The response of an isolated model foveal cone to a partially coherent sinusoidal intensity source is explored with the methods of photoreceptor optics. This extends the work with coherent sinusoidal intensity sources reported in part one [J. Opt. Soc. Am. A 11, 1193 (1994)]. The dielectric waveguide model of a foveal cone previously used to discuss the origin of the Stiles–Crawford effect and its wavelength dependence is applied to photoreceptor excitation by a partially coherent sinusoidal intensity source with variable spatial frequency and light wavelength. The results indicate that the photoreceptors themselves contribute to visual acuity through a wavelength-dependent response at each spatial frequency and that the response is dependent on the spatial coherence of the source. In particular the isolated photoreceptor has a characteristic low-pass-spatial-frequency filter effect for wavelengths between 410 nm and 654 nm (where two waveguide modes are carried). The coherent and the incoherent response curves are separated by differing amounts depending on the wavelength of the light. For wavelengths greater than 654 nm (the single-mode region) the incoherent curve also has the shape of a characteristic low-pass-spatial-frequency filter, while the coherent curve shows a flat response. The results are used to discuss human eye modulation-transfer-function measurements.
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