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technology Wavefront analysis


Insights into sight


adaptive optics systems use wavefront sensors as a feedback loop to correct for wavefront distortions. image courtesy of thorlabs


Our eyes are not perfect optical systems, so in order to study them, researchers are utilising wavefront sensing techniques, as greg Blackman discovers


S


eeing is said to be believing and for us humans this is certainly the case, with 80 per cent of information received by the brain coming


from the eyes. Our eyes are one of the most complex organs we have, allowing us to navigate in moonlight or bright sunlight. While our eyes are good for gathering visual information, they are flawed as optical systems, making them difficult to study. ‘The eye is adequate to look out through, but it’s poor for looking into,’ says Peter West of UK company 4D Optics. ‘For imaging the retina, the resolution is limited by aberrations caused by the eye itself.’ 4D Optics conducts research on correcting aberrations to image photoreceptor cells in the retina, in particular to develop instrumentation that could be used in clinical trials on diseases like age-related macular degeneration (AMD). The work is carried out with a major eye hospital. Just like any optical system, distortions in the


wavefront of light passing through the eye can be measured with a wavefront sensor. This provides information on the degree of aberration, which,


20 electro optics l APRIL 2011


when combined with adaptive optics, can be corrected for and a higher quality image produced. There are two main analytical properties of


a wavefront: the phase and the amplitude. A wavefront intensity analyser can measure the amplitude of light, but this article, however, is mainly concerned with analysing the phase, typically using a wavefront sensor. A common type of wavefront sensor is the


Shack-Hartmann sensor. Developed by Johannes Hartmann around 1900 and then improved upon by Roland Shack in the late 1960s, it uses a lenslet array to generate a spotfield, which is focused onto a photon sensor. The direction of light passing through each of the sample points is measured. The positions of the points of the foci are compared against spatially resolved patterns to gain a measurement of the wavefront and any distortions caused by imperfections in the optical system. 4D Optics is not using Shack-Hartmann


analysis, but curvature sensing. This involves measuring two samples along a propagating wavefront to identify the phase. ‘It’s a fearsomely complex piece of mathematics,’ West says. The instrument uses digital wavefront cameras and software from PhaseView to recover the phase from the intensity samples and measure aberrations in the wavefront. These aberrations are then corrected using a Hamamatsu spatial light modulator (SLM). West says the technique shows promise for


producing much higher resolution measurements of aberrations in the eye than techniques based around Shack-Hartmann analysis – PhaseView’s cameras offer around 10 times higher resolution than standard Shack-Hartmann sensors. The sensing technology is also fast. ‘The eye is subject to tiny movements at up to 30 to 40Hz,’ says West. ‘If you want to see those movements, the sample frequency has to be at 60Hz.’ This is achievable with one of PhaseView’s sensors. West says the technique has clinical potential


for the manufacture of custom correcting wavefront contact lenses and intraocular lenses (IOLs), as well as for studying the tear film, a film of tears spread across the cornea each time we blink: ‘The blinking mechanism is of great interest, in that there are a range of eye diseases that result in dry eyes.’ ‘The great advantage of PhaseView’s technology


is that it makes curvature sensing a viable technique,’ states West. ‘Curvature sensing has been used for a long time in astronomy, which is why we started looking at it for ophthalmology. Astronomers, however, can image for quite a long time, which is not possible in ophthalmic investigations. Even when the eye is apparently still, it’s moving; you’re making dozens of microsaccades a minute and there is drift and tremors. All of these are at the microscopic


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