## Abstract

Simple polynomial formulas to calculate the FWHM and full width at
1/*e*
^{2} intensity diffraction spot size and the depth of focus at a Strehl ratio
of 0.8 and 0.5 as a function of a Gaussian beam truncation ratio and a system
*f*-number are presented. Formulas are obtained by use of the
numerical integration of a Huygens-Fresnel diffraction integral and can be used to
calculate the number of resolvable spots, the modulation transfer function, and the
defocus tolerance of optical systems that employ laser beams. I also derived
analytical formulas for the diffraction ring intensity as a function of the Gaussian
beam truncation ratio and the system *f*-number. Such formulas can be
used to estimate the diffraction-limited contrast of display and imaging systems.

© 2004 Optical Society of America

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