determining the validity of the topographic lidar … determining the validity of the topographic...

I

DeterminingtheValidityoftheTopographicLidarEquation

by

EneaRrokaj

Aseniorthesissubmittedtothefacultyof

BrighamYoungUniversity–Idaho

inpartialfulfillmentoftherequirementsforthedegreeof

BachelorofScience

DepartmentofPhysics

BrighamYoungUniversity–Idaho

July,2016

III

BRIGHAMYOUNGUNIVERSITY–IDAHO

DEPARTMENTAPPROVAL

ofaseniorthesissubmittedby

EneaRrokaj

This thesis has been reviewed by the research committee, senior thesiscoordinator,anddepartmentchairandhasbeenfoundtobesatisfactory.________________ ____________________________________________________Date ToddLines,ThesisAdvisor________________ ____________________________________________________Date JonPaulJohnson,ThesisCommitteeMember________________ ____________________________________________________Date RyanNielson,ThesisCommitteeMember________________ ____________________________________________________Date RichardHatt,ThesisCommitteeMember________________ ____________________________________________________Date EvanHansen,SeniorThesisCoordinator________________ ____________________________________________________Date StephenMcNeil,DepartmentChair

V

ABSTRACT

DeterminingtheValidityoftheTopographicLidarEquation

EneaRrokaj

DepartmentofPhysicsBachelorofScience

ThepurposeofthisexperimentistodeterminethevalidityoftheTopographicLidarEquation.The lidar systemwas built using a class iii-b, 633 nm, 20mW helium-neon laser, aMode 1Celestron114AZ,D=114mmFL=900andadetectorwhichconsistsofacircuitonabreadboard,aDAQandalaptopforsignalanalysis.Datawascollectedinadarkroomwiththelaserbeingtheonlysourceoflight.Todeterminethevoltagegainedbythecircuit,thephotodiodewashitwithdifferent laserpoweroutputs (using theclass iii-b laserwithdifferentneutraldensity filters).Then thephotodiode integrated in the circuitwashitwith the same laser. Thiswasdone todeterminetherelationshipbetweenthevoltagethatthecomputerreadsandthepowerinputofthelidar.Thelaserilluminatedthesurfaceofaclipboard.Theexperimentwasrepeatedmanytimestodetermineifthedatawasconsistent.Thereturnpowerdetectedbythedetectorwasthenconvertedintoavoltagebyusingtherelationshipbetweenthevoltageandthepowerinput.ThiswasconsistentwiththereturnpowerpredictedbytheTopographicLidarEquationwithanerrorof0.48VwhichfallswithintheerrorpredictedbyLabView(0.52V).

VII

Acknowledgements

First,Iwouldliketothankmyfamilyforalltheirloveandsupport-especiallymysweet

wife Allison. Thank you for being patient, supportive and loving during countless nights of

studyingandtests.Withoutyou,Iwouldnotbeheretoday.TotheBYU-Idahofaculty,thankyou

foryourhardworkanddedication.AveryspecialthankstoToddLines,forthetime,dedication,

supportand insightfuladviceas Iwasworkingonthisproject.ToRichardHatt, thankyoufor

teachingmephysics and for believing inme evenwhen I did not believe inmyself. ToRyan

Nielsonthankyouforyourhelp.ToEvanHansen,thankyouforhelpingmewritethisthesis.To

JonPaulJohnsonthankyouforhelpingwiththealignmentoftheopticalsystem.Lastly,Iwould

liketothankmyfellowpeers.Physicsisdonebestwhendonewithfriends.Ihavemadesome

life-longfriendsduringmytimeatthisUniversity.ToNeishanAli,AngelRoystonandBenFarley-

thankyouforyoursupportandfriendship.

