Asymptotic approximation to the encircled energy function for arbitrary aperture shapes.

@article{Clark1984AsymptoticAT,
  title={Asymptotic approximation to the encircled energy function for arbitrary aperture shapes.},
  author={P P Clark and J. Woodford Howard and Edward R. Freniere},
  journal={Applied optics},
  year={1984},
  volume={23 2},
  pages={
          353
        }
}
Encircled energy can be easily estimated for imaging systems with oddly shaped apertures using a simple asymptotic approximation formula. A useful tool for the optical system designer, it linearly relates the encircled energy to the perimeter-to-area ratio of the aperture. Comparisons made with numerical calculations show that it is usefully accurate for circles in the focal plane as small in radius as λf #.