Like a scanning laser microscope, the image in an SLO is generated over time by recording the scattered light from a focused spot as it is raster scanned across the region to be imaged. As such, it does not collect an image using a film or a CCD array. Rather, the intensity of each pixel is recorded using a single, light sensitive detector, and the location of each pixel is inferred by outputs from the scanning mirrors. Typically, this information is combined by computer or frame grabber which is used to render the final image.
What Are the Limits of SLO?
Manufactured optical systems do not come close to matching the performance of the eye and visual system. But in terms of optical quality, the eye operates at a fraction of its potential. Irregularities in the corneal and lens surfaces as well as misalignments and relative tilts between the components generate aberrations that cannot be corrected by conventional methods. Like any ophthalmoscope, these imperfections impose limits on the highest resolution that can be achieved in an SLO. Most conventional SLO applications are not seriously limited by aberrations, however, as their resolution demands have traditionally not been very high. But with improved resolution, the scope of applications for SLO can be expanded greatly, as I hope to demonstrate in this paper.
Putting the AO into AOSLO
The first attempt to use AO in an SLO was made by Dreher and colleagues in 1989 3
. At that time a key component of an AO system, the wavefront sensor, was not used, and the AO system was capable only of correcting defocus and astigmatism. As such, the concept was laid out, but the improvements in image quality were modest. A short time later, out of the same lab, Liang et al. demonstrated for the first time a Shack Hartmann wavefront sensor for the eye 4
. With such a device, which could measure the eye’s aberrations quickly and accurately, all the pieces were finally in place. In 1996, David Williams at the University of Rochester assembled a team including Junzhong Liang and Donald Miller (PhD student at the time), who built the first AO ophthalmoscope, capable of correcting higher order aberrations 5
. The system employed a conventional imaging modality, using a flash lamp to illuminate the retina and a science grade CCD camera to record the image.
Extensive descriptions can be found in the literature on how an AO system works in general6
, or specifically for vision applications 7,8
. In brief, an AO system employs a wavefront sensor to measure the eye’s aberrations and a wavefront corrector to compensate for them. In most working instruments the wavefront sensor and the wavefront corrector are a Shack Hartmann sensor is a deformable mirror respectively.
A schematic showing the details on how AO is implemented in AOSLO is shown on . But before explaining the actual system, it’s worth first discussing how resolution is mediated in AOSLO. In an SLO, a light beam focuses to illuminate a small region of the retina. The light that scatters from the illuminated region back out of the eye through the system and is sensed by the detector yields the intensity value for that location, or pixel value. So, it follows that the resolution is governed, in part, by the size of the illuminated spot on the retina.
Figure 1 The AOSLO optical system relays light (indicated by dashed line) from the light source (LASER) through a series of lenses (L) and mirrors (M) to the eye. Points conjugate to the retina and pupil are labeled r# and p# respectively and numbered in sequence (more ...)
As shown in , there is a long train of elements between the eye and the light delivery and detection arms of the system. The light beam entering the system reflects off the deformable mirror and two scanning mirrors prior to entering the eye. The returning light passes through the same scanning mirrors and DM prior to reaching the detector. By virtue of the reversibility of light this returning light is ‘descanned’ prior to its arrival at the detector so that, even though the beam may be scanning across the retina, it is rendered stationary by the time it reaches the detector. This feature allows for placement of a key component, the confocal pinhole, prior to the detector in a plane that is conjugate to the focused spot on the retina. The confocality of SLO allows for optical sectioning. The basic concept of optical sectioning is illustrated in , whoch shows how only the light returning from the plane of focus can pass through the pinhole and reach the detector. Most of the light from other layers gets blocked.
Figure 2 Although the illuminating light is well-focused onto a specific plane in the retina, light scatters throughout its thickness (exaggerated in this figure for clarity). The optics are designed so that light scattered from the plane of focus can pass through (more ...)
The Role of AO
As mentioned at the beginning of this section, resolution is achieved, in part, by making the illuminated spot small. The lateral and axial resolution is improved further by making the confocal pinhole small, but that is only realizable if the light from the retina that is re-imaged onto the confocal pinhole plane is compact, otherwise very little light will make it to the detector. In fact, it is the double pass point spread function that reaches the confocal pinhole 9
and the more compact it is, the better the axial and lateral resolution. So, AO is used in both directions; to make the focused light on the retina more compact, and to reimage the returning light back onto the confocal pinhole. The resolution of the confocal SLO is given by:
are the point spread functions in and out of the eye respectively, and D(x,y)
represents the confocal pinhole. If the pinhole is tiny, then the PSF is simply the product of the ingoing and outgoing PSF, making the PSF smaller than the diffraction limit! When the confocal pinhole is optimized, the lateral and axial resolutions for a 6 mm pupil and 600 nm light are 1.9 and 33 microns respectively. With larger pupils or shorter wavelengths, the resolution improves further.
The scanning/descanning feature of SLO allows for unique implementation of wavefront sensing. shows how the returning light is divided between the detector and the wavefront sensor. The beam is stationary at this point, so although the beam scans an extended field, the wavefront sensor sees the light as though it is coming from a single direction. As such, any wavefront sensor exposure that is longer than one frame period will average the wavefront over the field. This does not improve the fidelity of the wavefront measurement (the wave aberration is nearly anisoplanatic over a typical imaged field) but it does offer the following advantages:
- Scanning the beam during the integration time of the wavefront sensor exposure improves the fidelity of the wavefront measurement by eliminating speckle and retinal feature artifacts from the wavefront sensor image.
- Since a large aperture beam is corrected on the entry path, the improvement in focus offered by AO renders the Shack Hartmann spots sharper once the AO system has closed the loop.
- Using the same light source for wavefront sensing and imaging obviates the need to compensate for chromatic aberration
The principle of reversibility of light mean that the single wavefront measurement can be used to correct the aberration in both directions - to sharpen the focused spot on the retina, as well as to sharpen the image of that focused spot on the confocal pinhole in the return path.