Convergent measure of focal extent, and largest peak intensity for non-paraxial beams

Second moment beam widths are commonly used in paraxial optics to define the focal extent of beams, and can be used to describe the spot size of point spread functions in microscopy. However, second moments of arbitrary beams are not guaranteed to be finite. I propose the focal concentration area as a measure of focal area, defined to be the ratio of total radial intensity to radial intensity regulated by a unit area Gaussian distribution. I use the Dirac delta limit of this distribution to establish a rigorous upper bound on the peak intensity of non-paraxial beams of a given total intensity, and show that this is achieved by the recently proposed 'proto-beam' solution. I discuss the generalisation to electromagnetic beams, and find the same lower bound as the scalar case. However, this bound cannot be achieved for any physical beam, and as such the physical lower bound must be higher. (paper)