resolution of the eye

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

The Nov. Journal of the Optical Society of America "A" has two very
interesting articles from Univ. Rochester on the resolution of the eye.
The first is a refinement of a test using a wave front sensor and computing
MTF from the results.  When the subject has large pupils, the classic
aberrations of astigmatism, coma, spherical aberration, and defocus do not
fully explain the measured aberrations.  The authors measure what they call
'irregular aberrations', which are different between observers but tend to
be similar in an individual's two eyes, with 'mirror symmetry' between the
eyes.  These IAs are from local variations within the eye.  They are not
significant when the pupil is small.  The article does not give res in
arcsec or lines per millimeter, but instead gives rather opaque values, for
MTF as a function of spatial frequency, and RMS wavefront error as a
function of Zernike order.

The same team has developed a truly amazing device they describe as an
adaptive optics system for the eye.  (Adaptive optics are used in
telescopes, the mirror is flexible and has a complex mechanical system that
very rapidly changes the shape of the mirrors surface in response to
atmospheric optics - the twinkling of stars.)  From their introduction: "We
have successfully corrected these (monochromatic) aberrations using
adaptive optics, providing normal eyes with supernormal optical quality.
Contrast sensitivity to fine spatial patterns was increased when observers
viewed stimuli through adaptive optics."   Later, "eyes with adaptive
compensation can resolve fine gratings (55c/deg) that were invisible under
normal viewing conditions.....Stimuli such as edges viewed through the
compensating deformable mirror have a strikingly crisp appearance".

They used an aluminized glass mirror, with 37 actuators (rods), 7mm apart,
in a square array behind the mirror.  Each rod could move up or down 2
millionth of a meter, which results in 8 millionths of a meter of wavefront
movement.  The rod's range of movement was divided into 4096 steps.  The
pupil of the eye was magnified 6.25 times at the mirror.  A laser beam was
focused onto the retina, and part of the beam was reflected out of the eye
and off the mirror to a wavefront sensor that controlled the actuators, in
a loop that reduced error to a minimum after 10 to 20 cycles.  A grating
then replaced the sensor, and the eye could view it (or anything else
placed there).

On the average, this system reduced P-V wavefront error by a factor of 4.
It can correct spherical aberration, coma, astigmatism, and defocus; and by
increasing the number of actuators, they think that other aberrations could
be corrected.  

The immediate use for this device is for imaging the retina.  The authors
were able to consistently resolve individual cones across the entire
retina, a first.
They did not say where you could buy one.

_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


------------------------------

End of TECH-3D Digest 230
*************************
*************************
 Trouble? Send e-mail to 
 wier@xxxxxxxxxxxxxxxxxxxxxxxx 
 To unsubscribe select one of the following,
 place it in the BODY of a message and send it to:
 listserv@xxxxxxxxxxxxxxxxxxxxxxxx 
   unsubscribe photo-3d
   unsubscribe sell-3d
   unsubscribe overland-trails
   unsubscribe icom
 *************************