Technical Lumen Maintenance Testing of Off‐grid Lighting ......a short description of each method,...
Transcript of Technical Lumen Maintenance Testing of Off‐grid Lighting ......a short description of each method,...
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Introduction
Consistent luminous flux over the lifetime of an LED
lightingproduct(referredtoaslumenmaintenance)isan importantperformancemetric. Lightingdesigners,manufacturers, importers, aswell as government and
non‐government organizations often require lumenmaintenance data to evaluate product quality. Forthosewhodonothavespecializedtrainingandaccess
to relatively high‐cost photometric equipment, thereexist simple, low‐cost devices that can accuratelymeasureLEDlumenmaintenance.Twosuchdevices,a
box‐photometer and a tube‐photometer, can beconstructed of widely available materials, using basictools,for$250‐$350.Inadditiontobeinginexpensive,
the tube‐ and box‐photometers occupy little space,and a minimally trained operator can make lumenmaintenancemeasurementsquicklywithbothdevices.
LEDLumenMaintenance
LEDsdonottendtofailcatastrophically.Instead,theyexperienceanirreversibledecreaseinlightoutputovertime called ‘lumen depreciation’. The inverse of
lumendepreciation,‘lumenmaintenance’isdefinedasthe percentage of the initial luminous flux outputremainingataspecifiedelapsedoperatingtime.1
Forexample,anLEDproductmaylose30%ofitsinitiallightoutputafterrunningfor2000hours.Itslumenmaintenancewouldthenbe70%at2000hours,alsowrittenasL70.Iftheproductinsteadretained90%ofitsinitiallightoutputafter6000hoursofoperation,itslumenmaintenancewouldbe“L90at6000hours”.
Modern off‐grid lighting technologies must becompetitive on the basis of cost‐per‐time of lighting
service if they are to be adopted by low‐incomeconsumers. Unfortunately, many low‐quality LED
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TechnicalNotesIssue8April2012
LumenMaintenanceTestingofOff‐gridLightingProductsThisTechnicalNotecoversmethodsformeasuringlumenmaintenanceofLEDbasedoff‐gridlightingproducts,focusingonlow‐costmethodstoevaluateaproduct’sabilitytoprovidelong‐termlightingservice.TheInformationcontainedinthisarticlebuildsonpreviousTechnicalNotes,seealso:http://www.lightingafrica.org/resources/briefing‐notes.html
products exhibit such rapid lumen depreciation that
the potential economic savings are never achieved.With this in mind, the Lighting Africa Quality TestMethod (LA‐QTM) and Initial Screening Method (LA‐
ISM) include an off‐grid lighting‐specific method formeasuring lumen maintenance that is derived fromexisting standards and internationally accepted
methods. Examples of lumen maintenance plots for“stable”andrapidlydepreciatinglightingproductsareshowninFigures1and2.
Figure1.Lumen maintenance plot for a product that meets Lighting Africa minimum standards.
Figure2.Lumen maintenance plot for a product that fails Lighting Africa minimum standards.
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ReferenceTestMethods
The test method established in LM‐80‐08 by the
Illuminating Engineering Society (IES) serves as theprimarymethodologicalreferencefordeterminingthelumenmaintenanceofindividualLEDcomponents.LM‐
80‐08specifiesthatanLED,LEDarray,orLEDmodulemust be operated for at least 6,000 hours,with datacollectionevery1,000hours,resultinginaminimumof
sixmeasurementsoverthecourseofthetest. Thesemeasurements are typically conducted with anintegrating sphere‐spectrophotometer system, as
described by the International Commission onIllumination(CIE)technicalreportonthemeasurementof luminous flux (CIE 84‐1989). LM‐80‐08 states that
LEDs are to be driven at a constant current that isspecified by the LED manufacturer, and must beregulated to within±3% of the rated value over thetesting period. The ambient air temperature is
maintained at 25oC ±2oC and airflow must be
minimizedinordertoreducetheeffectsofconvectiveheat transfer away from the LED package. A lumenmaintenance test report must include all of the
operating conditionsand thepercent change in initialluminousfluxateachmeasurementinterval.
LM‐80‐08 is an accurate method for measuring thechange in LED luminous flux for an LED source over
time. Alterations to the LM‐80‐08 procedure havebeen incorporated into the LA‐QTM and LA‐ISM inorder to allow and simplify testing of LED‐based off‐
grid lighting products. These changes help distinguishbetween products that experience rapid lumendepreciation and those that are able to provide
thousands of hours of lighting service while reducingthetime,expense,andcomplexityoflifetimetesting.
