Adaptive Optics for Retinal Imaging
What's the problem, or why do we need it?
The human eye is plagued by many optical imperfections. All these taken together caused Helmholtz (1821-1894) to say that if any optician sent him an instrument so full of defects, he would be justified in sending it back with the severest censure. 1
The image of the outside world is blurred by these imperfections and if it is severe, a person may need glasses or contact lenses.
Ophthalmologists look into the eye to detect and diagnose disease. The image of the retina they see is also blurred by the eye's optics and this blurring limits the level of detail they can see.
Adaptive optics provides a way to measure and correct the eye's optical imperfections, allowing us to look into the human eye with nearly diffraction-limited resolution.
Adaptive Optics Imaging of the retina
In the late 1990s, David Williams's lab at the University of Rochester developed an adaptive optics system to image the living human retina [PubMed abstract]. Their second generation system can image the retina with unprecedented detail.
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A tiny patch of my retina was imaged that was centered on my fovea. The patch was 0.975 mm across (or 3.25 degrees of visual angle) - the same size as the black spot on the penny.
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This is a composite of the area that we imaged (image size = 152 K).
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This is an expanded view of my fovea, magnified 10 times.
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These images were collected in June 2002 with Heidi Hofer.
Adaptive Optics Scanning Laser Ophthalmoscope
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Austin Roorda's laboratory at the University of Houston has constructed an adaptive optics scanning laser ophthalmoscope.
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A scanning laser ophthalmoscope uses the eye's optics as an objective lens to obtain microscopic images of the living retina. A confocal aperture and adaptive optical compensation work together to provide unprecedented axial and lateral resolution at video frame rates.
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The movie shows confocal optical sections through the different layers of my retina, starting at the inner limiting membrane. At first the nerve fibers are visible, followed by a blood vessel, within which some evidence of blood flow can be seen, and finally the photoreceptor layer.
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This movie shows a patch of retina 1.2 mm (4 degrees) superior to my fovea. The focal plane is at the inner segments of the photoreceptors. Only the cones are visible.
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These movies were collected in March 2002 in Austin Roorda's laboratory.
Adaptive Optics for Vision Science
Adaptive optics not only allows scientists to look INTO the eye with unprecedented detail, but it also allows the subject to look OUT at the world with a clarity that they have never experienced before.
You may think that the optical imperfections of the eye are all that prevent us from having "perfect vision." However, vision scientists have discovered that information processing in the brain also imposes strong limitations on our visual abilities.
This was eloquently stated a century ago by Helmholtz: "That which we have discovered in the way of inexactness and imperfection in the optical machine and in the image on the retina, is as nothing in comparison with the incongruities which we have just come across in the domain of the sensations." 2
Adaptive optics is a powerful new tool in the psychophysical arsenal, which can be used to probe the neural limits on spatial vision.
