically seven) light sensitive detectors and a masking element. The masking element has a pattern of translucent and opaque areas which are disposed to ensure that at any time at least one detector can be exposed to direct sunlight (if the sun is shining) through a translucent area and at least one detector is shaded from direct sunlight by an opaque area. Signal processing apparatus analyses signals from the detectors, such analysis including a comparison of the signal from the detector exposed to the most light and the signal from the detector exposed to the least light.