LumenMaintenanceTestingProcedure
The LightingAfricaprogramspecifies that theoff‐gridlightingproductbecontinuouslyoperatedforaperiod
of500hours forLA‐ISMand2,000hours forLA‐QTM.
The lightoutput ismeasuredatregular intervalsusing
a fixed‐geometrymeasurement cavity under constantconditions.2 The light output is quantified in units oflumens (for luminous flux measurements) or lux (for
illuminancemeasurements),dependingonthetypeoftestingdeviceused.
Foreachlightoutputmeasurement,thefollowingmust
bereported:
• Relative light output (initial measurement dividedbythemeasurementataspecifictimeinterval)
• Operatingtimeofthelightsource• Ambientairtemperature• Currentandvoltagedeliveredtothelightsource
Measurementfrequency
The LA‐QTM requires at least 17 light outputmeasurements are conducted over the course of the
test. The LA‐ISM requires at least 6 light outputmeasurements.Theinitialmeasurementistakenaftera 20minutewarmup period in order for the light to
reach steady state – if for any reason the power isinterrupted during the warm up period, a new 20minute warm up period must be given. The initial
measurement should be repeated at least once tocheck for an erroneous measurement. The secondmeasurement should be made 24 hours later to
determine if the product is likely to experienceextremely rapid lumen depreciation. If the relative
luminous flux at the second measurement isapproximately 95% or less, then the measurementfrequencyneedstobeincreasedtocapturethetimeat
which L70 is reached. The LA‐QTM indicates that thenext five light outputmeasurements should be takenat 48‐hour intervals, which are then followed by
measurements at one‐week intervals. LA‐ISMrecommendsthatmeasurementsbeconductedat100‐hour intervals. Discretion must be exercised to
determine the appropriate measurement frequency,according to the resolution desired as well asavailabilityofequipmentandoperators.
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Orientationofthelightingproduct
Overtheperiodofcontinuousillumination,thedeviceunder test should be oriented as it is intended fortypical use, in order to replicate the heat transfer
occurringduringnormaloperation.
RecommendedTestingDevices
LightingAfricarecommendsthreedifferentdevicesformeasuring lumen maintenance of an off‐grid lightingproduct,inorderofdecreasingcostandcomplexity:
• Integratingsphere‐spectrometersystem• Self‐builtbox‐photometer
• Self‐builttube‐photometer
Below,isashortdescriptionofeachmethod,includingrecommendationsandadviceforproperapplication.
IntegratingSphere
An integrating sphere is a hollow sphere whoseinternal surface is a diffuse reflector that spatiallyintegratesradiantflux.3Whileanintegratingsphereis
the most complex and expensive option for
determining lumen maintenance, it is also the mostversatile and can be used for several otherphotometric measurements including: radiant and
luminous flux, laser power, reflectance andtransmittanceofmaterials.AtypicalintegratingsphereisshownbelowinFigure3.
Figure3Integratingsphere(http://www.optronik.de/plm‐is‐its10.shtml).
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Ambientconditions
The ambient air temperaturemust bemaintained at25 °C ± 3 °C, with minimal airflow. LEDs are
temperature sensitive and large temperaturefluctuationsorexcessiveairflowcanresultinerrorsinlumenmaintenancemeasurements.
PowerSupply
The device under test (DuT) battery is replaced by aDC power supply set to deliver constant voltage
(measuredatthedevice)equaltothenominalbatteryvoltage.Thisvoltagemustberegulatedtowithin±3% for the duration of the testing period and measured with an accuracy of ± 0.2% at each photometric measurement. The power supply should have aresolution of at least 0.01V and 0.001A, and becapableofremotevoltagesensing(usedwitha4‐wire
measurement technique) in order to compensate forresistanceandvoltagedropsinwiresandconnectionsto the lighting device. For power supplies without
remote sensing, the voltage can be measured veryclosetothelightingdeviceundertesttoconfirmthatthe voltage input is equal to the nominal battery
voltage.Paired,multi‐strandwireofthetypetypicallyusedforspeakers (18‐22AWGor0.258‐0.823mm2)is acceptable for connecting the lighting product to
thepowersupply–heaviergaugewire (>0.823mm2)ispreferred.
