Understanding the diffractive bifocal contact lens.
How a diffractive bifocal contact lens works is not intuitively obvious. This is because the diffractive bifocal operates by an unusual blend of refraction and interference. The present article seeks to clarify how it achieves its two focal powers. Most of this article, however, is devoted to the conceptually simpler case of diffraction gratings in order to develop an intuition for how a double focus is produced. Single slit, double slit, and multiple slit gratings are considered using two formalisms: Feynman's "arrow" method (also called phasors) and the Fourier method. When dealing with linear diffraction gratings we are interested in the light intensity in the focal plane. By a simple redefinition of variables, all of the mathematics for diffraction gratings can be applied to circular lenses. For this latter case instead of looking at the light intensity in the focal plane, we look at light intensity on the optic axis. The appearance of two focal points is a simple consequence of a combined interference and diffraction pattern. The goal of this article is to present an introduction to Fourier optics and an analysis of the bifocal contact lens in a reader-friendly manner.
Diffractive multifocal intraocular lens image quality.
The diffractive multifocal intraocular lens is implanted in the eye to replace a cataractous crystalline lens. Axially separated images from two diffraction orders provide near and distance vision. Overall image quality is affected by both the quality of the in-focus component and the energy distribution between images. Modulation transfer function and energy distribution data are presented. Measurement accuracy is limited by the diameter of the point spread function and by the difficulty in separating the focused and defocused components.
Contrast discrimination by the human visual system
In vision of everyday scenes, features requiring detection are frequently observed in the presence of suprathreshold background structures. Detection of such features is a contrast discrimination task and is often necessary for the subsequent process of recognition. In order to provide a description of this task, contrast discrimination measurements were determined for targets having luminance profiles which were localised in both space and spatial frequency. The investigation extends earlier work on this topic by measurement of contrast discrimination levels for different base contrasts, sizes, luminances and aspect ratios of the targets. For all conditions, it is shown that the functional variation of the contrast discrimination level with base contrast can be described, approximately, by a single curve with one scaling constant. This constant is specified by the contrast threshold level of the target. A model is proposed to describe the contrast discrimination process. The model contains two noise sources, one with noise level proportional to the target contrast and the other with noise level proportional to the contrast threshold level of the target. Predictions from the model adequately describe the experimental data of contrast discrimination against base contrast and, in addition, fit data on the probability of discrimination against the level of contrast difference. An example is given of a simple application of the model to the determination of the number of discriminable steps in contrast as a function of the spatial frequency of a sinusoidal grating target.