Degree of diffraction for monochromatic light beams

A parameter, called the degree of diffraction, is defined to describe the diffractive spreading of a monochromatic light beam. The same as the degree of paraxiality that was introduced by Gawhary and Severini in Opt. Lett. 33, 1360 (2008), the degree of diffraction depends only on beam's angular spectrum. With this definition, it is possible to quantitatively compare the diffractive spreading of different light beams.