Integral estimator of broadband omnidirectionality
  By using the notion of wavelength- and angle-averaged reflectance, we assess in a systematic way the performance of finite omnidirectional reflectors. We put forward how this concept can be employed to optimize omnidirectional capabilities. We also apply it to give an alternate meaningful characterization of the bandwidth of these systems. 
A. G. Barriuso, J. J. Monzon, L. L. Sanchez-Soto and A. Felipe