Biomedical Engineering Reference
In-Depth Information
Diverging
beam
Collimated
beam
Converging
beam
FIGuRE 9.11
Interference.patterns.for.a.divering,.collimated,.and.converging.beam.
If. the. interference. fringes. are. not. straight. lines,. but. distorted. with. wiggles,. it. indicates. higher-
order.aberrations.in.the.beam..Sometimes,.straight.lines.are.observed.but.not.equally.spaced..Again,.
this. is. the. indication. of. higher-order. aberrations.. Depending. on. the. extent. of. the. wiggles. or. the.
straight-line.ofset,.the.optical.components.may.need.to.be.replaced.to.achieve.a.higher-quality.col-
limated.beam.
If.the.beam.is.astigmatic,.it.is.possible.to.collimate.it.in.one.direction,.but.not.in.the.orthogonal.direc-
tion..As.a.good.practice,.the.beam.should.be.tested.and.sheared.both.horizontally.and.vertically.
9.6.4 Autocollimator
An.autocollimator.is.an.optical.instrument.that.is.used.to.measure.small.angles.with.very.high.sensitiv-
ity..he.autocollimator.projects.a.beam.of.collimated.light..An.external.relector.relects.all.or.part.of.the.
beam.back.into.the.instrument.where.the.beam.is.focused.and.detected.by.a.photodetector..he autocol-
limator.measures.the.deviation.between.the.emitted.beam.and.the.relected.beam..It.is.a.very.useful.tool.
in.precision.alignment.and.laser.collimation.
9.6.5 Viewing card
It.is.helpful.to.have.a.small.(1″.×.1″).piece.of.white.paper.or.an.infrared.card.that.can.be.used.as.a.viewing.
card..Placing.the.card.in.the.optical.beam.will.help.to.track.the.optical.paths,.relected.light,.scattered.
light,.and.difused.light..Placing.the.card.close.to.the.center.of.an.optical.component.provides.informa-
tion.on.the.rough.centering.of.the.beam.on.the.optical.component..If.the.beam.is.in.the.collimate.space,.
the.spot.size.on.the.card.along.the.beam.path.should.stay.the.same.size..Otherwise,.the.optical.compo-
nent.needs.to.be.realigned.so.that.the.beam.size.does.not.change.along.the.beam.path..his.is.a.quick.
check.in.troubleshooting.the.optical.system.alignment..In.addition,.the.card.can.help.track.unwanted.
light.in.the.system.so.that.a.beam.block.can.be.used.to.terminate.any.undesired.beam.or.an.iris.used.to.
ilter.out.scattered.light.to.reduce.the.background.optical.noise.
9.6.6 iris (Diaphragm)
An.iris.is.a.very.useful.component.in.constructing.and.aligning.an.optical.system..It.is.a.thin.disk.with.
an.adjustable.opening.at.its.center..he.center.of.the.iris.coincides.with.the.optical.axis.of.the.optical.
path..It.may.serve.several.functions.in.a.complex.optical.system..First,.the.size.of.the.opening.regulates.
the. amount. of. light. that. passes. through. the. optical. path.. Second,. it. stops. unwanted. light,. especially.
randomly.scattered.light,.from.reaching.the.imaging.camera..his.decreases.the.background.noise.and.
increases.the.imaging.contrast..hird,.it.is.an.efective.visual.tool.for.alignment..If.the.iris.opening.is.
adjusted.to.be.slightly.smaller.than.the.beam.size.of.the.alignment.laser,.a.ring.of.light.can.be.viewed.on.
the.iris..If.the.ring.is.not.uniform.on.the.iris,.it.indicates.that.the.iris.is.not.centered.on.the.optical.path..
Adjustment.in.either.the.
x
.or.
y
.direction.is.needed.until.the.circular.ring.is.uniform..he.other.way.to.
use.an.iris.to.assist.alignment.is.to.close.the.opening.to.a.very.small.circle..Check.the.circular.dots.on.
each.optical.element.in.the.subsequent.downstream.optical.path.ater.the.iris.to.see.if.all.of.the.optics.