VIII

TableofContents

Chapter1Introduction...........................................................................................................11.1 ABriefIntroductiontoLidarSystems...................................................................................11.2LidarApplications.......................................................................................................................3

Chapter2Methods.................................................................................................................42.1BasicSystemDesignofaLidar....................................................................................................42.2TheTransmitter..........................................................................................................................5

2.2.2LaserSafety.................................................................................................................................52.3TheReceiver...............................................................................................................................6

2.3.1TheTelescope.............................................................................................................................62.3.2TheDetector...............................................................................................................................7

2.4MathematicalModelforTheLidar............................................................................................10

Chapter3CalculationsandResults........................................................................................113.1CalculatingtheGeometricFormFactor.....................................................................................113.2PowerInputCalculation...........................................................................................................133.3VoltageDetectedbytheDetector.............................................................................................143.4CalculatingthePowerInputfromtheResponsivityoftheDetector..........................................15

Chapter4Conclusions...........................................................................................................164.1Results......................................................................................................................................164.2FutureWork.............................................................................................................................20

IX

Figure1Basicdesignofthelidarsystem.......................................................................................2Figure2Basiclidardesignusedfortheexperiment......................................................................5Figure3Thetelescope...................................................................................................................7Figure4Circuitdesign....................................................................................................................7Figure5Detectorresponsivity.......................................................................................................9Figure6LabVIEWcode.................................................................................................................10Figure7Overlappingfieldofviewsofthelaserandthetelescope.............................................12Figure8Opticalalignment...........................................................................................................12Figure9Mathematicacodefortheerrorpropagation................................................................14Figure10FrontpaneloftheLabVIEWprogram...........................................................................15Figure11Voltagecalculatedat3.35m........................................................................................17Figure12Voltagecalculatedat2.50m........................................................................................18Figure13Therelationshipbetweentherangeandvoltage........................................................19

1

Chapter1Introduction1.1 ABriefIntroductiontoLidarSystems

Lidar systems are extremely useful deviceswith a large range of applications. They are

broadly used in industry, in space exploration and technology, meteorology, atmospheric

sciencesandbythegovernmentformilitarypurposes.Thereisaresearchgroupatthephysics

departmentofBYU-Idahothatdoesresearchinatmosphericsciences.ForthisprojectIbuilta

lidarsystemtohelpstudentswhowishtoconductresearchinatmosphericscienceshaveaccess

toadevicetohelpthemperformthatresearch.ThemainfunctionofthelidarIbuiltistocalculate

distances, however with proper modification it can also be used to perform the functions

mentionedabove.

Alidar(lightdetectorandranging)issimilartoradarwhichisatoolabletodetectdistance,

velocity,andthelocationofanobjectusingradiowaves.Radiowaveshavealargewavelength

(0.001mto300,000m)andthereforeareveryusefulindetectinglargeobjects.However,alidar

usesalaser(UVtoIRfrequency)insteadofradiowavestodetectthetarget.Alidarcandetect

thevelocity,distance,andlocationofanobject.Lidaroftenuseslightfrequencieswellwithinthe

visiblelightwavelengthandasaresultitcandetectverysmalltargets.Alidarisalightsensor

withitsownlightsource.Itoperatesbysendingatimedpulseoflightfromthelasertothetarget

andbackagain.Itconsistsoftwomaincomponents:atransmitterandareceiver.Thetransmitter

hasalightsource(laser)andanopticalsystem(thepulseofthelidar).Thereceiverconsistsofa

pulsegenerator(usuallyatelescope)andadetectorsystem(thedetectorcapturesthelightand

converts it intoasignal).Thedifferencebetweena lidarandaradar isthata lidarcandetect

2

objectswithanabsolutelocationwhereasradarcandetectanyobjectinthepathoftheradio

wave.Figure1showsthebasicdesignforalidarsystem.(ImageCredit:Lidar

Notes,ToddLines)

Figure1Basicdesignofthelidarsystem

ThelidarsystemIbuiltforthisexperimentconsistsofaclassiii-bhelium-neonlaserwith

awavelengthof633nm,apoweroutput,lightintensityofthelaserasafunctionofsurface,of

20mWandaNewtoniantelescope(brand:Celestron).ADAQconnectedtoabreadboard(with

acircuit)andalaptopthatoperatesaLabVIEWprogramwasalsousedduringthisexperiment.

