Nathan Doble and David Williams - Center for Visual
Science, University of Rochester
optics (AO) for astronomy has been an established discipline for a couple of decades.
The last five or so years has seen successful application of this technology to
imaging the human eye. Current vision AO systems have generally used the low volume,
high cost AO components found on telescopes. The mirrors alone are expensive ($1000
per channel), physically large and lack the necessary stroke required for correcting
the aberrations of many eyes.Through CfAO, a collaboration is underway between
the University of Rochester, Lawrence Livermore National Laboratory (LLNL), BostonMicroMachines
Corporation (BMC), UC Berkeley and Lucent Technologies to investigate alternative
low cost wavefront correctors for the next generation of ophthalmic instrumentation.
Various mirror designs are being explored but for a commercial
instrument the mirror would need a corrective range greater than 1um, require
over 100 channels and be small, ideally comparable to the pupil diameter (7mm).
As a first step, a BMC MEMS mirror has been incorporated into the current Rochester
AO testbed and a system containing a Hamamatsu liquid crystal spatial light modulator
(LC-SLM) has been built by LLNL for delivery to UC Davis.
The BMC MEMS mirror: 144 actuators defined under a 3.3mm2 active area with
a mirror stroke of 2um. Photograph BMC.
technology is appealing because the devices are very small (see comparison to
the dime above) and would eventually allow high volume, low cost manufacture.
One area that needs further development is the mirror stroke. Several CfAO groups
have designs for higher stroke devices, including Lucent and UC Berkeley. The
BSAC team at Berkeley has a segmented mirror design with over 10um of stroke operating
at a low voltage (70-80 volts).
Initial results with the BMC MEMS have been very promising.
For the first time, the AO loop was closed with a MEMS device instead of a conventional
mirror. The results before and after correction are shown for a subject with a
Before and after AO results for a 4.6mm pupil with the BMC MEMS mirror as the
The system built by LLNL for UC Davis uses an optically addressed
Liquid Crystal Spatial Light Modulator (LC-SLM) to provide the wavefront correction.
It avoids the need for large stroke due to the fact that it is segmented and can
be phase wrapped. A photo of the UC Davis system is shown at right. The LC-SLM
can be seen in the back right of the breadboard.
|A low cost AO system for vision science would
be very useful in earlier diagnosis and monitoring of treatments for many retinal
diseases. In addition, such a system could be used as a means of showing patients
the benefits of a vision correction procedure, such as LASIK or contact lenses.
The ongoing research described here will eventually lead to several prototype
AO equipped phoropters. These devices will be tested at several CfAO nodes with
a view to commercialization.
The Liquid Crystal Spatial Light Modula-tor (LC-SLM)
AO system designed by LLNL for delivery to UC Davis. Photo-graph Scot Olivier,