A one or two channel radiometer with a long fibre optic probe extension. The light is collected from the tip of the probe at right angles to its long axis. Thus the probe can be inserted in short grass sward and collect down-welling light. The filters and diodes are housed in a small ABS box, which is neatly mounted with the measuring unit to make a compact hand-held system. The response of this probe can be can be chosen from the standard range of sensors, eg. PAR Quantum response