MoredetailswillbeprovidedonthecircuitdetectorandtheLabVIEWprograminthesectionsto

follow.

3

1.2LidarApplications

Oneapplicationofalidarsystemistodeterminetheaerosolheightdistribution.Byusing

ananalyticsolutiontothelidarequation:𝑃"# =%&'(∗*∗+,∗-

.∗/,∗ 𝜁 ∗ 𝐺 ∗ 𝑇

Thetermspowerinputandpoweroutputwillbeusedabundantlyinthispaperthusitis

importantthattheirmeaningisexplained.Poweroutputisthelightintensity(asafunctionof

surface)generatedbythelaser.Itisgivenbythespecksheetsofthelaser.Powerinputisthe

returnlightintensity(asafunctionofsurface)detectedbythephotodiode.Specificdetailsabout

the topographic lidar equation will be provided in the sections to follow. Considering the

molecularpropertiesoftheatmosphere,andbyconsideringthescatteringofaerosolsFernald,

HermanandReaganwereabletodeterminethetheheightoftheaerosolintheatmosphere.

(Fernald,Herman,&Reagan,1971)1

Lidarsystemsareusefuldevicesinwarfareandmilitaryapplications.Theycandetermine

thechemicalcompositionofsmokecloudsandbydoingso,onecanknowwhatkindofweapon

was detonated to cause the smoke cloud. A newmethod was developed to determine the

chemical compositionof the smokeclouds.Themethod relieson theknowledgeof the total

transmissionthroughthedistantrangeobservedandonapproximationofthebackscatter-to-

extinctionratioineachrangecell(unlessthevalueofbackscatter-to-extinctionratioisknownto

bethesameforeachrangecell).Inthismethod,thefirstorderelasticscatteringsufficesandas

aresulttheusuallidarequationisused.(Godecke,etal.)2

4

Anotherapplicationoflidarisinatmosphericandenvironmentalsciences.Itcandetect

pollutantintheatmosphere.Byusingthenarrowspectralbandwidthofheterodyneradiometers,

one can construct highly selective pollutant sensing devices. The availability of pollutant

absorptionlinesandwavelengthdependenceofheterodynesensitivityfavortheinfra-redregion.

ThissystemisespeciallyefficientindetectingNOpollutionsintheatmosphere.(Menzies,1972)3

Lidar are also used for calculating distances and three dimensionalmapping. Photon-

counting laser mapping is a technique that uses timed photon pulses to illuminate targets.

Combining this technique with the lidar equation, one can calculate distances with great

accuracy. Knowing the time of arrival of each photon to a nanosecond precision allows the

constructionofthree-dimensionmapping.(Priedhorsky,Smith,&Ho,1996)4

Anotherapplicationoflidarisincalculatingthewindspeedattheoceansurfaceusinga

satellite-born lidar. For this method, the data from a single-frequency space-born lidar is

combinedwithsatelliteimaginarytodeterminetheaerosolextinctioncoefficientsandthewind

speedattheoceansurface.(Weinman,1988)4

Chapter2Methods

2.1BasicSystemDesignofaLidar

Figure2showsthebasicdesignofthelidarsystemIbuiltforthisexperiment.Alaseris

mountedonthetelescope.Whenalignedproperly,thelightfromthelaserbeamwouldreflect

offatargetintothetelescope.Thetelescopeservesasacollectingapparatusforthelight.The

light then comes out of the telescope through the eye piece. The eye piece consists of a

converginglenswhichfocusesthelightatafocalpoint.Atthefocalpointthereisaphotodiode

5

whichconvertsthelightintoelectriccurrent.Thephotodiodeispartofthedetectorsystem.In

thenextsectionsdetailsaboutthetransmitterandthereceiverusedinthisexperimentwillbe

provided.

