Abstract
The classical Huygens–Fresnel theory of diffraction breaks down for aperture dimensions less than four to five wavelengths. Keller’s Geometrical Theory of Diffraction, on the other hand, is known to fit the exact solution very well for slit-widths as small as two wavelengths. A comparative study of the correlation of the Keller and Kirchhoff theory with measurements of the far-field pattern behind slit-widths 1.092 μ, 1.21 μ, and 2.67 μ, ruled in an aluminum film of thickness 120 nm, has been undertaken. The theory is evaluated for thin slits and for thick round-ended slits of wavelength dimensions, and also for dielectric and conducting edges. The latter are compared with edge-diffraction measurements. The results lead to the conclusion that, at optical frequencies, the inability to fabricate edges of precisely describable size and shape have the effect of extending the relative usefulness of Kirchhoff theory to apertures of wavelength dimensions.
© 1965 Optical Society of America
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