Resolution Efficiency

The historical way of characterizing IOLs was to measure the resolution efficiency which is also available with the OptiSpheric^® IOL. The principle is based on examining the image of an USAF 1951 test chart in the focal plane of the IOL in air using an aperture stop of 3.0 ±0.1 mm. From this examination, the finest pattern (e.g. Group G, Element E) which can be resolved is determined. The resolution efficiency is then calculated by comparison of the spatial frequency of the resolution limit with the cut-off frequency of the diffraction limited system.

This subjective way of determining the resolution of the IOL can be replaced by an objective measurement procedure based on the MTF measurement by taking into account the human eye contrast sensitivity function (CSF).

MTF-Measurement

The Modulation Transfer Function (MTF) is a parameter describing objectively the performance of optical imaging systems. Additionally MTF can be calculated from the lens design data which provides the manufacturers the possibility to compare the image quality of the manufactured lenses with the design expectations.

The resulting image produced by a lens or another optical system will always be somewhat degraded due to aberrations and diffraction phenomena. As a consequence, bright areas will not appear as bright as they do in the object, and dark or shadowed areas will not be as black as those observed in the original patterns.

The MTF is describing the ability of an optical system to transfer the details of an object to the image in terms of contrast and has the value 1 for a perfect contrast reproduction

and the value 0 for a system being unable to produce any image contrast.

The measuring process includes a collimator projecting a target with a suitable pattern typically a slit. The IOL under test collects the light from the target and transfers it into its own image plane. The high performance microscope lens picks up this image and focuses it onto the high resolution CCD-camera. The intensity profile of the target is scanned electronically in both the radial and tangential directions. The data is collected and, by using Fourier Transform techniques, the MTF is calculated and displayed on the PC-monitor in real-time.

The software calculates and displays the MTF value at selected spatial frequencies, the theoretical diffraction limited MTF-graph and the effective MTF-graph.

Instead of using a slit as reticle, evaluation of special target patterns with proprietary design developed by TRIOPTICS gives different advantages:

Using a cross target for example has the advantage of giving simultaneous measurement of the MTF in tangential and sagittal directions.

Using square targets allows for the simultaneous measurement of EFL and MTF in both sagittal and tangential directions. In this way OptiSpheric^®IOL provides consistent accuracy and astigmatism information at an unmatched speed of measurement.

When using a pinhole target, the MTF is obtained by the analysis of the Point Spread Function (PSF) of the lens instead of the line spread function.

This method has a significant advantage compared with the slit method since it shows
the MTF of the lens in all azimuth directions.

Toric IOL can be described much faster and by this way the astigmatism axis are easily determined. Nevertheless this measurement method require a very high resolution CCD camera being more expensive than the one for the standard slit method.

MTF of IOL is recommended to be tested in situ. The Standard or Advanced Model Eye, whose own optical aberrations are negligible, may be used to hold the IOL.

Back Focal Length

The Back Focal Length (BFL) is the distance from the lens vertex to the rear (back) focal point of the lens. To measure the BFL, in a first step the Measuring Head equipped with microscope objectives or achromats is focused to the vertex of the lens (sample) surface. The illuminated reticle of the measuring head is projected on the lens vertex, reflected back and focused again on the CCD-camera.

In a second step the accurate finding of the focal plane position of the lens (sample) is accomplished in the same way as in the power and MTF measurement.

The distance between these two positions is accurately measured to determine the BFL.

Radius of Curvature

The measurement of radius of curvature is similar to the BFL measurement. Focusing is now performed on the lens vertex and in the center of curvature of the surface under test. The distance is measured in the same way as the BFL.

The best focus positions for lens vertex and center of curvature are again found using the autofocus feature of the OptiSpheric^®IOL.