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Brighton Webs Ltd statistics for energy and the environment Home Index About The resistance of light dependent resistor falls falls as the illuminance increases. They can be used to build a simple, low cost photometric device. They appear to work well with medium to high light levels (greater than 10 lux). Unlike photodiodes they do not saturate at high light levels. The spectral response is similar to that of the human eye with the greatest sensitivity being in the green range (approx. 500 550 nm). The relationship between illuminance and resistance is nonlinear being a straight line when plotted on a loglog graph. Whilst some suppliers do provide datasheets, information can be limited to the range of resistance at at light levels and a typical resistance at some representative point. Thus it can be necessary to perform some form of calibration, although without reference to some absolute standard, the results should be used for measuring relative illuminance, rather then absolute illuminance. This page is evolving as experiments are completed and should be viewed as "lab report" on a single component rather than a generic "datasheet". Calibration The relationship between illuminance and resistance is given by: I O and R O are the reference values of illuminance and resistance respectively, in the example below they are 100 lux and 20k respectively. The problem is to obtain the value of A which will allow us to determine illuminance from resistance which can be readily measured with a meter. Typically the value of A is in the range 0.7 to 0.9. Light Source A convenient means of providing a variable light source is to use an incandescent light bulb and vary the intensity by taking measurements at different distances, the inverse square law can then be used to create a table of relative illuminances. The inverse square law is often expressed in this form:

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  • 5/8/2015 LightDependentResistor

    http://www.brightonwebs.co.uk/electronics/light_dependent_resistor.aspx 1/7

    BrightonWebsLtdstatisticsforenergyandtheenvironment

    Home Index About

    Theresistanceoflightdependentresistorfallsfallsastheilluminanceincreases.Theycanbeusedtobuildasimple,lowcostphotometricdevice.

    Theyappeartoworkwellwithmediumtohighlightlevels(greaterthan10lux).Unlikephotodiodestheydonotsaturateathighlightlevels.Thespectralresponseissimilartothatofthehumaneyewiththegreatestsensitivitybeinginthegreenrange(approx.500550nm).Therelationshipbetweenilluminanceandresistanceisnonlinearbeingastraightlinewhenplottedonalogloggraph.Whilstsomesuppliersdoprovidedatasheets,informationcanbelimitedtotherangeofresistanceatatlightlevelsandatypicalresistanceatsomerepresentativepoint.Thusitcanbenecessarytoperformsomeformofcalibration,althoughwithoutreferencetosomeabsolutestandard,theresultsshouldbeusedformeasuringrelativeilluminance,ratherthenabsoluteilluminance.

    Thispageisevolvingasexperimentsarecompletedandshouldbeviewedas"labreport"onasinglecomponentratherthanageneric"datasheet".

    Calibration

    Therelationshipbetweenilluminanceandresistanceisgivenby:

    IOandROarethereferencevaluesofilluminanceandresistancerespectively,intheexamplebelowtheyare100luxand20krespectively.TheproblemistoobtainthevalueofAwhichwillallowustodetermineilluminancefromresistancewhichcanbereadilymeasuredwithameter.TypicallythevalueofAisintherange0.7to0.9.

    LightSource

    Aconvenientmeansofprovidingavariablelightsourceistouseanincandescentlightbulbandvarytheintensitybytakingmeasurementsatdifferentdistances,theinversesquarelawcanthenbeusedtocreateatableofrelativeilluminances.Theinversesquarelawisoftenexpressedinthisform:

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    Thus:

    Iftheilluminanceofalightsourceismeasuredas100luxatadistanceof1metre,theilluminanceat2metreswillbe25lux,thecalculationisshownbelow:

    ThisallowsustousethesquareofthereciprocalofthedistanceasmeasureofrelativeilluminanceandwithalittlehelpfromExcelwecanestimatethevalueofAintheequationabove.

    TestData

    Thetablebelowisasampledataset:

    D(metres) R(k) ln(D2) ln(R)

    3.71 96.4 2.622 4.569

    3.04 74.4 2.224 4.309

    2.33 46.4 1.692 3.837

    1.68 27.6 1.038 3.318

    1.30 18.2 0.525 2.901

    0.94 10.0 0.124 2.303

    0.64 4.4 0.893 1.482

    Agraphofthesevalueslookslikethis:

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    Theslopeofthisline,whichisthevalueofAis0.87.

    UsingthevalueforAandthetypicalresistancegivesaformulawhichallowsaresistancemeasurementtoconvertedintoilluminance,forthetestdevicesthisis:

    Thisgivesthefollowingloglogcurveforilluminanceandresistance:

    Atthetimeofwriting,thereliableworkingrangeofthisparticulardevicehasnotbeenexplored,itmaybe,thattherangeislessthanthatimpliedbytheabovegraph.

