The structure of the Maxwell spot centroid

Albert Le Floch^a,b, and Guy Ropars^a,c
aLaser Physics Laboratory, University of Rennes, 35042 Rennes Cedex, France
^bQuantum Electronics and Chiralities Laboratory, 20 Square Marcel Bouget, 35700 Rennes, France
^cUFR SPM, University of Rennes, 35042 Rennes Cedex, France
Dedicated to Prof Jay M Enoch

The dark entoptic Maxwell spot centroids seen through a blue filter, which coincide with the blue cone-free areas centered on the foveas, are shown to exhibit a particular structure. When observing through the green part of a blue-green exchange filter in a foveascope, after a fixation through the blue part, a small orange disc is seen around the centre of the pale green memory after image corresponding to the blue cone-free area of the retina. Using artificial pupils with different diameters, we show that the defined small circular pattern corresponds to the Airy disc due to the Fraunhofer diffraction through the pupil. Typically, for an eye with a 3 mm diameter pupil, the Airy disc exhibits a diameter of about 8 μm at the centre of the usual 100-150 μm Maxwell centroid. Fixation tests show that the towering central maximum of the Airy pattern irradiance corresponds to the preferred locus of the fixation of the eye at the centre of the blue cone-free area of the fovea. The preferred locus of fixation in human vision thus appears to be located in the only area of the fovea where the large chromatic dispersion is cancelled, optimising the eye acuity. © Anita Publications. All rights reserved.
Keywords: Entoptic images, Fovea centralis, Maxwell spot, S cones, Chromatic dispersion, Airy pattern, Line of sight.

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