Biomedical Engineering Reference
In-Depth Information
nonlinear..However,.comparing.results.obtained.from.geometrical.optics,.rigorous.difraction.theory,.
and.experiments.yields.similar.conclusions.[9]..his.means.that.the.geometrical.approximation.can.be.
used.to.speed.up.the.entire.wavefront.reconstruction.process.
7.1.1.4 Digital reconstruction and Lensless Approaches
A. newer. method. recently. proposed. by. Hattori. and. Komatsu. [11]. and. Sekine. et. al.. [12]. involves. the.
dispensation.of.the.lenslet.array.found.in.the.SHWS..he.advantage.is.that.there.is.more.eiciency.as.
compared.to.using.a.microlens.array.in.a.conventional.SHWS.[10]..Instead,.a.two-dimensional.grating.
replaces.the.lenslet.array..Another.variation.of.this.principle.is.the.development.of.a.digital.SHWS.by.
replacing.the.lenslet.array.with.a.spatial.light.modulator.(SLM).[13]..A.new.technique.based.on.in-line.
holography.has.been.proposed.by.Lai,.King,.and.Neiield.[14]..In.principle,.all.the.above.methods.can.be.
used.to.sense.the.aberrations.of.the.eye,.and.thereby.provide.continuous.feedback.to.the.active.element.
for.aberration.correction.
7.1.2 Wavefront correction
he.active.element.imparts.the.correction.to.the.aberrated.input.wavefront..his.is.achieved.by.physi-
cally.changing.the.optical.path,.as.with.a.deformable.mirror.(DM),.or.through.adjusting.the.phase,.as.
with.a.liquid.crystal.spatial.light.modulator.(LC-SLM)..his.section.briely.characterizes.the.principle.
of.operation.behind.the.various.methods.of.wavefront.correction.
7.1.2.1 Deformable Mirrors
he.category.of.DMs.covers.a.wide.range.of.instruments..For.DMs,.the.incident.wavefront.is.changed.
through.physically.altering.the.optical.path.length.by.means.of.a.deformable.surface.that.is.relective.
and.mounted.on.actuators..here.are.two.main.classes.of.DMs—segmented.and.continuous. mirrors,.
depending.on.the.construction.
In.the.segmented.DM,.the.DM.consists.of.small.mirrors,.each.mounted.on.an.actuator..he.actuators.
can.generate.solely.piston.(push/pull).motions.or.have.an.added.tip/tilt.capability..Naturally,.the.DMs.
with.added.tip/tilt.actuators.are.better.able.to.correct.the.aberrations..Segmented.DMs.have.the.advan-
tage.of.being.free.from.the.coupling.that.afects.continuous.sheet.DMs.as.each.actuator.is.able.to.move.
independently.from.its.neighbor..For.the.same.reason,.the.stroke.of.a.segmented.DM.is.oten.larger.than.
that.of.a.continuous.sheet.DM.and.is.oten.better.suited.for.correcting.higher-order.modes.of.high.fre-
quencies,.although.the.versions.with.tip/tilt.are.well.placed.to.correct.both.high-.and.low-order.modes..
However,.segmented.DMs.sufer.from.difraction.efects.that.occur.due.to.the.edges.between.the.mirror.
segments..At.the.same.time,.the.relatively.large.size.of.a.DM.means.that.the.pupil.of.the.eye.needs.to.be.
magniied.signiicantly..his.results.in.a.larger.overall.size.of.the.optical.setup,.which.is.a.disadvantage.
for.a.clinical.instrument.
Continuous.DMs.consist.of.a.thin,.continuous.sheet.with.actuators.on.the.underside.that.cause.a.con-
formational.change..In.such.devices,.each.actuator.is.never.truly.independent.of.the.others.because.of.an.
inluence.function,.although.this.efect.can.be.minimized..his.function.describes.the.coupling.between.
actuators.and.how.a.conformational.change.of.one.actuator.afects.the.entire.shape.of.the.continuous.
sheet..As.with.segmented.mirrors,.the.conformational.change.in.the.mirror.is.efected.through.actua-
tors..hese.need.not.be.piston.like.actuators.and.can.be.of.the.bimorph.type.[15],.wherein.two.strips.of.
dissimilar.metals.that.react.diferently.to.an.applied.voltage.are.bonded.together;.electrostatic.actuators.
that.rely.on.electrostatic.attraction.[16];.and.also.magnetic.actuators.[17].
Newer.technology.exists,.based.on.microelectromechanical.mirrors.(MEMs)..MEMs.have.the.advan-
tage. that.the. DM. can. be.built. smaller,. faster,.and. cheaper. than.conventional. DMs.. he.technology. is.
based.on.established.silicon.surface.micromachining..his.confers.the.advantage.of.cost,.reliability,.and.
a.smaller.aperture.size.that.is.more.suited.for.AO.work,.especially.in.ophthalmic.applications..Some.of.
the.earliest.instances.of.MEMs.being.used.as.the.active.element.in.an.AO.are.the.works.of.Zhu.et.al..[18],.