During testing, test personnel should be aware of
nominal drive current and voltage levels for DuT’s.Anydiscrepancybetweenexpectedandactualcurrentor voltage values should be noted, and the test
apparatus should be inspected for possible causes.Sincethetestingprovidesaconstantvoltage,andnota constant current, it is not usually possible to
regulate the power being delivered to the LEDsource(s). The DuT electronics regulate this power,and there can be significant drift in this power
regulation that can affect the light output of thedevice.
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An integrating sphere is advantageous for use in
lumen maintenance testing because it is lesssusceptiblethanothercavitiestomeasurementerrorassociated with changes in the location and
orientation of the light source within the cavity.Nonetheless, when conducting lumen maintenancemeasurements,thelightingproductshouldbelocated
inexactly thesamealignmentandorientationwithinthespheretominimizethepotentialforthiserror.
Thedrawbacksofanintegratingsphere,especiallyfor
users who do not specialize in photometry, includethe relatively high cost of a sphere and associatedcomponents, the need for frequent calibration,
relatively time‐intensivemeasurements, theneed forspecializedtrainingtooperatethesystemandanalyzethedata, and relatively large spatial requirements (a
1mdiameter integratingsphere,sufficient for testingmostoff‐gridlightingproducts,typicallyoccupies4m2offloorspace).
Accurate luminous fluxmeasurementsmadewith anintegrating sphere require calibration of the systemaccording toa ‘standard lamp’of known radiant flux
andcorrectionofthelumenmeasurementtoaccountforself‐absorptionofthebodyofthelightingproduct.Themethods used to calibrate and correct luminous
fluxmeasurementsmadewith an integrating spherearenotcoveredhere,astheyrequireamuchmorein‐depth explanation, which will be covered in a
forthcomingTechnicalNote.3
Box‐Photometer
A box‐photometer is an integrating photometeremployinganarbitrarilyshaped,hollowboxorcavitythatcanbeusedtocomparetheluminousfluxoflightsources of the same type. The box‐photometer isuseful and convenient for measuring the relativechangeinlightoutputofalightsource,asrequiredtoevaluate lumenmaintenance. Aninteriorviewandathreedimensionalrenderingofabox‐photometerareshowninFigure4.4
A typical low‐cost box‐photometer is a hollow box
constructedofwoodwiththeinnersurfacescoveredinwhitemattepaint.Multiplecoatsofpaintimprovetheuniformity and reflection of the walls. The finished
interior surface is intended to reflect lightasdiffuselyand homogeneously as possible. Caremust be takento apply the paint such that the surface texture is
smooth. Midway up one corner of the box is a portthat allows an illuminancemeter sensor to “see” intothe box. A baffle that is constructed of a thin, rigid
material(0.5cm‐thickplywoodisappropriate)isplacedin front of the sensor port so that light emitted from
the device under test cannot shine directly on thesensor.Thelightsourcetobemeasuredisplacedonastand that is located in the center of the box. When
closed, the box must not allow any ambient light toenter.RefertoAppendixAfordetailedplansforabox‐photometer that is suitable for lumen maintenance
testingoftypicaloff‐gridlightingproducts.
Relative light output measurements with a box‐photometer do not require calibration or device‐
specific corrections to deliver quick and accurateresults. Sincenocalibrationor correction is required,the light source is simply placed in the center of the
box and the illuminance measured by the lux/foot‐candle meter is recorded. It is extremely importantthat for each illuminance measurement, the light is
placed in the box in exactly the same location, withexactly the same orientation. Even the slightestadjustment in the location or orientation of the light
sourcebetweenmeasurementscanresultinsignificanterrorintherelativelightoutputmeasurements.
Figure4.Box‐photometer(left:Interiorview,right:3‐Dview)
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Two approaches can be taken when using the box‐photometer to conduct lumen maintenance
measurements. Theboxcanbededicatedtoasinglelight source, or the device can be used to measuremultiplelightsources.
Dedicatedbox‐photometer
A single product sample remains in the box for theentire durationof the test. Thebenefit of dedicatingthe box to a single light is that the product can be
fixed in the box, thereby reducing the possibility ofmeasurement error due to changes in alignment. Inaddition, ifa luxmeterwithdata loggingcapability is
used, then the illuminance can be continuouslymonitoredandrecordedwithminimaloperatoreffort.Care should be taken to check that the light source
does not build up heatwithin the cavity, as this canaffect both the source and the sensor.Most off‐gridproducts,however,willnotdissipateenoughpowerto
havesignificantthermaleffectsinthebox.
