Biomedical Engineering Reference
In-Depth Information
Choose aberration
test function
Apply aberration
Acquire image
Calculate image metric
Estimate correction phase
Apply correction
FIGuRE 10.2
Flow.chart.of.the.indirect.wavefront.sensing.scheme..A.sequence.of.images.is.acquired,.each.with.a.
diferent.aberration.applied..For.each.image,.the.quality.metric.is.calculated..From.the.obtained.metric.values,.the.
correction.phase.is.estimated..he.dashed.line.represents.an.optional.repeat.of.the.process,.if.full.correction.was.
not.achieved.ater.the.irst.cycle.
inluence.the.metric.through,.for.example,.a.functional.description.of.the.microscope's.image.forma-
tion.process.in.the.presence.of.aberrations.
tion.problem.informs.the.design.of.the.sensorless.AO.scheme..In.particular,.we.show.how.the.choice.
of.the.modal.expansion.used.to.represent.aberrations.plays.an.important.role.in.the.eiciency.of.the.
process.
10.3 Modal representation of Aberrations
In
.
Section.10.2
,.we.explained.how.the.optimization.metric.
M
.can.be.represented.as.a.function.of.the.
control.signals.that.drive.the.actuators.of.the.DM..However,.it.is.usually.more.convenient.to.express.a.
wavefront. as.a.sequence.of.basis. functions.or.aberration.modes..For. example,. an.arbitrary. wavefront.
Φ(
r
,θ).is.expressed.as
∑
Φ θ =
( , )
r
a X r
i
( , )
θ
(10.1)
.
.
i
i
with.
a
i
. representing. the. mode. coeicients. and.
X
i
. the. basis. functions.. If. the. DM. is. used. in. a. linear.
operating. regime,. then. the. modal. coeicients. are. related. to. the. DM. control. signals. by. a. linear.
transformation.
he.aim.of.modal.wavefront.sensing.is.to.obtain.the.coeicients.
a
i
.directly..his.contrasts.with.zonal.
sensing.methods,.such.as.the.Shack-Hartmann.sensor,.where.the.measured.wavefront.is.constructed.
from. separate. measurements. from. diferent. regions. of. the. wavefront. (Hardy. 1998).. he. choice. of. the.
basis. modes.
X
i
. can. be. inluenced. by. various. factors,. both. mathematical. and. practical.. For. example,.
