Adaptive optics: astronomy, military might, and retinal imaging.

Abstract

Light traveling through any optical media other than a vacuum is distorted by the media. We call this distortion aberration. Aberration occurs for many reasons, including nonuniform indexes of refraction in the media, differential bending of light by “lenses” in the media, interface changes in the media, and scatter. This produces defocus, astigmatism, coma, spherical, and other types of waveform aberration. Aberration produced by the optical media limits our ability to clearly capture an image of an object. Atmospheric distortion imposes significant aberration for images collected by astronomical telescopes such as the Hubble Space Telescope. Beam divergence results in considerable aberration for data collected by free-space optical communication systems. Fortunately, this became a problem for light-based systems used by German anti-aircraft units in the Second World War. Adaptive optics is a technique to correct for optical aberration, resulting in incredibly clear, accurate images. This complex and nuanced technology comprises two simple components. The first component analyzes the aberration. The second component corrects for it. If we know exactly what an object really looks like, we can then analyze the aberration produced by the optical media while viewing the object through the media. To do this, we use reference images— either natural or projected ones. In the case of retinal images, we can laser project an image onto the retinal plane and analyze the aberration the media produces. Once we analyze the aberration produced by a particular optical system, we can deform a mirror in a way that the image reflected off the mirror has the aberration corrected. We can then get incredibly clear images. The mirror is deformed by a complex computer algorithm based on feedback from a sensor analyzing the aberration. As the optical media parameters producing the aberration change, so does the shape of the correcting mirror. The resulting image resolution is so fine that the instrument can be used to visualize individual rods and cone photoreceptors, retinal pigment epithelium cells, and corpuscles within the retinal vascular system. This technology is still in the early phases of development and is not yet widely commercially available. One can only speculate as to the important clinical applications adaptive optics will produce.