Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGuRE 8.5
(a).Conventional.way.of.illuminating.a.scattering.sample.with.a.lens.(not.shown)..he.lens.causes.the.
light.rays.to.converge.outside.the.sample,.but.owing.to.scattering.in.the.sample,.the.light.does.not.come.to.a.geo-
metric.focus..(b).An.alternative.to.geometric.focusing,.using.a.lens,.is.interferometric.focusing,.using.a.spatial.light.
modulator..When.the.phase.delays.for.each.of.the.incident.beamlets.are.adjusted.correctly,.the.beamlets.interfere.
constructively.in.the.sample,.coming.to.an.interferometric.focus.at.the.luorescent.bead..(From.Vellekoop,.I..M.,.E..
G..van.Putten,.A..Lagendijk,.and.A..P..Mosk,.
Opt Express.
.16,.67-80,.2008.)
8.3 History of Deformable Mirrors in Adaptive optics
It.is.helpful.to.discuss.the.history.of.AO.in.astronomy.and.vision.science,.since.the.use.of.DMs.for.AO.in.
both.these.ields.has.a.longer.history.and.has.helped.to.guide.the.application.of.DMs.for.AO.in.biological.
imaging.systems.
Astronomer.Horace.W..Babcock.at.the.Mount.Wilson.and.Palomar.Observatories.irst.proposed.a.
concept.for.AO.in.1953.to.improve.astronomical.image.resolution.(Babcock.1953)..Light.waves.from.a.
distant.star.passing.through.the.earth's.atmosphere.become.aberrated.by.the.dynamic.variations.in.the.
refractive.index.of.the.air..his.causes.the.star.image.to.blur.and.jitter.with.time,.since.the.light.rays.
bend.to.travel.along.the.minimum.optical.path.distance.routes.through.the.atmosphere,.according.to.
Fermat's.principle,.as.discussed.in.Chapters.1.and.2..Light.rays.are.refracted.when.passing.through.areas.
of.diferent.indices.of.refraction..A.familiar.example.of.bending.of.light.by.refraction,.and.one.that.is.
important.for.biological.imaging,.is.that.seen.at.an.interface.between.air.and.water..hese.transmissive.
media.have.diferent.indices.of.refraction.(1.0.and.1.5,.respectively).and.the.efect.on.the.light.path.is.
clear,.as.shown.in.
Figure.8.6
.
In.astronomy,.the.changes.in.the.index.of.refraction.are.due.to.temperature.variations.that.lead.to.
density.variations.in.the.air..hese.variations.are.random.and.luctuate.rapidly.according.to.the.laws.of.
