Measurement of Power, Add Power and Cylinder in Compliance with ISO 11979
The OptiSpheric® IOL bench is using the imaging of an object through the lens under test. The resulting image is analyzed in order to evaluate the parameters of the lens (Power, Imaging quality through MTF calculation, energy distribution, Strehl ratio, and more…). Using this imaging technique is similar to realistic conditions and allows testing any type of IOLs designs.
Different types of target are projected to infinity through a collimator. The lens under test gives therefore an image of the target in its focal plane. A measuring CCD microscope is used for analyzing the image by the help of an autofocus mechanism. The instrument also includes different aperture sizes and different imaging objectives chosen automatically by the system according to the power of the lens under test. The wavelength used for the measurement is 546 nm as requested in the ISO 11979 standard.
Power Measurement Principle
Power, Add power and Cylinder are measured from the magnification of a double slit test target. The Powers are determined in each focal planes of the IOLs, two different focal planes in case of a bifocal lens, two different crossed focal planes in case of a toric lens, four different focal planes in the case of a bifocal toric IOL, etc… The Add Power is calculated by subtraction of the Near and Far Powers and the Cylinder is calculated by subtraction of the crossed directions powers of a toric lens.
MTF Measurement Principle
The measurement of the imaging quality is done through the evaluation of the Modulation Transfer Function (MTF). This objective and accurate method gives a full frequency resolved measurement of the optical quality.
The OptiSpheric® IOL measures the MTF on axis. It uses a single illuminated slit as object which is imaged on the focal plane of the sample. Due to diffraction and aberrations, there will not be a perfect slit image in this plane but a broadened slit, the so called Line Spread Function (LSF). The Fourier analysis of the LSF, giving the contribution of each spatial frequency, corresponds to the MTF of the sample. Alternatively to a slit it is also possible to use a cross (i.e. two perpendicular slits) giving the MTF in two image directions simultaneously or a pinhole giving as image a Point Spread Function (PSF) containing the complete MTF information in all directions.