A prescription for obtaining the three-dimensional irradiance of any periodic pattern is given and applied to obtaining the irradiance and the depth of focus (DOF) for lines-and-spaces patterns. As is well known, the DOF for such patterns can be made large by restricting the angular spread of the illuminating light. At a fixed k₁, the so-called tip-to-tip distance between two opposing line-ends can be made smaller if the numerical aperture of the imaging optics is increased.