Abstract: A multi-mode optical fiber delivers light from a radiation source to a multi-focal confocal microscope with reasonable efficiency. A core diameter of the multi-mode fiber is selected such that an etendue of light emitted from the fiber is not substantially greater than a total etendue of light passing through a plurality of pinholes in a pinhole array of the multi-focal confocal microscope. The core diameter may be selected taking into account a specific optical geometry of the multi-focal confocal microscope, including pinhole diameter and focal lengths of relevant optical elements. For coherent radiation sources, phase randomization may be included. A multi-mode fiber enables the use of a variety of radiation sources and wavelengths in a multi-focal confocal microscope, since the coupling of the radiation source to the multi-mode fiber is less sensitive to mechanical and temperature influences than coupling the radiation source to a single mode fiber.