Biomedical Engineering Reference
In-Depth Information
Pupil
subregions
Mask summary
Set of masks
(a)
•
3
×
1 Independent masks
•
N
= 3 subregions
•
Each mask: 1/3 pupil area,
corresponds to 1 subregion
(b)
•
3
×
3 Independent masks
•
N
= 9 subregions
•
Each mask: 1/9 pupil area,
corresponds to 1 subregion
(c)
•
Overlapping masks
•
N
= 9 subregions
•
Each mask: 1/3 pupil area
(d)
•
3
×
3 w/ 2
×
1 Stepped
overlapping masks
•
N
=
18 subregions
•
Each mask: 1/9 pupil area
FIGuRE 13.14
Examples.of.the.independent,.overlapping,.and.stepped.overlapping.mask.approaches.to.AO.cor-
rection..he.objective.rear.pupil.is.represented.by.the.large.square,.and.the.shaded.rectangles.represent.the.fraction.
of. the. pupil. turned. on. at. speciic. points. during. beam. delection. measurement.. (a). Independent. mask. approach:.
three.nonoverlapping.masks,.each.covering.1/3.of.the.total.pupil.area..(b).Independent.mask.approach:.nine.non-
overlapping.masks,.each.covering.1/9.of.the.pupil.area..(c).Overlapping.mask.approach:.six.overlapping.masks,.each.
covering.1/3.of.the.pupil.area,.lead.to.unique.wavefront.estimations.in.nine.pupil.subregions..(d).Stepped.overlap-
ping.mask.approach:.a.mask.covering.1/9.of.the.pupil.area.is.translated.in.horizontal.steps.equal.to.half.the.width.
of.the.mask,.which,.in.vertical.steps.equal.to.its.height,.lead.to.18.unique.subregions..(Ji,.N.,.et.al.,.
Nat. Meth
.,.7,.2,.
141-147,.2010.)
which. can. then. be. used. directly. for. wavefront. reconstruction,. using. algorithms. long. established. for.
Shack-Hartman.wavefront.sensors.
13.5.2 Phase Measurement vs Phase reconstruction
Figure. 13.15
.
shows. an. example. of. AO. correction. through. a. 250. um. brain. slice. with. the. phase. deter-
mined. either. by. direct. measurement. or. phase. reconstruction.. For. the. same. number. of. distinct. pupil.
areas,. phase. measurement. provides. a. better. correction. than. phase. reconstruction,. because. the. direct.
interferometric.measurement.afords.a.higher.accuracy..Keeping.the.“on”.subregion.size.the.same,.the.
performance.of.phase.reconstruction.approach.can.be.improved.to.the.same.level.by.overlapping.the.
subregions.to.increase.the.sampling.density.of.the.wavefront..A.critical.advantage.of.the.phase.recon-
struction.approach,.however,.is.that.it.is.insensitive.to.sample.photobleaching.and.more.tolerant.of.low.
signal-to-noise.conditions.
