A tree scale distributed multipoint measuring system of photosynthetically active radiation (PAR) has been designed. The system is divided into a central unit, measuring units and sensor-amplifiers. Measurements can be carried out with a maximum number of 32 measuring units each comprising 24 sensor-amplifiers. The feasibility of the system has been demonstrated with measurements within a Scots pine canopy using a 12 m telescopic mast with horizontal cross-booms. Two sensor arrays of 4 m in length comprising about 40 sensors were divided into 10-sensor sub-arrays of 1, 2, and 4 m in length. The simultaneously measured PAR averages over the sub-arrays differed considerably from the 4 m reference arrays. The maximum difference was 100% in the case of the 1 m sub-arrays, 80% in the case of the 2 m sub-arrays and 54% in the case of the 4 m sub-arrays. When averaged over the 6 h measuring period around noon, the respective maximum differences were 27, 15 and 3%. © 2001 Elsevier Science B.V.