We present a study on the improvement of the external Quantum Efficiency (QE) of Gallium-Nitride-based Light Emitting Diodes (LEDs) by the use of the Single Mirror (SMLED) design [N.E.J. Hunt et al., Electron. Lett. 28, 2169 (1992)]. Three different substrate emitting LEDs are compared by measurements and simulations. An increase in the external QE by more than a factor of two is demonstrated, due to the enhancement of the Light Extraction Efficiency for the SMLED design. Using the SMLED design leads to values of the ext. QE of 10.9% (in air) and 23.2% (in glycerine, simulating a device in epoxy), while simulations predict Light Extraction Efficiencies (LEE) of 28% and 50% (in air and packaged) respectively, taking into account only the emission through the substrate. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhem.