Biomedical Engineering Reference
In-Depth Information
depends.solely.on.the.NA..It.has.been.termed.confocal.efect,*.since.the.sample.and.the.reference.light.
need.to.have.the.same.focus.to.gain.maximum.signal..he.total.axial.response.function.of.OCT.is.deter-
mined.by.the.combined.efects.of.coherence.gating.and.confocal.iltering..For.short.coherence.lengths.
and.low-NA.systems,.the.axial.resolution.of.a.confocal.microscope.will.essentially.be.determined.by.the.
coherence.length,.while.for.long.coherence.lengths.and.high-NA.systems.it.is.determined.by.the.NA..
he.highest.axial.resolution.can.be.achieved.for.large.spectral.widths.in.combination.with.high.NAs.
OCT.is.usually.concerned.with.determining.the.density.of.scatterers.and.thus.measures.the.ampli-
tude. of. the. coherence-gated. backscattered. light,. which. is. the. coherent. sum. of. contributions. from. all.
scatterers.in.the.CV,.as.described.earlier..he.amplitude.is.taken.as.a.measure.of.the.local.backscattering.
coeicient.of.the.sample,.which.can.be.linked.to.structures.of.interest.such.as.cell.boundaries.or.intra-
cellular.organelles.such.as.nuclei.or.mitochondria.
However,.due.to.the.interferometric.nature.of.the.detection,.not.only.the.amplitude.but.also.the.phase.
of. the. backscattered. light. can. be. measured.. It. is. the. phase. resulting. from. integrating. the. electromag-
netic. ield. over. the. full. acceptance. angle. of. the. objective. lens. and. summing. over. all. scatterers. in. the.
CV..his.phase.will.change.if.the.optical.path.length.to.the.CV.changes.or.if.the.contributing.scatterers.
move.inside.the.CV..However,.when.scanning.through.a.sample,.the.phase.measurements.from.diferent.
.spatial.positions.will.have.no.relation.with.each.other,.unless.the.optical.path.lengths.of.the.scatterers.in.
the.two.volumes.are.highly.correlated..As.such,.a.phase.measurement.in.OCT.usually.does.not.contain.
much.spatial.information.about.the.sample.and.is.only.interesting.when.dynamic.changes.in.optical.path.
are.to.be.observed,.due.to.either.mechanical.movement.or.refractive.index.changes.(Akkin.et.al..2009).
14.3 Background/Prior Work: Wavefront Sensing
Using a Shack-Hartmann Sensor
he.Shack-Hartmann.sensor.(Figure.14.3).is.based.on.the.principle.that.an.aberrated.wavefront.which.
contains.components.only.up.to.a.certain.spatial.frequency.can.be.decomposed.into.many.tiny.wave-
front.patches,.each.of.which.is.well.approximated.by.a.lat.wave.with.a.certain.wavefront.tilt..If.the.local.
tilt.of.all.these.patches.is.measured.individually,.the.global.wavefront.can.be.reconstructed.by.itting.the.
data.to.a.suitable.functional.form..For.example,.a.least-squares.it.to.the.local.slopes.of.a.certain.number.
of. Zernike. modes. can. provide. a. Zernike. decomposition. of. the. wavefront.. To. measure. the. local. tilts.
simultaneously,.the.wavefront.is.passed.through.an.array.of.tiny.lenslets,.each.corresponding.to.one.of.
the.aforementioned.patches.of.the.wavefront..Each.lenslet.focuses.its.incoming.wavefront.on.a.position.
that.depends.linearly.on.wavefront.tilt..Without.tilt,.the.light.is.focused.on.the.optical.axis.deined.by.
this.lenslet,.whereas.strongly.tilted.wavefronts.will.focus.at.a.certain.lateral.distance.from.the.optical.
FIGuRE 14.3
Principle.of.a.Shack-Hartmann.sensor.
* . his.efect.is.comparable.to.the.confocal.pinhole.in.a.confocal.laser.scanning.microscope,.with.the.diference.that.the.
size.of.the.“pinhole”.in.OCT.is.deined.by.size.of.the.reference.beam.focused.on.the.detector—that.is,.difraction-limited.
at.best.
 
 
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