Biomedical Engineering Reference
In-Depth Information
in.
P. palinurus
.and.the.weevils.discussed.earlier,.where.the.multiple.relections.are.from.
differently.oriented.planes.within.the.scales.
27
25.2.3 Whiteness
It.has.long.been.known.that.there.is.no.naturally.occurring.pigment.that.causes.a.white.
color;. white. is. more. simply. the. lack. of. color. selection.. As. such,. whiteness. in. nature. is.
caused. by. irregularly. scattering. wavelength-independent. structures. in. otherwise. trans-
parent.materials.
28
Good.examples.of.this.are.the.
Pierid
.butterlies,.such.as.the.small.cabbage.white,.
Pieris
rapae
,. or. the. white. patches. on. the. black. jezebel,.
Delias nigrina
,. which. have. small. beads.
around.150.nm.wide.and.200.nm.high.that.scatter.the.light..It.can.be.seen.by.removing.
these.beads.that.they.are.responsible.for.the.white.of.the.scales.
29,30
However,.the.brightest.whites.have.been.observed.in.the.scales.of.the.beetles.
Lepidiota
stigma
.and.
Cyphochilus
.spp..(Figure 25.12a)..As.with.the.weevil,.the.scales.contain.a.struc-
ture.that.is.responsible.for.the.observed.color,.although.in.these.scales.the.substructure.
has.been.shown.to.consist.of.a.highly.irregular.ibrous.structure.that.has.no.periodicity.
(Figure 25.12b.and.c).
31
The.sections,.as.seen.in.Figure 25.12,.show.a.disordered.system.inside.the.scale,.with.an.
average.volume.occupancy.of.61.3%,.varying.from.58.5%.to.64.5%..A.volume.occupancy.
of.this.range.has.been.shown.to.maximize.scattering.ability.while.avoiding.overcrowding.
that.would.otherwise.decrease.the.scattering.eficiency..The.iber.size.is.typically.300.nm,.
varying.from.250.nm.to.315.nm.
31
Relection.spectra.were.collected.using.a.Datacolor.ELREPHO
®.
photospectrometer..This.
spectrophotometer.is.based.off.a.152.mm.diameter.integrating.sphere.that.has.two.mea-
surement.devices..The.irst.is.a.reference.device.focused.on.the.barium.sulfate-coated.inner.
wall.of.the.sphere,.and.the.other.is.focused.on.the.sample..Both.detectors.are.oriented.such.
that.they.only.detect.light.from.the.targeted.areas,.and.the.signals.from.each.are.fed.into.
a.SP2000.dual.256.diode.array.
The.light.is.from.a.CIE.D65.daylight.source.based.off.a.xenon.bulb.that.is.passed.through.
a.pulsed.shutter,.an.optical.ilter,.and.then.into.the.sphere..This.coniguration.has.a.spectral.
working.range.of.360-700.nm..The.device.is.speciically.designed.to.measure.the.relectiv-
ity.of.white.substances.
Measurements. taken. from. the. Datacolor. photospectrometer. allow. calculations. of. the.
whiteness.and.brightness.of.the.sample.under.examination..These.measurements.showed.
(a)
(b)
(c)
FIGURE 25.12
(a). Photograph. of. two. white. beetles,.
Lepidiota stigma
. (left). and.
Cyphochilus
. sp.. (right).. (b). Scanning. electron.
microscopy. image. of. a. scale. from. the. surface. of.
Cyphochilus
. sp.. (c). TEM. section. from. a. scale. from.
Lepidiora
stigma
..Scale.bars:.3.μm,.1.μm..(From.Vukusic,.P.,.et.al.,.
Science
,.315(5810),.348,.2007.).(With.permission.)
