Abstract: A universal launch cable assembly for testing multimode fiber optic cable includes a first connector for interfacing with a light source; a second connector for interfacing with a fiber optic cable to be tested; and a stepped index fiber optic cable having an optical fiber of a diameter larger than the diameter of the fiber in the cable to be tested and being disposed in a curved path and having a numerical aperture for generating a secondary uniform, symmetrical, idealized light source. Also disclosed is an intergrated idealized light source system using the univeral launch cable and having means for adjusting the output of the light source as a function of the diameter of the fiber in the cable to be tested.