Saturday, September 24, 2011

Measuring the eye

Our eyes are like windows, I’ve been told. We use them to look out at the world, and doctors use them to look into our body; a non-invasive health check. But our window is not really top-quality, which blurs the seeing in both directions. Glasses and contact lenses have long been used to better see the world. They correct low order errors, like defocus and astigmatism, and give improved vision.

Of course, some glasses just serve as inexpensive identity props
 more funky ones let you see wrackspurts.
For doctors to look into our eyes, we turn to imaging systems that compensate these aberrations and image the retina, blood vessels and other features in the eye. The efficiency of correction both ways depends much on how accurately the aberrations are measured.

In 1961 Mikhail Smirnov developed a subjective technique to measure the aberrations of the eye. The individual would look at two incoming beams of light, use the on-axis beam as reference and change the position/angle of the off-axis beam to try and fuse the two beams at the retina. This gave the slope of the wavefront and allowed step-by-step measurement of the eye’s aberrations.

Double pass methods, that image a spot projected through the eye on the retina and reflected back, can be used to get the MTF of the optics. Combined with multiple images and phase retrieval type computation they can also give the aberrations (phase). Interferometry, where coherent laser fringes are projected on the retina, along with contrast sensitivity can also be used to get an estimate of the MTF. In the Tscherning aberrometer, a perfect grid casts a shadow on the retina which is imaged back by a fundus camera. The distortion of the perfect grid gives the wavefront. Corneal topography lets you specifically measure the power, shape and thickness of the cornea.

My favorite is the Shack-Hartmann wavefront sensor, where, a point source projected at the retina is imaged by a lenslet array onto the sensor. The lenslet array produces multiple little images of the spot. For perfect optics, the spot in every sub-image is dead center. For imperfect optics, the displacement of the spots from the center of each lenslet is indicative of the slope of the wavefront. This technique gives a fairly accurate measure of aberrations of the eye.

Shack Hartmann Wavefront Sensor for the eye. (Image from Wikipedia)

All these and more technologies have been developed to “just” measure the optics of our eye. Once the measurement is done, correcting our optics for improved visual perception is a whole field in itself. Glasses and contacts have come a long way. We have customized contacts, intra-occular lenses with custom correction, customized refractive surgery, the options are many. How much this correction benefits visual performance is also a huge field of research. After all, there's no point in more correction if it's overkill.

Imaging the eye has also required much innovation. The error sensed in the wavefront from the eye must be corrected and compensated accurately enough to improve the quality of a retinal image. Dynamic lenses and phase plates, as well as more expensive liquid crystal spatial light modulators and MEMS deformable mirrors can be used to correct the wavefront and give almost perfect image resolution. The progress in imaging the eye has made it possible to see features at even cellular level, making it possible to detect and diagnose diseases at very early stages.

Some of my background is in retinal imaging and adaptive optics, and there seem to be a whole bunch of talks at FiO this year discussing these topics. They have a symposium on Wednesday 10/19 celebrating 50 years of measuring the eye’s aberrations. This is followed by sessions on models of eye disease (FWO) and optical design of animal eyes (FWV). Now shouldn’t spark off some cool imaging system design ideas..!

Thanks for stopping by, folks! See you around!