 The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized.Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency.The refractive index of the package material should also match the index of the semiconductor, to minimize back-reflection.