Abstract

The often cited criterion for the depth of focus is expressed solely in terms of the f/No. of the lens and the wavelength of light. Under the assumptions that the designer knows in advance the highest line density (resolution) required in the image, a more precise criterion can be formulated. This, for example, applies to the field of photolithography associated with microelectronics. Starting from the block diagram representation of an incoherent imaging system, the incoherent impulse response is obtained. The resulting analytical expression is posed in a normalized form, obviating the specification of any actual physical lengths. The procedure for the application of the Rayleigh criterion to the normalized incoherent impulse response is described for a general lens aperture topology. The resolution expression is shown to depend on a normalized out-of-focus defect parameter. This analysis then is applied to a number of aperture topologies. Specifically, rectangular, annular spherical, and Gaussian spherical apertures are examined. For an aperture without polar (i.e., circular) symmetry, the two-point resolution is dependent on the polar angle. From the analysis for the spherical Gaussian aperture we obtained, up to a constant, agreement with the expression for the resolution-independent depth-of-focus by assuming the diffraction-limited upper limit for realizable resolution.

© 1987 Optical Society of America