Biomedical Engineering Reference
In-Depth Information
Bifano.and.coworkers.[19-21],.and.Vdovin,.Sarro,.and.Middelhoek.[16]..A.comparison.of.the.various.
types.of.DMs.can.be.found.in.Devaney.et.al..[22].and.Loktev,.Monteiro,.and.Vdovin.[23].
7.1.2.2 Liquid crystal Spatial Light Modulators
he.previous.methods.relied.on.changing.the.surface.proile.of.the.mirror.to.account.for.the.physical.
optical. path. length.. he. optical. path. length. corresponds. to. an. additional. phase. term,. which. can. be.
introduced.with.an.LC-SLM.operating.in.the.phase-only.mode.
he.earliest.reported.works.on.the.application.of.liquid.crystals.in.wavefront.correction.were.from.
Russia.[24,.25]..In.general,.the.LC-SLM.consists.of.a.thin.layer.of.birefringent.liquid.crystals.sandwiched.
between.two.glass.plates.with.a.particular.“brushing”.on.both.glass.plates.to.ensure.the.alignment.of.
the.liquid.crystals..he.liquid.crystals.twist.to.realign.their.long.axis.to.match.the.direction.of.the.elec-
tric.ield.and,.by.doing.so,.cause.a.change.in.the.refractive.index..he.entire.surface.of.the.LC-SLM.is.
typically.divided.into.pixels,.each.individually.addressed..he.methods.of.addressing.each.pixel.can.be.
optical.or.electronic.through.conventional.LCD.display.technology..he.optically.addressed.systems.are.
pixel-less.and.do.not.sufer.from.difraction.efects.whereas.the.electronically.addressed.versions.have.
a. ill-factor,.and. certain. types. have. ill-factors.of. 90%. or. even. equivalent. to. 100%,. when. we. take. into.
account.how.the.electric.ield.varies.between.pixels.
he.fact.that.the.liquid.crystals.are.birefringent.imposes.the.condition.that.the.incident.beam.has.to.
be.of.a.speciic.polarization.for.the.LC-SLM.to.operate.eiciently..his.does.limit.the.use.of.LC-SLMs.
in. adaptive. optics,. especially. for. ophthalmic. applications,. since. the. back-relection. from. the. retina. is.
unpolarized..Recent.work.[26].has.shown.that.the.incident.polarization.state.does.not.afect.the.results,.
and.the.work.undertaken.by.Shirai.et.al..[27].has.demonstrated.that.the.depolarized.light.relected.from.
the.retina.can.still.be.phase.modulated,.albeit.with.a.loss.of.intensity.that.they.circumvented.through.
the.use.of.highly.sensitive.detectors.
he. advantages. of. using. an. LC-SLM. is. that. new. technology,. such. as. the. liquid-crystal-on-silicon.
spatial.light.modulators.(LCOS-SLM),.utilizes.CMOS.and.current.LCD.television.technology,.therefore.
making.them.cheaper,.with.better.modulation.characteristics.and.a.higher.pixel.count..he.most.recent.
LCOS-SLM.as.at.the.time.of.writing.is.1920.×.1080.pixels,.a.full-HD.SLM.
he.disadvantages.of.SLMs.in.general.are.that.they.require.a.polarized.input.for.eicient.phase.modu-
lation,.are.not.broadband.and.require.calibration.for.diferent.wavelengths,.and.sufer.from.difraction.
efects.due.to.the.pixelated.nature.of.the.technology..his,.in.turn,.reduces.the.amount.of.power.relected.
back. into. the. system.. here. is. also. a. need. to. carry. out. phase. wrapping. to. achieve. phase. modulation.
above.2π..he.refresh.rate.is.also.locked.at.60.Hz.since.the.SLM.is.oten.driven.through.the.video.card.of.
a.computer..While.this.may.be.less.of.a.concern.for.ophthalmic.applications,.it.is.insuicient.for.scan-
ning.microscopy.
7.2 Biological imaging in the eye
he.eye.is.an.imaging. system.and.consists.of.a.lens.(the.cornea).and.a.detector.(retina)..he.lens.can.
accommodate. various. imaging. conditions,. being. able. to. adjust. the. power. through. the. contraction. of.
the.ciliary.body..his.causes.the.image.to.form.on.the.retina,.where.the.photoreceptors.convert.light.to.
electrical.impulses,.which.are.then.relayed.to.the.brain..Like.man-made.imaging.systems,.the.eye.is.not.
perfect.because.the.cornea.may.contain.imperfections.that.give.rise.to.higher-order.aberrations..At.the.
same.time,.the.ciliary.body.may.be.unable.to.adjust.and.bring.the.image.to.the.surface.of.the.retina,.as.
in.the.common.conditions.of.myopia.or.hypermetropia..Added.to.this,.the.retina.is.curved.and.the.most.
sensitive.part.of.the.retina.is.typically.at.5°.of.the.central.axis..As.an.optical.system,.the.eye.is.always.far.
from.perfect..While.spectacles.and.Lasik.can.help.correct.for.myopia.or.hypermetropia,.they.oten.leave.
the.higher-order.aberrations.(which.are.always.present).uncorrected.for..he.result.is.that.while.spec-
tacles.and.Lasik.may.be.suicient.for.day-to-day.use,.the.imaging.of.the.retina,.which.is.important.for.
