To achieve high throughput with flexible glass or plastical optical fibres, multiple fibres are often arranged in a bundle. This is also desirable when light sources are not point-like and thus cannot be focused to small diameters. However light losses are inevitable for two reasons:
1. For a given fibre, the maximum usable area is defined by the core and cladding diameters. These diameters determine the fibre packing factor (FPF).
2. When fibres are arranged in a bundle, not all of the bundle cross-sectional area is filled with fibres, even when tightly packed arrangements are used. The particular packing arrangement defines the bundle packing fraction (BPF).
