Airy disk
In optical aberration theory, it is common to describe an imaging system as "diffraction-limited" if the Airy disk radius is larger than the RMS spotsize determined from geometric ray tracing (see Optical lens design). The Gaussian profile approximation provides an alternative means of comparison: using the approximation above shows that the RMS width formula_49 of the Gaussian approximation to the Airy disk is about one-third the Airy disk radius, i.e. formula_55 as opposed to formula_56.