Biomedical Engineering Reference
In-Depth Information
14
Coherence-Gated
Wavefront Sensing
14.1.
Introduction.......................................................................................253
14.2.
Background/Prior.Work:.Coherence.Gating................................254
14.3.
.Background/Prior.Work:.Wavefront.Sensing.Using.a.
Shack-Hartmann.Sensor..................................................................257
14.4.
Methods..............................................................................................258
Considerations
14.5.
Application.........................................................................................263
Correction. •.
Speckle.Size.and.Multiple.Scattering
Jonas Binding
Max Planck Institute
for Medical Research
14.6.
Critical.Discussion............................................................................267
Markus Rückel
BASF SE
14.7.
Conclusions........................................................................................268
14.1 introduction
In.optical.microscopy,.signal.size.and.resolution.can.be.impaired,.particularly.in.biological.specimens,.
by.absorption.and.scattering.loss.and.by.specimen-induced.aberrations..Although.absorption.loss.and.
high-angle.scattering.can.be.reduced.to.some.extent.by.choosing.the.excitation.wavelength.appropri-
ately,. they. are. also. frequently. compensated. by. increased. excitation. intensities.. Optical. aberrations,.
caused.by.refractive.index.inhomogeneities.within.the.sample,.can.be.compensated.by.wavefront.shap-
ing.with.active.elements,.which.has.intrigued.researchers.for.several.decades.(Sheppard.and.Cogswell.
1991)..Nowadays,.an.increasing.supply.of.deformable.mirrors.(DMs).and.liquid.crystals.allows.the.cor-
rection.of.optical.aberrations.with.varying.temporal.and.spatial.resolution.
In. astronomy,. where. atmospheric. turbulences. produce. optical. aberrations,. limiting. the. perfor-
mance. of. large. terrestrial. telescopes,. methods. for. fast. wavefront. measurement. have. been. developed.
that.allow.for.the.determination.of.the.optimal.correction.settings.from.the.analysis.of.a.strong.(natu-
ral.or.artiicial).guide.star.in.the.ield.of.view..Such.a.guide.star.serves.as.a.point-like.emitter,.whose.
light.passes.through.the.unknown.atmospheric.distortions..his.light.is.then.analyzed.by.a.wavefront.
sensor—for.example,.a.Shack-Hartmann.wavefront.sensor..he.such-determined.aberrations.can.serve.
to. increase. the. optical. resolution. within. a. certain. patch. of. the. sky. around. the. location. of. the. guide.
star,.making.much.weaker.and.closely.spaced.objects.discernible..In.particular.for.solar.astronomy,.
variants.of.these.methods.have.been.developed.to.allow.the.use.of.laterally.extended.objects.as.a.refer-
ence.(von.der.Lühe.1983;.Soltau.et.al..1997;.Rimmele.and.Radick.1998;.Poyneer.2003).based.on.image.
cross-correlation.
253
