1. This concept may appear to be more restrictive than is justified, in that a bundle of rays which is not symmetrically centered on the axis might seemingly be confined by total reflection to the “light pipe” of Eq. (1). But any such “bundle” necessarily exhibits diffraction effects which will distort it into the steady-state configuration which is defined here.
2. R. W. James,The Optical Principles of the Diffraction of X-Rays(G. Bell and Sons, Ltd., London, 1954), p. 200.