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.
Quantitative performance of bifocal and multifocal intraocular lenses in a model eye: point spread function in multifocal intraocular lenses.
OBJECTIVE
To compare the optical properties of bifocal diffractive and multifocal refractive intraocular lenses.


METHODS
A model eye with a pupil 4.5 mm in diameter was used to determine the point spread function (PSF) of the distance focus and near focus of a diffractive bifocal intraocular lens (IOL) (model 811E; Pharmacia Inc, Columbus, Ohio) and of a refractive multifocal IOL (model SA40N; Allergan Optical Inc, Irvine, Calif) to compare them with PSFs of foci of corresponding monofocal lenses. For interpreting the PSFs the through focus response, the modulation transfer function, and the Strehl ratio were evaluated.


RESULTS
The intensity of the distance focus of the bifocal diffractive lens reached 58.5% and the near focus attained 42.7% of the intensity of a corresponding monofocal lens. The maximal halo intensity surrounding both foci was approximately 4.5%. The distance peak of the refractive multifocal IOL was 73.4% and the near peak 25.1% of a corresponding monofocal lens. The out-of-focus image overlaying the distance focus of the refractive multifocal IOL was approximately 3% of the light intensity of the distance focus, whereas the PSF of the near focus of the multifocal IOL is substantially affected by out-of-focus images. The computed modulation transfer functions show better results for the monofocal lenses, similar results for the tested distance foci, and clear advantages for the bifocal diffractive near focus.


CONCLUSIONS
Modulation transfer functions reveal comparable properties for distance vision and a superiority of the bifocal diffractive lens over the refractive multifocal lens for near vision.
The relation of food quantity to rumination behavior.
Preliminary work suggested that the quantity of food ingested by retarded individuals who usually ruminated following meals was related to the frequency and duration of ruminating responses. This possible relation was experimentally examined by systematically varying food quantity from regular portions to satiation levels for three retarded individuals who exhibited high levels of ruminating. A clear functional relation of food quantity to ruminating emerged, with satiation procedures producing rapid and large decreases in the relatively high frequencies and durations of ruminating characteristic of baseline food quantity conditions.