In northern climate it is difficult to attain high yearly solar fractions, due to the combination of high load and low insolation in winter. Attempts to increase collector area result in additional costs and overheating problems. Collectors with internal reflectors present one possible solution of matching the load over the seasons and the available solar insolation and eliminate the overheating problems during the summer. This thesis work presents analytical methods to describe the optical efficiency of LACs (Load Adapted Collector) for beam, diffuse and ground reflected radiation. The optical design is based on the angular distribution of the irradiation, the load, temperature levels and the angular dependent optical efficiency. Two collectors LAG I and LAG II have been built according to the modelling results. The first collector model LAC I consists of two troughs, while the second collector model LAC II consists of four troughs. The two collector models have a concentration ratio of 3. The optical performance of load adapted collectors LACs is mainly dependent on collector geometry. With this dependency on collector geometry, we have with this kind of collectors high optical efficiency during the spring, autumn and as high as possible during the winter and low optical efficiency during the summer. This variation of optical efficiency creates the aimed seasonal variation of collector output. The calculated optical performance of LAC I and LAC II is described by ?0,beam,h 1)=0.73, ?0,beam,L 2)=0.24, ?0,beam,h =0.64, ?0,beam,L =0.41, for both of them respectively. 1) Optical efficiency at high incidence angle. 2) Optical efficiency at low incidence angle.