Figure2Basiclidardesignusedfortheexperiment

2.2TheTransmitter

Thetwomaincomponentofthetransmitterarethelaserandthepulsegenerator.For

this experiment a pulse transmitter will not be used. The purpose of this experiment is to

determinedistancesinshortrangeswithrelativelylowbackgroundnoise.Thelaserusedforthis

experimentisahelium-neonclassiii-blaserwithawavelengthof633nmandapoweroutputof

20mW.ThelaserbeamisaGaussianbeam.

2.2.2LaserSafety

Laserstandsforlightamplificationbysimulatedemissionofradiation.Likeanytypeof

radiationlightradiationshouldnottreatedlightly.Someofthecommonlaserhazardsareskin

burnsandeyeinjuries.Somecommonskininjuriesarethermalskininjuriesfromahighpower

laser.Thelaserusedforthisexperimenthas20mWpower.Whileworkingwithlasersanother

6

potential hazard is deeper burn penetration caused by lasers with a wave length of 1 µm.

Photochemicalskininjuriesareimportanttoconsiderwhileworkingwithlasers.Sunburn

forscatteredUVlightandthepossibilityofskincancerfromlongtermUVexposurearethetwo

main photochemical hazards. However, for this procedure skin burns will not be a relevant

becausethelaserisonlyat20mWandthewavelengthiswellwithinthevisiblespectrum.

Eyesareverysensitivetohighfrequencyradiation.Whileworkingwithlasersitisalways

recommendedtousinglasereyesafetygoggles.Whileworkingwithlasersoneshouldneverlook

intodirectlyintothelaserbeam.Lookingintoaspecularreflectionofthelaserbeamshouldbe

avoidedaswell.Diffusedreflectionisalsoverydangerousandlookingdirectlyintoscatteredlight

can cause damage to the retna. Lasers can cause serious eye injuries and can even result in

permanentvisionloss.Whenaligningthedevice,lookingthroughtheeyepieceofthetelescope

shouldbeavoided.(Pitsburg,2014)6

Class iii-b lasers operating within the visible spectrum are relatively safe to operate.

However,everytimethelidarisinoperationlasereyesafetygoggleshouldbeworn.

2.3TheReceiver

The receiver consists of collective apparatus, which is a Mode 1 Celestron 114 AZ,

D=114mmFL=900mmtelescopeandthedetector.

2.3.1TheTelescope

Thetelescopeisanintegralpartofthelidarsystem.Itsmainpurposeistocollect

thereflectedlightofthetargetandfocus it intothephotodiode.However,thetelescopeisa

greattooltohelpaligntheopticalsystemandtooptimizethegeometricformfactor.Detailswill

beprovidedinthenextsectionsaboutthegeometricformfactorandopticalsystemalignment.

7

Figure3showsthelidarsystemusedforthisprocedure.Thedetectorhasmanysectionsandit

willbeexplainedindetailthenextsection.

2.3.2TheDetector

Themainpurposeofthedetectorsystemistodetectthelightcomingoutoftheeyepiece

ofthetelescopeandconvertingthatlightintoasignalthatcanbeanalyzed.Thedetectorconsists

ofacircuitonabreadboardandaDAQ.Thecircuitconsistofaphotodiode,anop-amp,andtwo

resistors.Figure4showsthedesignofthecircuitboard.

Figure3Thetelescope

Figure4Circuitdesign

8

Aphotodiodeisconnectedinseriestoanop-amp.Aphotodiodeisap-njunction.When

photonswithsufficientenergyinteractwiththephotodiodeitcreatesanelectron-holepair.The

holemovestothenextpair,thuscreatingacurrent.Thephotodiodeworksontheprinciplesof

thephotoelectriceffect.Photodiodescangenerateacurrentindarkaswell.Thedarkcurrentfor

