Theory of single- and multiple-entry reflectometric measurements. The intensity distribution at the pupil plane of light returning from a small area of bleached retina is imaged on a CCD camera for a series of entry pupil locations (top panel, data from subject SM, 0 deg, 543 nm, horizontal axis). The total amount of guided light is maximum for the location in the pupil toward which the photoreceptors are pointing. The circled image corresponds to the optimal entry pupil location, and is the image used for single entry reflectometric measurements. Since these measurements are based on a single image, we refer to them as *single-entry measurements* (left bottom panel). The intensity distribution for this image is fit to a constant added to a 2-D gaussian function, from which we obtain rho value : *ρ*_{s}. As the entry position of the illuminating beam moves away from the optimal entry pupil, less light is captured by the cones and guided back. The measurements based on a series of images across the pupil are referred to as *multiple-entry measurements* (right bottom panel). The total amount of guided light as a function of entry pupil position is fit with a gaussian function, with rho value: *ρ*_{mx}. The final rho, *ρ*_{m}, is computed as the mean of the estimates for the horizontal and vertical axis (*ρ*_{mx} and *ρ*_{my}).