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The threshold selection method of N. Otsu (ibid., vol.SMC-9, p.62-6, 1979) is shown to break down for a certain range of object-to-background pixel population ratios. Modifications to Otsu's method are proposed to overcome some of its limitations. The findings are also relevant to the closely related methods of T. Ridler (1978) and H.J. Trussell (1979).