thephotodiodeusedinthisexperimentgeneratesa0.03V.However,thecurrentgeneratedfrom

thephotodiodeisnotstrongenoughfortheDAQ.Thephotodiodeisconnectedinseriestothe

op-amp.Theop-amphastwopowerinputsandtworelayvoltagepins.Oneofthevoltagerelay

pinsisconnectedtothephotodiode.Anop-ampamplifiesthevoltagedifferencebetweenthe

tworelayvoltagepins.Theamplifiedvoltagecomesoutoftheoutputpinoftheop-amp.Anop-

amp cannot amplify the voltagedifference tobehigher that it receives from the twopower

inputs. Tominimize the voltage difference between the two input leads of the op-amp and

thereforemaximizingthevoltagegainedbytheop-amp,theoutputoftheop-ampisconnected

toa9megaohmresistorinparallelanditgoesbackintotheotherinputpin.Theoutputpinis

alsoconnectedtothepowerinputoftheDAQAssistant.Toavoidgroundfloatingthephotodiode

iscontactedtoaresistorandconnectedtoground.Todeterminethevoltagegainbythecircuit,

thephotodiodewasilluminatedwithdifferentlaserpoweroutputs(usingtheclassiii-blaserwith

differentneutraldensityfilters).Thenthephotodiodeintegratedinthecircuitwasshinedwith

thesamelaserpoweroutputs.Thisistodeterminetherelationshipbetweenthevoltagethatthe

computerreadsandthepowerinputofthelidar.Thedatawasthenenteredinexcelandacurve

wasfittedtothedata.Figure5showsthedatacollectedtodeterminetheresponsivityofthe

detector.There isonedatapointwhich isskewedandthat isbecauseoneofneutraldensity

filterswasmisplaced.

9

Figure5Detectorresponsivity

TheotherpartofthedetectoristheDAQ.Forthisexperimenta20KHzDAQAssistantis

used(NationalInstruments).DAQisveryusefulfordataacquisitionandcanconnectthelidarto

thelaptop.Figure6showstheblockdiagramoftheLabVIEWcodeusedforthisexperimentis

showedbelow.TheLabVIEWprogramforthisprojectconsistofaDAQAssistantthatcollectsthe

data from the circuit board, theoutputof theDAQ is connected to awaveform chart and a

numericindicator.Thiscodeisinsideofawhileloopwhichhashasawaitbuttonadjustingthe

saplingfrequencytoathousandsamplespersecond.Thewhileloophasalsoastopbuttonwhich

is connected toanumeric control adjusting theprogram to stopatadesired time.TheDAQ

Assistantisconnectedtoastatisticpanelthroughaconverterbutton(convertingdynamicdata

coming out of the DAQ Assistant to integer data used by the statistic panel. Each time the

10

programrunsthroughtheloopitsendsdatatothestatisticpanel.Twonumericindicatorsare

connectedtothestatisticspanel.Oneofthenumericindicatorsisconnectedtothearithmetic

meanandtheotherisconnectedtothestandarddeviation.Thearithmeticmeanisthemeanof

the1000datapointcollectedduringthesecondiftheprogramisrunningforonlyonesecond,

the standard deviation calculates the uncertainty in the data collected by the detector. The

experimentwasperformedinadarkroomwiththelaserbeingtheonlysourceoflight.Sincethe

systemdoesnothaveapulsegeneratoritcannotdistinguishdifferentpulses.Itisnecessaryto

avoidanybackgroundnoisemakingthelightdetectedbythedetectorthereturnlightifthelaser.

