An angular differential measurement method was used to test transmission efficiencies of optical fibers. The angular measuring accuracy is about 0.2°. Solar radiation was used as a parallel incident beam. The solar power fluctuation during each period of trial (usually several minutes) was measured to be less than 2%. Several transmission curves were obtained corresponding to different fiber lengths, all showing very similar transmission characteristics. A typical transmission curve for the optical fibers is given in Fig.6.

Due to the UV and IR absorption of solar spectrum, only 89% transmission efficiency was measured at the angle of 0o. The efficiency was further reduced to 50% at the rim angle of 23o. A major part of the transmission loss was caused by the imperfect total internal reflections along side surface of optical fibers. As shown in Fig.6, it was dependent on polar angles. Rays of large angles suffered from higher losses. From basic principles of optics, it is known that Fresnel reflections on the end-faces of a light guide depend also on input beam angles, which could be avoided by antireflection coating techniques in the future.