What diameter beam will an optical fiber output?

Optical fibers do not produce beam-like outputs, but rather quickly diverging cones of illumination. They are very good for transmitting light because the fiber core is of a higher index of refraction than the cladding. This index difference not only keeps light in the fiber, but also defines the largest angle of light that the fiber can accept. Due to symmetry principles in fiber optics, the output angle of a fiber is approximately the same as the input angle. The full acceptance angle is defined as the maximum allowable input/output angle for each optical fiber and is directly related to the numerical aperture specification (NA). Our typical acrylic fibers will accept a cone of light approximately 61°, 56° or 35°, which correspond to 0.51, 0.47 and 0.3 NA values respectively. If the input angle, say 30°, is smaller than the acceptance angle, say 61°, then the output angle will still be 30°, not the 61° that one might think. If a fiber is overfilled, the output angle will be slightly less than the acceptance angle due to losses.