technology Wavefront analysis
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Boston Micromachines’ deformable mirrors. It has also recently released a sensor incorporating a CMOS detector capable of measuring wavefronts at frame rates up to 450Hz. The ability to close the loop between sensing
and correction quickly is one of the big challenges Michael Feinberg, director of product marketing at Boston Micromachines, identifies for engineering high-quality adaptive optics systems: ‘The speed of the sensor is a limiting factor in many AO systems.’ As an alternative to sensor-based AO systems,
the phase of light can also be measured with a metric-based analysis, in which a property such as contrast or intensity is used to quantify how far in or out of phase the wavefront is. A hill-climbing algorithm is used to manipulate the mirror or corrector, perturbing it in one direction and then the other to determine whether the metric has got better or worse. ‘A sensorless technique would typically be used in light-starved environments,’ notes Feinberg. Boston Micromachines is about
to release a sensorless AO kit, called the wavefront sensorless AO demonstrator. This uses a photodetector and Boston Micromachines’ deformable mirror.
intraocular lenses Intraocular lenses (IOLs) are implants in the eye, typically used to replace the crystalline lens in cataract surgery or in other refractive surgery. Designs of IOLs are becoming increasingly complex, as Dr Iris Erichsen of Trioptics explains: ‘In the past, IOLs were spherical with only one radius of curvature. Now multifocal IOLs are being designed. These have several zones on the lens with different radiuses of curvature, enabling the patient to see objects nearby and far away. There
are now many different multifocal designs.’ Trioptics manufactures both Shack-Hartmann
sensors and interferometers. Its OptiSpheric instrument, which has a reticule through which the magnified image is measured to provide information on the focal length of the lens, is designed to satisfy the ISO standard for IOL characterisation. It also has the advantage of being able to measure the properties of a lens in different focal planes as the detector operates directly in the image plane. ‘Using a Shack-Hartmann or any wavefront sensor, it’s hard to separate the image planes as these sensors only measure a single wavefront,’ says Erichsen.
But Erichsen notes that the big advantage of Shack-Hartmann sensors compared to the OptiSpheric system is that the lateral resolution is
in principle, interferometry
can be more accurate than shack- hartmann wavefront sensors
lost when imaging over the full aperture of a lens with OptiSpheric: ‘The system always measures objects in infinity, whereas, with a wavefront sensor, the measurement is made in the exit pupil of the system. Therefore, a 2D image of the complete lens is recorded and, for example, if there is a defect at the edge of the lens, it can be seen.’ Compared to an interferometer, the
repeatability and speed of wavefront sensors is particularly important for ophthalmic applications like testing contact lenses and IOLs, says Dr Johannes Pfund, managing partner at optical metrology company Optocraft: ‘In principle, interferometry can be more accurate than Shack- Hartmann wavefront sensors and is the technique
of choice in, say, semiconductor inspection, but generally not in ophthalmology.’ Interferometry is typically slower, requiring a number of measurements compared to a Shack- Hartmann sensor that gathers all the information in a single shot. However, there is interferometry equipment suitable for testing IOLs. Phasics’ (Palaiseau, France) SID4 sensors, which operate via four-wave lateral shearing interferometry, are used in contact lens or IOL characterisation. ‘Particularly for IOL measurements, the
dedicated system provides a complete and fast analysis of all the important IOL parameters (aberrations, MTF, power map, etc) in accordance with ISO standards,’ comments Raphaël Serra, sales manager at Phasics. ‘The advantage of this system is that, thanks to the high resolution of the phase map and the intensity map, we can obtain a very precise quality analysis of the IOL in only one measurement.’ The technique records the interference pattern
from four identical waves, replicated from the incident beam with a 2D diffraction grating. Any aberrations in the beam result in distortions in the interference grid, which can then be measured.
iols of the future Wavefront sensing is playing an important role in developing new IOL designs. Professor Marcos at CSIC is testing IOL designs for presbyopic corrective procedures in which an optimal aberration pattern is determined using wavefront sensing and adaptive optics prior to manufacturing the lens. Presbyopia is the loss of accommodation as the eye ages, corrections for which involve multifocal IOLs that induce aberrations to extend the depth of focus of the eye. Marcos says that by using the AO setup, new lenses can be designed and tested before being manufactured. l
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