The condition that prevails when the Numerical Aperture of an optical source, such as a laser, light-emitting diode, or optical fiber, exceeds that of the driven element, e.g., optical Fiber core.

The condition that prevails when the Beam Diameter of an optical source, such as a laser, light-emitting diode, or optical fiber, exceeds that of the driven element, e.g., Optical Fiber core. Note: In optical communications testing, overfill in both numerical aperture and mean diameter (core diameter or spot size) is usually required.