Figure6LabVIEWcode

2.4MathematicalModelforTheLidar

Forthisproject,thelidarwouldcalculateonlythesignalscatteredfromasurface.The

equationusedforthisexperimentisthestandardtopographiclidarequationwhichcalculates

thereturnpowerofthelight.(Lines,2015)7:

𝑃"# =%&'(∗*∗+,∗-

.∗/,∗ 𝜁 ∗ 𝐺 ∗ 𝑇(1.1)

Thevariablesaredeterminedasexplainedabove:

11

T=0.3𝜌=0.05L=0.114mR=5.83mG=0.921𝜁 = 0.8𝑃89: = 20𝑚𝑊

T is the transmission coefficient of the atmosphere which determines how the

atmosphereeffectsthetransmissionoflightfromthelasersourcetoadistanceRandback.𝜌is

thesurfaceemecivityof the target. Itdetermineshowmuchof the lightshining the target is

reflectedback.Listhereceiveraperturediameter.Ristherangethelidarcovers;itisthedistance

fromthelightsourcetothetarget.Gisthegeometricformfactorwhichdetermineshowwell

the optical system is aligned- that is, how much of the reflected light falls into the return

apparatus.𝜁 is the system spectral transmission factor,which is the ratio of the radiant flux

transmittedthroughtheatmosphereandthereturnfluxthroughit.𝑃"#isthereturnlightpower

ofthelaser.𝑃89:isthelightpowersendbythelaser.

Chapter3CalculationsandResults3.1CalculatingtheGeometricFormFactor

Todeterminethepowerinput,thegeometricformfactormustbecalculatedwithsome

accuracy.Thegeometricformfactordetermineshowmuchofthereturnedlightpowerisactually

focusedonthephotodiode.Anotherwayofexplainingthegeometricformfactorisbylookingat

howmuchoftheilluminatedsurfacebythelaserthefieldofviewofthetelescopeencompass.

Figure7helpsvisualizethisconcept.

12

Figure7Overlappingfieldofviewsofthelaserandthetelescope

Theprojectionofthelaserontheilluminatedsurfaceisconcentriccircleswiththecenter

circlebeingthebrightest.Whenthelaserwasoperatingtheilluminatedsurfacewasmarkedwith

amarker(theperimeterofthecentercircle).Thenthelaserwasturnedoffandthedrawncircle

wasviewedwiththetelescope.Togetageometricformfactorcloseto1,theentirecircledrawn

onthesurfacemustbeviewedthroughthetelescope.Bycalculatinggeometricformfactorone

candeterminehowwell theopticalsystemisaligned.Figure8visuallyrepresentsthe ideaof

aligningtheopticalsysteminsuchawaythatthefieldofviewofthetelescopecanencompass

theilluminatedsurface.Forthisexperiment,theopticalsystemwasalignedinsuchawaythat

theentireilluminatedsurfacecouldbeviewedwiththetelescope.

Figure8Opticalalignment

SincethecollectingapparatusisaNewtoniantelescope,ithastwomirrorsinfrontofeach

otherwiththefrontmirrorblockingsomeofthelightcomingintotheapparatus.Thelaseruses

13

aGaussianbeam,thereflectedbeam(thebrightcircleofthebeam)exceedsthediameterofthe

telescope. To calculate the geometric form factor, the difference of the surfaces of the two

mirrors suffices. Then to determine the geometric form factor the difference in the mirror

surfacesisdividedbythebigmirrorarea.Thebigmirrorofthetelescopehasadiameterof0.114

mandthesmallmirrorhasadiameterof0.032m.Belowisshownthemathematicalformto

calculatethegeometricformfactor:

𝐺 =𝜋 0.114A𝑚A − 0.032A𝑚A

𝜋 ∗ 0.114A𝑚A = 0.921

Erroranalysis:

𝜎𝐺 = (𝜕𝐺𝜕𝐴 𝜎𝐴)

A + (𝜕𝐺𝜕𝑎 𝜎𝑎)

A = 0.0042

3.2PowerInputCalculation

Todeterminethevalidityofthetopographiclidarequation,thelidarilluminatedasurface

acrossoneoftheroomsintheRomneybuilding.Thesurfacewasthebackofaclipboardwitha

surfacereflectivityofthe0.05.Thedistanceacrosstheroomwas5.83m±0.02m.

