Optical performance of decentered monofocal intraocular lenses.

A model eye water cell was used to evaluate the optical performance of biconvex, meniscus, and plano-convex (spheric and aspheric) monofocal poly(methyl methacrylate) intraocular lenses when the lenses were centered and when they were decentered 1 mm and 2 mm. Resolution, induced astigmatism, and modulation transfer function measurements were performed for all lenses with the more convex surface of the lens oriented toward the incident light. The same measurements were performed for the plano-convex and meniscus lenses in the reverse orientation. The lens shapes least affected by decentration were the biconvex and spheric plano-convex with the convex surface oriented toward the incident light. When centered, the aspheric plano-convex lenses had the best overall contrast performance based on the modulation transfer function measurements. However, once decentered the performance of the aspheric lenses approached that of the meniscus lenses in the reverse orientation, the lens shape which had the worst performance.

Survey of the optical quality of intraocular lens implants.

Intraocular lenses (IOLs) from 15 U.S. firms were tested for conformance to the requirements of the American National Standards Institute Z80.7 standard for IOLs. A total of 162 IOLs were tested for resolution, astigmatism, and accuracy of labeled power. Average resolving power was 78% of the diffraction limit, much better than the ANSI minimum requirement which is typically equivalent to 30% of the diffraction limit. This suggests that the ANSI Z80.7 requirement could be significantly tightened with little effect on current production practices. Only one IOL exhibited astigmatism in excess of 0.25 diopter. Differences between measured and labeled power in excess of 0.50 diopter were found in 22 lenses, indicating that accuracy of refractive power may be the most commonly missed optical requirement of the standard.

Optical quality testing of monofocal intraocular lens implants with a 3 mm and a 4 mm aperture.

The optical quality of 81 monofocal poly(methyl methacrylate) intraocular lenses (IOLs) from eight U.S. firms was tested using a 3 mm aperture, as recommended by the American National Standards Institute Z80.7 standard for IOLs, and a 4 mm aperture. The use of the larger aperture had no effect on the measurements of refractive power and astigmatism. When examined with the 3 mm aperture, the average resolving power of the IOLs was 81% of the diffraction limit; when examined with the 4 mm aperture, the average resolving power was 67% of the diffraction limit. Use of the larger aperture would ensure that a larger area of the IOL had been examined. Incorporating the larger aperture into the ANSI minimum resolution requirement would not appreciably affect the rejection rate for currently manufactured monofocal lenses.

Transmission of a Gaussian scanning beam through circular, square, and slit apertures: Part II. Experimental results for transmission through a circular aperture.

To perform an analysis of the hazards associated with scanned laser radiation to determine compliance with either federal or user standards, it is usually necessary to determine the amount of energy entering a specified aperture. A mathematical model has recently been developed that may simplify the hazard analysis. Experimental data and the mathematical model were compared for stationary and scanning Gaussian laser beams transmitted through a stationary 7-mm-diameter circular aperture, representing the dilated pupil of the human eye. Data were obtained for three values of laser beam diameter. The data and the mathematical model were found to agree within the estimated experimental error when the laser beam was concentric with the aperture, the situation typically encountered in compliance measurements. For two of the three laser beam diameters studied, as the eccentricity of the laser beam with respect to the center of the aperture was increased, the difference between the data and the mathematical model was found to increase. At the higher degrees of eccentricity, the percentage errors were shown to be highly sensitive to the errors in the determination of relative position.

Design and evaluation of a null lens for testing the optical performance of silicone intraocular lenses.

A two-element null lens, optimized for a 20-diopter silicone intraocular lens (IOL), was designed and tested. The performance of the null lens was examined for silicone IOL's with in situ powers from 16 to 24 diopters. Improvements in resolution, resolution efficiency, and modulation transfer function were obtained over the tested power range.

Minimum resolution specification of intraocular lens implantsusing the modulation transfer function.

We investigated the use of modulation transfer function (MTF) measurements to provide a standard test of minimum optical quality of intraocular lenses. We used a water cell with plane entrance and exit windows. This geometry is independent of lens material but relatively simple to implement. We investigated the choice of aperture stop, and 3.0 mm was deemed a suitable choice of stop diameter. Minimum acceptable performance must be specified if this technique is to be adopted as a standard method. The MTF of an ideal lens defocused 1/2 wave is suggested as a possible reference. Strehl ratios, although desirable because they can be measured directly without determining MTF's, were found to be unsuitable. These ratios tend to emphasize the high-frequency response, and the observed ratios are typically too low to provide assurance that the low-frequency response is as high as desired. MTF integrals or contrast at spatial frequencies of particular interest were found to be useful benchmarks of acceptable optical quality.

Viable bacteriophage in CO2 laser plume: aerodynamic size distribution.

The size of CO2 laser generated plume particles containing viable bacteriophage, phi X174, was determined with 2 models (10-830 and 15-500) of Andersen bioaerosol cascade impactors. Samples were collected during 100 one-second laser exposures (approximately 68 W/cm2) of a bacteriophage-agar substrate with and without a space-confining hood. The hood appeared to facilitate collection of the largest particles (P < 0.1). In addition, Andersen model 15-500 was a more efficient collector of the largest particles, a phenomenon which is likely a function of the dynamic nature of the laser plume as well as impactor design. We found that laser plume particles containing viable bacteriophage are very large, in one instance exhibiting a count median aerodynamic diameter (CMAD) and mass median aerodynamic diameter (MMAD) of 23 and 55 microns, respectively. Furthermore, the appearance of viable bacteriophage in these plume particles constitutes an extremely rare event. The limitations of cascade impactor design when used to analyze particles with high water concentrations generated in a laser plume are discussed.

In vitro production of viable bacteriophage in carbon dioxide and argon laser plumes.

Both CO2 and argon laser ablation of an agar substrate containing high titres of bacteriophage phi X174 create plumes which disperse viable phage particles. Irradiances at the beam impact site ranged from 73 to 215 W/cm2 for the CO2 laser and from 40 to 227 W/cm2 for the argon laser. To increase the absorption of argon laser radiation, oxidized hemoglobin was added to the target material. Plume-borne viable phage were observed to be associated with particles large enough to settle out from the plume within 100 mm of the beam impact site. The ratio of the number of dispersed viable phage to the number of viable phage potentially dispersible by a single, 1-second laser exposure was on the order of 10(-6) to 10(-5).