    ResponsetoFilteredLight

    Whilstknowingsomethingabouttheintensityofalightsourceisuseful,thespectrumisalsointeresting.Anexampleistheselectionoflightsourcesforinvestigatingsolarpanelperformance.Ifafilterisplacedbetweenthelightsourceandthelightdependentresistor,itsresponsetodifferentwavelengthscanbeinvestigated.Thegraphbelowshowstheresultofapplyingdifferentfiltersandvaryingtheintensitybychangingthedistancebetweenthesourceandthelightdependentresistor.

    Whilstthemagnitudeoftheresponsetovaryingwavelengthsisdifferent,thesensitivityisthesame.

    CosineResponse

    Whenmakingobservationsofarayoflightstrikingasensor,itisnecessarytoknowhowthe

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    relationshipbetweentheangleofincidenceandtheresponseofthesensor.Theideally,thisshouldconformtoLambertscosinelaw:

    Thegraphbelowshowstheresultsofanexperimentinwhichalightsourcewasrotatedaroundalightdependentresistor:

    ThereddotsshowtheexperimentalresultsandthebluelineiswhatLambertscosinelawpredicts.Allowingfordefectsintheexperimentalmethod,thereisareasonableagreementbetweentheoryandpractice,thusitmaybepossibletousealightdependentresistorinapplicationswhereangleofincidenceisoneoftheparameters.

    TemperatureResponse

    Thecomponenttestedhadsimilarbehaviortoathermistor.Ifthelevelofilluminancewasheldconstant(byplacingthedeviceinadarkroomwithan100wattincandescentlightbulb)andthetemperaturevaried(byimmersingthedeviceinabowlofhotwaterwhichwasallowedtocool),theresistanceincreasedasthetemperaturedeclined.Theresultsofthisexperimentasshownbelow,thefirstgraphshowsresistanceandtemperatureplottedagainsttime:

    Thesecondgraphshowsresistanceplottedagainsttemperature:

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    Therelationshipbetweentemperatureandresistancewasnotcompletelyinvestigated.ItappearsthattheresistanceofanLDRisafunctionofbothilluminanceandtemperature.Inmanyapplicationsthisisnotaproblemifthedeviceismountedinsuchawaythatvariationsintemperaturearesmallandthelevelofilluminancedoesproduceextremevariations(inthecaseofthecomponenttested,thismightbe20k1k).However,whentheresistanceisverylow,asisthecasewhenthecomponentisorientedtowardsthesun,asituationwherethetemperatureofthecomponentwillriseduetosolarheating,theeffectsoftemperaturechangeappeartobesignificant.

    Application

    TheauthorsinterestinLDRisisaspartofapieceofequipmentwhichisintendedtomeasurediffuseirradiancefromacloudedsky.Theilluminanceunderacloudyskyisdiffuse,thusaspectroscopecannotbeused,howeveranLDRusedinconjunctionwithsomefragmentsofred,greenandbluestainedglasscanbeinstructiveasshowninthegraphbelow:

    Therelationshipbetweenthe100wattincandescentbulbandthe8wattCFLismoreorlesswhatisexpected.Theilluminantefficiencyofthe8wattCFLappearstobyhigherthantheCFLmainlyduetothelowerintensityattheredendofthespectrum.ThisbecomesclearwhenaCFLisusedasanenergysourceforadevicewhichrespondstothenearinfraredasthereisnodiscernableheating.Whilsttheobservationsmadeunderclearandcloudskiesshouldbtreatedwithcaution,itcanbeinferredthatthemaindifferenceisintensity,ratherthanspectrum.

    ResponsetoSolarRadiation

    Thegraphicsbelowshowtheresistanceofthesampledeviceduringperiodsofclearsky.Onaclearday,itispossibletomakeareasonableestimatetheglobalhorizontalirradiance(GHI).Alsoonclear

  • 5/8/2015 LightDependentResistor

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    andovercastdays,thefluctuationinsolarirradianceduetoatmosphericconditionsisminimal,thusitispossibletocorrelatetheresponseofasolardevicetoanearbyweatherstation.Thefirstgraphicshowsirradianceandresistanceplottedoveramorning.Sadly,cloudconditionsdidnotallowthecollectionofacontinuousstreamofreadingsandthemeasurementshadtobetakenovertwodays.

    Thesecondgraphicisaloglogplotofresistanceagainstirradiance.

    Asthedatapointsformastraightline,itwouldseemthatanLDRcanbecalibratedtoprovideareasonableestimateofsolarirradiance.Therearetwocaveatstothis.ThefirstisthatanLDRisnotabroadbanddevice,havingaresponseroughlyintherange3001100nm,whilstthespectrumofsunlightisfrom1504000nm.Thesecondistheeffectoftemperature.Atthetimethesereadingsweretaken,thetemperatureresponseofthesampledeviceisunknown.

    Itisplannedtorepeattheaboveexperimentunderanovercastsky.

    PageUpdated:13Sep2010

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