𝑃"# =%&'(∗*∗+,∗M∗N∗O∗-

.∗/,= 2.11 ∗ 10PQ𝑚𝑊

Thecalculations for theerrorpropagation for thepower input (returnpower)wascalculated

usingaMathematicacode.Figure9showstheMathematicacode:

14

Figure9Mathematicacodefortheerrorpropagation

Theerrorpropagationforthepowerinputis:

𝜎𝑃"# = 8.20 ∗ 10PR𝑚𝑊

3.3VoltageDetectedbytheDetector

Asthereturnlightwasfocusedonthephotodiodeacurrentwasgeneratedinthecircuit

whichwasthenamplifiedbytheop-amp.TheDAQassistantreadthevoltageacrossthecircuit.

After many trials of alignment, the apparatus was ready for operation. LabVIEW read the

arithmeticmeanforthevoltage,generatedbythecircuit,tobe3.15Vandtheerrorinthedata

collectiontobe0.52V.Thatis,thereisnoiseinthesystemsothevoltagewouldfluctuateby0.5

V.Themainsourceofthenoisecomesfromtheinternalresistanceofthephotodiode.Thereare

othertypesofnoisebuttheyarebeyondthescopeofthisexperiment.Figure10showsthefront

paneloftheLabVIEWprogram.

15

Figure10FrontpaneloftheLabVIEWprogram

Theexperimentwasrepeatedmanytimesandtheresultswereconsistent.Table1shows

theresultsfromthedifferenttrialsatarangeof5.83m.

Table1Experimenttrialsatarangeof5.83m

3.4CalculatingthePowerInputfromtheResponsivityoftheDetector

Whendeterminingtheresponsivityofthedetector,therelationshipbetweenthevoltage

readbytheLabVIEWprogramandthepowerinputwasestablished.Themathematicalequation,

16

whichwas derived in excelwhen fitting the data into a curve, below shows the relationship

betweenvoltageandthepowerinput:

𝑉 = 0.4018 ∗ 𝐿𝑜𝑔 𝑃"# + 9.7721 = 2.67𝑉(3.1)

Chapter4Conclusions

4.1Results

Thevoltagecalculatedfromtherelationshipbetweenthevoltageandthepoweroutput

iswithintheerrorcalculatedbyLabVIEW.Thisexperimentwasrepeatedmanytimesandthe

datacollectedwasconsistent.Theexperimentwasconductedinadarkroomwherethelaser

wastheonlysourceoflight.Theexperimentwasperformedattwootherdistances(at3.35m

andat2.50m).Forthe3.35mthepowerinputwascalculatedtobe6.40 ∗ 10PQ𝑊.Usingthe

equation for the responsivity of the detector it was predicted to get 3.12 V. Then LabVIEW

measuredthevoltagetobe3.16Vwithanerrorof0.099V.Figure11showstheresultsofthe

3.35mtrial.

17

Figure11Voltagecalculatedat3.35m

Theexperimentwasrepeatedmanytimes.Table2showstheexperimenttrials.


Usingthetopographiclidarequationitwaspredictedapowerinputof1.149 ∗ 10PY𝑊.

Thepredictedpowerinputwasconvertedintovoltsusingtheequationfortheresponsivityof

18

thedetector.Thevaluepredictedfromtheresponsivityequationis3.35V.Figure12showsthe

voltagecalculatedat2.50.

Figure12Voltagecalculatedat2.50m.

Theexperimentwasagainrepeatedmanytimes.Table3showsthevaluesofthetrials

forthisrange.

19


Fromthedataanalysisarelationshipbetweentherangeandthevoltagegeneratedfrom

thedetectorsystemwasestablished.Thefirstsetofdata(at5.83m)collectedhadastandard

deviation bigger than the other two sets of data. This is due to the learning curve in data

collection,thedevicewasnotwarmedwhenthedatawascollected.Theothertwosetsofdata

(3.35mand2.50m)showedaconsistentstandarddeviation.Thedevicewaslefttowarmup

before the datawas collected. The lidar system cannot operate at a range smaller that 1m

because thedetector systemsaturates (12V). Figure13 shows the relationshipbetween the

rangeandthevoltage.

