- The optical performance of remote lighting systems and recent innovations in solar fiber- optic concentrators is acutely sensitive to transmission losses in their optical fibers. Typically, these multi-mode fibers are expected to incur small losses over long distances for broad- spectrum light sources. Experimental results reveal substantial light leakage within the nominal numerical aperture of optical fibers that have been deemed suitable for these applications. The same fibers exhibit negligible attenuation in their core. Of particular interest is the dependence of this leakage on:(a) incidence angle,(b) the optical properties of the core and the cladding, and (c) fiber length. We present laboratory measurements of fiber angular transmission, along with a theoretical model.