Biomedical Engineering Reference
In-Depth Information
8.4.1 Figure of Merit for Adaptive optics System Performance
To.properly.specify.a.DM,.it.is.important.to.understand.the.quantitative.performance.objectives.of.the.
AO.system..For.this,.the.traditional.approach.is.to.characterize.the.point-spread.function.of.the.system.
relative.to.that.which.would.be.attained.if.there.were.no.aberration..he.Strehl.ratio.is.a.handy.scalar.
metric.
Strehl.ratio.is.deined.as.the.ratio.of.the.peak.of.the.aberrated.point.spread.function.(PSF).to.the.peak.
of.the.theoretical.PSF.with.no.wavefront.aberrations.(φ.=.0):
( )
PSF
PSF
θ
.
S =
(
)
0 0
,
ϕ
=
0
A.Strehl.ratio.of.above.0.8.is.a.typical.threshold.for.what.is.termed.difraction.limited.imaging..Under.
such.a.condition,.points.in.the.object.form.very.sharp.images.at.the.focal.plane,.where.the.width.of.the.
PSF.is.limited.by.difraction.as.opposed.to.the.aberration.blur.(see.Figures.8.13.and.8.14).
he.Strehl.ratio.is.related.to.variance.of.the.wavefront's.phase.departure.from.perfectly.lat.or.per-
fectly.spherical..Using.Marechal's.approximation,.for.small.σ P
(
)
PSF
PSF
0 0
0 0
,
,
(
)
2
.
σ
S =
)
e
p
(
(
)
2
S =
e
σ
.
p
where.σ P .is.the.standard.deviation.of.the.phase,.in.radians.
FIGuRE 8.13 Images.taken.with.the.IRCAL.camera.and.adaptive.optics.at.the.Lick.Observatory.Shane.Telescope..
Let :. he. image. is. with. the. adaptive. optics. system. of.. Right :. he. imaging. is. with. the. adaptive. optics. system. on..
(Credit:.James.Graham,.UC.Berkeley.)
θ ~ λ / r 0 ~ 1 arc sec
θ ~ λ / D
Long exposure
image
Short exposure
image
Image with
adaptive optics
FIGuRE 8.14 Image.of.a.bright.star,.Arcturus,.taken.with.the.1.m.telescope.at.Lick.Observatory..he.speckles.
in.the.short-term.exposure.are.each.at.the.difraction.limit.of.the.telescope..(Credit:.Lawrence.Livermore.National.
Laboratory.and.NSF.Center.for.Adaptive.Optics.)
 
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