Figure13Therelationshipbetweentherangeandvoltage

20

Atthispointthelidarisefficientonlyincalculatingrangesinthedark.Thisexperiment

provedthetopographiclidarequationtobevalid.Theatmosphericextinctioncoefficientiszero

insidetheroom.However,whenthelidarisusedtocalculatebiggerranges,especiallyifused

outdoors, the atmospheric extinction coefficient increases. The increase in the extinction

coefficientshouldbetakenintoconsiderationwhencalculatingerrorpropagation.

With the equipment provided, this experiment was a success. Considering that the

sampling frequency of theDAQwas 20 KHz, this lidar system is efficient in calculating short

ranges.Ifbiggerrangesneedtobecalculated,aDAQwithfastersamplingfrequency(around1

MHz)isnecessary.Todetecthigherlevelsofpoweroutputabetterop-ampisnecessarytoo.The

detectorsaturatesatabout12V.Tocalculateabiggerrangeapulsegeneratorisneeded,aswell

asaLabVIEWprogramthatcandosignalrecognition.Outdoors,thereismorebackgroundnoise

so it is necessary to recognize the pulse sent out by the lidar. If the lidar is used solely for

atmosphericsciencesanerroratabout1kmintherangeissufficient.

4.2FutureWork

Lidarareveryusefuldeviceswithavarietyofapplicationsasmentionedabove.However,

thelidarIbuiltcanbeimprovedtoperformmorecomplextasks.Apulsegeneratorisnecessary

in distinguishing the pulse produced by the laser from background noises. For a lidar to be

successfulatcompletingthistask,acomplexpulsemustbeproduced.Thatis,toproduceapulse

thatiseasilydistinguishablefromanybackgroundnoise.

TheLabVIEWprogramcodedforthisexperimentonlyrecognizesavoltageinput.Itisvery

efficientindetectingthereturnpowerwhenthelaseristheonlysourceoflight.However,this

21

codewouldnotbeusefulifthelidarwastobeoperatedoutdoors.LabVIEWprovidespowerful

toolsforsignalprocessingunderExpress>Signalprocessing.

A20mWlasercoversadistanceofover100miles,butamorepowerfullasercancover

larger distances. The laser can be easily replaced from the collective apparatus if needed to

calculatelargerdistances.

Thedetector canbe improved tooby replacing theop-ampand thephotodiodewith

betterones.Thecircuitdesignusedisefficientandtheresponsivityofthedetectorhasproved

the topographic lidar equation valid.However, the circuit design canbe improvedand if the

photodiodeandtheop-amparereplacingitmightbenecessarytoreplacethecircuitdesignto

maximizetheresponsivityofthedetector.

Dr.ToddLinesdirectsaresearchgroupofstudentswhoconductresearchinatmospheric

sciences-especiallyicescatteringintheatmosphere.Sofarthedataforthisresearchgroupis

providedbyMichiganStateUniversity.However,withthesuggestedmodificationtothelidar,

conductingicescatteringresearchintheatmospherecanbecollectedusingthelidarIbuilt.This

lidarcanalsobeusedforotherresearchprojectssuchas3Dmapping,chemicalanalysisofthe

pathtraveledbythelaserandlongrangecalculation.However,ifneededtousetheoutsidethe

physicsbuildingperditionmustbeacquiredbythesafetyofficerifBYU-I.

23

Bibliography1. Fernald,F.G.,Herman,B.M.,&Reagan,J.A.(1971,November30).Determinationof

AerosolHeightDistributionsbyLidar.JOURNALOFAPPLIEDMETEOROLOGY,11,482-489.

2. Godecke,G.H.,Loveland,R.B.,Lines,R.T.,Gonzales,R.,Thompson,J.,&Yee,Y.P.(n.d.).Alidarinversiontechniqueusingtotalattenuationsystemconstant.

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