Biomedical Engineering Reference
In-Depth Information
To.a.certain.extent,.AO.systems.for.biological.imaging.of.the.eye.can.be.said.to.have.taken.of.with.
the implementation.of.the.Shack-Hartmann.wavefront.sensor.(SHWS).and.subsequent.demonstration.
of.the.improvement.to.the.images.of the.fundus.with.AO.[2,.3]..Perhaps.the.most.popular.among.wave-
front.sensing.devices.is.the.SHWS..here.are,.however,.other.modes.of.sensing.the.wavefront.and.the.
following.section.touches.on.some.of.these.various.methods.
7.1.1.1 Shack-Hartmann Wavefront Sensor
Currently,.one.of.the.most.popular.methods.of.measuring.the.aberrations.of.the.eye.in.the.ield.of.AO.is.
with.an.SHWS..he.SHWS.is.a.simple.device.consisting.of.a.lenslet.array.that.focuses.the.incident.beam.
onto.a.charge-coupled.device.(CCD).placed.at.the.focal.plane.of.the.lenslet.array..Each.lenslet.produces.
a.focal.spot.that.is.detected.over.a.region.on.the.CCD.and.is.a.group.of.pixels.(subdetection.area).cor-
responding.to.the.aperture.of.the.lenslet..If.the.incident.wavefront.is.planar,.the.foci.are.all.centered.on.
the.appropriate.pixels..If.the.wavefront.is.aberrated,.the.foci.shit.across.the.subdetection.area,.accord-
ing.to.the.type.of.aberration..he.slope.of.the.wavefront.is.given.by.the.ratio.of.the.shit.over.the.focal.
length.of.the.lenslet.
he.irst.implementation.of.an.SHWS.in.ophthalmology.was.in.1994.by.Liang.et.al..[2]..In.this.experi-
ment,.they.used.a.1.5.mW.laser.at.632.8.nm.focused.onto.the.retina..he.relected.light.and.wavefront.
aberrations.imparted.by.the.eye.were.then.detected.by.a.home-built.SHWS..As.the.retina.was.acting.as.
a.difuse.relecting.source,.it.was.not.possible.to.retrieve.the.wavefront.errors.arising.from.the.irst.pass.
through.the.eye..At.the.same.time,.they.reported.that.the.double.pass.by.relection.meant.that.coma.was.
canceled.although.they.found.that.the.third-order.coma.could.not.be.ignored.in.their.results.[2].
7.1.1.2 curvature Sensor
Curvature.sensing.has.been.used.in.astronomical.research.but.was.irst.introduced.to.examining.the.
aberrations.of.the.eye.in.2006.by.Diaz-Douton.et.al..[4]..Curvature.sensing.is.based.on.the.principle.that.
changes.in.the.local.wavefront.curvature.result.in.variations.in.the.local.intensity.as.a.detector.traverses.
the.focus..In.the.paraxial.approximation,.the.amount.of.curvature.at.the.input.aperture.bears.a.linear.
relationship.to.the.intensity.changes.at.two.diferent.planes.of.defocus.(one.before.and.the.other.ater.
the.focus)..Since.the.defocus.between.the.two.planes.is.small,.the.detected.variations.are.considered.to.
be.well.localized.over.the.same.cluster.of.pixels.on.the.detector..he.advantages.of.the.curvature.sensor.
are.that.it.oten.costs.less.than.an.SHWS.and.has.a.higher.dynamic.range.
7.1.1.3 Pyramid Sensor
he.pyramid.sensor.is.another.novel.method.of.measuring.the.wavefront.aberrations.of.the.eye..First.
developed.for.the.sensing.element.in.an.astronomical.telescope.by.Ragazzoni.and.Farinato.[5],.it.was.
irst.applied.in.measuring.the.aberrations.of.the.human.eye.in.2002.by.Iglesias.et.al..[6]..In.this.version,.
the.pyramid.wavefront.sensor.was.modiied.such.that.it.was.able.to.sense.the.wavefront.variations.from.
an.extended.source.instead.of.a.point.source,.thereby.obviating.the.need.to.oscillate.the.pyramid..Unlike.
the.SHWS,.the.pyramid.sensor.can.adjust.its.sampling.rate.and.dynamic.range.through.the.oscillations.
of. the. pyramid. [6],. allowing. for. a. higher. update. frequency. to. the. active. element.. In. 2006,. Chamot,.
Dainty,.and.Esposito.[7].built.a.pyramid-sensing-based.AO.system.for.the.eye,.operating.at.a.frame.rate.
of.55.Hz.over.a.6.mm.pupil..More.recently,.Daly.and.Dainty.[8].have.achieved.a.frame.rate.of.83.Hz.in.
both.open-.and.closed-loop.operations,.with.RMS.errors.below.0.1.µm.for.a.6.mm.pupil.
In.principle,.the.pyramid.wavefront.sensor.is.similar.to.that.of.the.Foucault.knife.edge.test..A.pyra-
mid. of. transmissive. material. is. placed. such. that. the. apex. of. the. pyramid. is. at. the. focus. of. the. beam.
whose. wavefront. variation. over. the. aperture. is. what. is. to. be. studied.. Using. geometrical. optics,. the.
converging.rays.are.split.into.four.images,.which.are.detected.on.diferent.parts.of.a.CCD..Each.image.
is.therefore.an.image.of.the.aperture..he.variations.in.intensity.over.the.pixels.and.for.each.quadrant.
will.allow.the.wavefront.to.be.reconstructed..One.drawback.of.the.pyramid.wavefront.sensor.is.that.it.is.
not.suitable.for.large.aberrations.as.the.relationship.between.the.local.slope.and.the.wavefront.becomes.
