The position of the first minimum or stationary value which appears in the irradiance beyond the end of the geometrical image in the image plane of an objective with a narrow annular aperture when a line element of uniform radiance lies in the object plane is examined for its relation to the length of the line element. The line element is assumed to be self-radiant or its equivalent by scattering. A table of the relation between line length and the position of the first minimum or stationary value is presented for lengths 2K in the range 0≦2K≦5.728 Airy units. Line elements as short as 0.4 Airy unit appear measurable by this method.
LUTHER W. SMITH and HAROLD OSTERBERG, "Length of Self-Radiant Lines by Use of a Critical Point in the Image," J. Opt. Soc. Am. 54, 595-597 (1964)