The special equipment requirements and limitations for bifocal intraocular lens Modulation Transfer Function (MTF) measurement using a charged coupled device detector are discussed. Proper MTF measurement requires that at least 90% of the energy passing through the lens aperture be imaged within the sampled area of the sensor. Spectral filtering of optical density greater than 2.5 sufficiently eliminates out of band radiance. Well corrected relay optics of numerical aperture at least twice that of the lens under test and a calibrated image scale are important for accurate results. Significant modulation losses at spatial frequencies well below the Nyquist sampling occur due to pixel geometry, blooming and charge transfer inefficiency. For a bifocal intraocular lens, where the overall blur pattern may be as large as 0.5mm, these factors can limit the ability to accurately compute modulation at spatial frequencies greater than 100 cycles/mm. Correction of equipment induced modulation losses can be realized through improved data processing. Use of MIT measurement for quality control of the Storz TRUE VISTATM bifocal intraocular lens is discussed. Results agree with theoretical expectations to within 10% modulation. Sensitivity to defects such as astigmatism and zone defects are demonstrated at low spatial frequencies.

The STORZ TRUE VISTA bifocal intraocular lens was evaluated in a laboratory setting and during clinical trials. The Modulation Transfer Function and Through Focus Response were measured in a simulated eye chamber for selection of optimal add zone size, location, power and pupil dependency. Results indicate the optimaldesign to be an annular add power of+4D having a 1 .5 millimeterinner diameter and a 2.6 millimeter outer diameter. Optical performance was predicted using computer ray trace and Fourier analysis. Laboratory testing confirmed design performance achieved through the manufacturing process. Clinical results correlate well with Through Focus Response but less well with Modulation Transfer Function. 0-8194-11

The HYDROVIEW® intraocular lens is a biconvex foldable hydrogel lens configured as a conventional one-piece design. The unfolding time is approximately constant with lens dioptric power at an average of2.4 seconds, and increases with the length oftime the lens is held in air prior to implantation. Mechanical recovery can be modeled as a non-linear critically damped harmonic oscillator, which demonstrates the controlled manner ofunfolding. The Modulation Transfer Function at 30 cycles/degree reaches an average of 95% of its pre.-fold value within one hour after being released from folding and recovers completely to its pre-fold values within 24 hours. Interferometrically measured root-mean-square transmitted wavefront error and Strehl Ratio both recover to pre-fold values within 24 hours after folding. Dehydration for a period of 10 minutes followed by folding, then rehydration and unfolding was found to have no significant effect on lens power (<0. 15 Diopter) or resolution. Neither temperatures in excess of 120°C, which may be used during sterilization, nor temperatures as low as -85°C were found to have any significant effect on lens power and resolution.

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