The Application of Adaptive Optics (AO) to the Human Eye
The concept of AO was first proposed by the astronomer Horace Babcock in 1953. However, it was not until the late 1960s / early 70s that the first system was implemented, first by the military then followed subsequently by the astronomy community. The first step towards the application of AO to the human eye was the work of Dreher et al. (1989), who employed a deformable mirror (DM) to give a static correction of astigmatism in a scanning laser ophthalmoscope. Later work by Liang et al. (1994) saw the first use of a Hartmann-Shack wavefront sensor (HS-WFS) for measurement of the human wavefront aberration who then used a HS-WFS in conjunction with a DM (Liang & Williams, 1997) to produce some of the first in vivo images of the cone photoreceptors. Today AO has been successfully applied to several retinal imaging modalities employing a variety of DM and WFS technologies.
Key AO Components
Similar to the AO systems used for other applications such as astronomy and communications, a vision science AO system is comprised of three main parts:
- The Wavefront Sensor (WFS): Most vision science AO systems employ a Hartmann-Shack WFS. Typically, the ocular wavefront is sampled at 10-20 Hz with closed loop bandwidths of 1-3Hz which is sufficient to correct most of the ocular dynamics.
- The Wavefront Corrector: These optical devices correct the aberration profile measured by the WFS. These are typically deformable mirrors (DMs) whose surface is deformed to take on the conjugate profile of the measured aberration.
- The Control Computer: Takes the output of the WFS and converts it to voltage commands that are sent to the wavefront corrector.
There are three main ophthalmic imaging modalities that have successfully employed AO (i) flood illuminated-fundus cameras that take a short exposure image of the retina, (ii) confocal laser scanning ophthalmoscopes (cSLOs) that acquire the image by rapidly scanning a point source across the retinal surface and (iii) optical coherence tomography (OCT) which again scans a point source, but uses low coherence interferometry to form the image.
Figure 3. Schematic of the AO flood illuminated (flash) fundus camera in use at our laboratory. (Headington et al., 2011)
Figure 3 shows the optical layout of our AO flood illuminated (flash) fundus camera (Headington et al., 2011). The WFS beacon is used to measure the ocular aberrations; a small incident beam from a superluminescent diode (SLD) at 820nm is focused to a ~10µm diameter spot on the retina. The scattered light exits through the dilated pupil and is redirected by the DM, through the dichroic beamsplitter into the WFS. The aberrations are sampled at 20 Hz and the required correction profile is sent to the DM. The system is fast enough to track and correct dynamic ocular aberration changes at a frequency of ~1Hz. Once the aberrations have been corrected, the retinal image is acquired. The particular imaging wavelength (between 500-800 nm) is chosen to highlight a particular retinal feature. Typical retinal image sizes are 1-3º (0.3-0.9 mm diameter).