Box‐photometerformultiplesamples
Theboxisusedtomeasuremultiplelightingproductsthat are operated outside of the box between
measurements. Using the box‐photometer tointermittently measure multiple samples is morecommonthanthe“dedicated”approach,as itgreatly
increases the testing throughput of the box‐photometer.Inthiscase,thetestoperatormusttakeextreme care to place the light in exactly the same
location and orientation for each subsequentmeasurement. Several methods can be used toensure replicate product placement within the box,
including:• using a photo for reference that shows the
placementofthelight• usinga custom jigwithin thebox thatholds the
lightinexactlythesameorientation• markson theproductand test stand toassist in
placementandalignment
Measuringluminousfluxwithabox‐photometer
Luminousfluxisusuallymeasuredwithanintegratingsphere in accordancewith CIE 84‐1989. However, ifan integrating sphere is not available, a box‐
photometercanbeusedformeasuringluminousflux.Inpractice,however,measuring lumenoutputwithabox‐photometer is not recommended since it is
relatively cumbersome and inherently more errorprone than an integrating sphere‐spectrometersystem. Modern lighting laboratories generally rely
onintegratingspherestomakelumenmeasurementsonaccountoftheeaseofuse,measurementaccuracy,andminimalimpactofspatialirregularities.
Tube‐Photometer
A tube‐photometer is a simple, hand‐made deviceusedtomeasuretherelativechangeinilluminanceof
a light source over time. The apparatus consists oftubewithacapatoneend.Theendcapfitssnuglyonthe tube such that it holds the illuminance meter
sensorinafixedpositionandrestrictsstraylightfromenteringthetube.Lightemittedbyalampundertestis directed into the open end of the tube and the
illuminance incidenton the sensorhead ismeasuredbytheluxmeter. Adiagramofthetube‐photometerisshowninFigure5.
Figure5.Drawingofatube‐photometer,indicatingthebasicdevicecomponents.
Materialsforthetubeandendcapcanbeprocuredatvery low cost, often free of cost. Cardboard tubes
that are commonly used in rolls of fabric, paper andplastic sheets tend to be widely available andinexpensive, if not free. Alternatively, inexpensive
plasticpipecanbeused.Atypicaltubediameteris6
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cm and an appropriate length for the tube‐
photometer is 0.5 m, although these dimensions donotneedtobestrictlyadheredto.Ideally,theendcapismachinedfromarigidmaterial likewoodorplastic
that fits snugly into the tube and mates with thephotometer sensor such that the sensor is fixed inplace. Research has shown that if precisemachining
equipmentisnotavailable,anacceptableendcapcanbe simply constructed fromcardboard andpackagingtape.5 Refer to Appendix B for instructions for
constructingtheendcap.
Unlike the integrating sphere and box‐photometer,the luminous flux of a light source cannot bedetermined by the tube‐photometer. The tube‐
photometer is used solely for evaluating the lumendepreciation characteristics of a light source. Byplacingthedeviceundertestflushwiththeopenend
of the tube, the orientation anddistance of the lightsource from the sensor head can be replicated for eachilluminancereadingtakenthroughoutthelumen
maintenance test. As long as the same orientationbetweenthelightsourceandsensorismaintainedforeach reading, an accurate measurement of relative
changeinluminousfluxcanbeachieved. Similar to the box‐photometer, the tube‐photometercan be dedicated to measuring the lumenmaintenance of a single light source, or used to
measuremultipletestsamples.
Dedicatedtube‐photometer
Atube‐photometerisdedicatedtoasinglelightthatisfixedtotheopenendofthetube.Extremecaremust
betakentoaffixthetestproducttothetubesuchthatrepositioningdoesnotoccurbetweenmeasurements.
Tube‐photometerformultipletestsamples
Asingletube‐photometerisusedtomeasuremultiple
product samples that are not attached to the open
endofthetube.Rather,thetestoperatorholdsthelight source flush to the open end of the tube.Maintainingthe lightsourceflushwiththetube,the
operator makes slight adjustments to the anglebetweenthelightsourceandthetube,searchingforthe maximum illuminance measured by the light
meter.Thisapproachrequiresameterthathastheability to record and display the maximumilluminance reading. The test operator continues
“searching” until the maximum illuminance readingdisplayedon the lightmeterno longer increases foranyorientationof thedeviceunder test. Forhighly
directional light sources like small task lights andtorches (flashlights), all of the emitted light can beshined into the open end of the tube. Wider
distribution light sources (lanterns/ambient lights)mayrequiretheoperatortoselectaspecificregionin
whichtosearchforthemaximumilluminance.
