Researchers have developed a way of mathematically removing aberrations in the eye by using wavefront-sensing techniques. At the University of Murcia (Murcia, Spain), physicists have not only demonstrated the concept both theoretically and through computer simulation, they have carried out experiments both in an artificial eye and in vivo. Many improvements are necessary before the technique can be used in practice, but researchers say the initial results are promising.
Imaging through the eye is difficult for two reasons: first, because the eye is a complicated optical system in its own right and, second, because it is continuously changing. The latter problem is due to eye movements, microfluctuations in the shape of the eye's lens as it tries to focus at different distances (accommodation), and the constantly varying tear film (the layer of liquid across the front of the eye). These aberrations must be taken into account in order to capture an image of the retina.
The University of Murcia ophthalmoscope works by combining information that is captured in two different ways at the same time as ensured by an electronic shutter (see Fig. 1).1 The optical system sends light from the imaged retina to two charge-coupled-device (CCD) detectors. One of these detects the focused image, while the other detects many small copies of the image focused by a lenslet array.
The latter provides a mechanism for the wavefront shape to be calculated after image processing, which involves several steps. First, the central lenslet image has to be found (see Fig. 2). Next, this image is correlated with the other lenslet images (using a Fast Fourier Transform to implement the correlation algorithmically), followed by a secondary crosscorrelation step designed to refine the results acquired from the first. These steps produce a set of coordinates: the positions of the surrounding lenslet images with respect to the central one. Finally, these coordinates are used to produce a map of the aberrated wavefront, which is in turn used to calculate the point spread function (PSF) of the eye.
