A modular semiconductor light source assembly includes a semiconductor light source, such as a light emitting diode, which is mounted on a substrate which supplies electricity to the light source and which assists in removing waste heat therefrom. Substantially all of the light emitted by the LED is transferred to a lens by a light pipe, the cross section of the light pipe increasing from the light source to the lens and the lens having a general D-shape such that the light pattern formed by the lens is constrained in a first direction orthogonal to a second direction. The assembly can be combined with other similar assemblies or other light sources in a light fixture to produce a desired overall beam pattern such as a automobile headlamp low beam or high beam pattern.