Biomedical Engineering Reference
In-Depth Information
Variable
aperture
Sample
632 nm, 543 nm, 473 nm
Laser or collimated Xenon
White light sources
θ
Focusing lens
Detector
FIGURE 25.2
Schematic.of.the.apparatus.used.to.determine.accurately.the.relectivity.of.a.surface..The.sources.used.were.
a.series.of.lasers.and.an.Ocean.Optics.HPX-2000.white.light..By.combining.white.light.spectra.with.absolute.
values.obtained.for.single.wavelengths,.it.is.possible.to.remove.features.caused.by.short-.and.medium-range.
irregularities.in.the.sample.geometry.
speciied.angle.using.the.apparatus.shown.in.Figure 25.2..In.order.to.obtain.absolute.mea-
surements,.the.white.light.source.was.replaced.with.a.series.of.laser.sources..This.made.it.
possible.to.measure.the.proportion.of.the.incident.light.that.was.relected.by.the.sample,.
and.this.information.could.be.used.to.calibrate.and.then.validate.the.data.obtained.using.
the.white.light.source.
Regions. of. the. sample. were. found. to. typically. contain. 17. layers,. although. areas. were.
found. where. the. number. ranged. from. as. few. as. 8. to. as. many. as. 23. layers.. TEM. mea-
surements. showed. that. the. layers. varied. in. thickness. from. approximately. 80. nm. to. 130.
nm,. with. the. red. region. demonstrating. dimensions. proportionally. larger. than. those. in.
the.green.region..The.use.of.least.squares.algorithms.to.match.experimentally.measured.
absolute. relectance. proiles. to. theoretical. values. generated. using. recursive. calculations.
of.Fresnel.formulae.for.the.individual.layers.allowed.the.real.and.imaginary.parts.of.the.
refractive.indices.to.be.established.
Previous.studies.have.regularly.concluded.refractive.indices.in.the.region.of.1.56,.with.
other.work.proposing.values.for.the.individual.layers.varying.from.1.3.to.2..This.sample.
also.demonstrated.refractive.indices.in.this.region,.with.values.of.
n
.=.1.68.and.
k
.=.0.03.for.
the.light.contrasted.layers.and.
n
.=.1.55.and.
k
.=.0.14.for.the.dark.contrasted.layers..While.
these.values.are.reliable,.they.are.unable.to.be.used.to.deine.the.material.that.comprises.
the.layers.within.these.structures.
Despite.many.years.of.study,.the.composition.of.the.layers.observed.in.the.epicuticle.of.
iridescent.beetles.is.still.largely.unknown..The.material.is.often.reported.to.contain.a.high.
proportion. of. chitin,
7,8
. although. it. has. also. been. reported. that. chitin. is. restricted. to. the.
bulk,.with.the.epicuticle.remaining.chitin-free.
9
.Chitin.is.a.polymer.composed.of.N-acetyl-
D-glucosamine.and.D-glucosamine.monomer.subunits.linked.by.β-1,4-glucosidic.bonds,
9,10
.
and.it.forms.three.types.of.highly.ordered.crystalline.structures,.termed.α,.β,.and.γ,.depen-
dent.on.the.orientation.of.the.chitin.chains.
11
.However,.with.very.few.exceptions,
12
.chitin.is.
not.found.in.the.animal.kingdom.in.a.pure.crystalline.form,.but.rather.as.a.complex.matrix.
involving.signiicant.contributions.of.proteins,.minerals,.and.other.organic.compounds,
9,10
.
with. the. insect. cuticle. shown. to. contain. 40%-76%. protein.
12,13
. This. binding. of. proteins,.
minerals,.and.pigments.can.result.in.signiicant.changes.to.the.physical.and.optical.prop-
erties.of.resulting.chitin-based.materials..It.has.been.shown.that.the.presence.of.uric.acid.
within.these.materials.can.signiicantly.increase.the.measured.refractive.index.
14
