The total cavity loss in several ruby lasers has been determined experimentally from a set of threshold measurements. Scattering loss and end-mirror reflectance were measured separately and the difference between these losses and the total attributed to diffraction. Both plane-parallel and confocal resonators were employed. Diffraction loss in the plane-parallel cavities was 25±8% per pass while diffraction loss in the confocal cavity was 8±8%. These values are in agreement with theoretical analysis within experimental error.
R. L. AAGARD, "Losses in a Pulsed Ruby Laser," J. Opt. Soc. Am. 53, 911-913 (1963)