When using this approach, the test operator mustsearch diligently for the maximum value, onlyrecordingthemax luxvaluewhenallorientationsof
the light source have been evaluated. The operatormustalsopreventstray light fromenteringthetubeby covering the light source and tube opening with
anopaque clothor by taking themeasurement in adarkroom.
Box‐andTube‐PhotometerReliability
An analysis of the box‐ and tube‐photometerconfirms that the devices can be used to accurately
measuretherelativechangeinlightoutputoftypicaloff‐grid lighting products.5 The study compares
relative light output measurements made with thebox and tube to those made with an integratingsphere. Results indicate that these methods are
acceptable substitutes for an integrating spherewhenconductinglumenmaintenancemeasurementsof light sources with varying physical shapes, sizes,
lumenoutputs,andspatiallightdistributions.
The analysis does warn that errors can result fromimproper use of the tube‐ and box‐photometers.Specifically, test operators must take care to avoid
changes in the light source location and orientationwithin the box. When testing multiple samples, theoperator must be aware of error due to false
identification of the maximum illuminance. Theseerrorscanbeavoidedbyrepeatingthemeasurementprocedure and training test operators in the proper
useoftheequipment.
Absolutevs.RelativeTesting
Relative testing measures the change in a measuredtest quantity. For lumen maintenance testing, the
procedures described herein measure the relativechangeinlightoutputofalightsourceoverasettimeperiod. Equipment calibrations are less stringent for
relativeteststhanforabsolutetests.
Absolute testing, as the name implies, measuresabsolute quantities (lumens, lux, etc. represent
quantitiesofvisiblelight).Absolutetestresultsrequirecalibrated instruments that have been properlychecked and adjusted for accuracy. Ultimately, a
properlycalibratedinstrumentwillhaveitscalibrationperformance ‘traceable’ to a proper standardmaintainedbyaprofessionalstandardsorganization.
For example, an illuminance meter may be out of
calibrationand ‘off’by5 lux.Absolutemeasurementswill thus be inaccurate. However, this same meter
maybeusedtomeasurethechangeinlux(asduringalumenmaintenancetest)aslongasitisstableandnochange has occurred to the meter during the test
period(so itmustremain5 lux ‘off’attheendofthetest, too). In thisway, relative testing can cancel theerrorincalibrationofthetestequipment.
Conclusion
Lumen maintenance is a critical performance metric
thatmustbemeasuredwhenevaluatingthequalityofLED‐based lighting products. The box‐ and tube‐photometeraresimpleandaffordabledevicesthatcan
be constructed and used by non‐specializedtechnicianstoaccuratelymeasurelumenmaintenance.Relative lumen maintenance measurements made
with the box and tube require less calibration thanabsolutemeasurements.Theaccuracymeasurementsmade with these devices, however, is highly
dependent on stable operating conditions as well asconsistentlocationandorientationofthelightsource.
References
1. For more information about the LED lumendepreciation and lifetime, refer to Issue 2 of
LightingAfricaBriefing Notes: LED LumenDepreciationandLifetime
2. Refer to the Lighting Africa Quality Test Method
(LA‐QTM)fora complete description of the Long‐Term LumenDegradationTest, available fordownloadat www.lightingafrica.org
3. Labsphere. (2008). A Guide to Integrating SphereTheory and Applications. Retrieved January 2011.http://www.labsphere.com/uploads/technicalguid
es/a‐guide‐to‐integrating‐sphere‐theory‐and‐applications.pdf
4. Basic instructions for construction of a box‐
photometer are included in Lighting AfricaQualityTestMethod(LA‐ QTM) available fordownloadatwww.lightingafrica.org
5. Carlsen, C. (2011). Analysis of Low‐cost testingmethods for LED lumen maintenance of off‐gridlightingproducts. Humboldt State Universityhttp://humboldt‐dspace.calstate.edu/handle/2148/766
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APPENDIXA:Box‐PhotometerConstructionLumenMaintenanceTestingOff‐gridLightingProducts
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APPENDIXA:Box‐PhotometerConstructionLumenMaintenanceTestingOff‐gridLightingProductsIssue7January2012
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TUBE‐PHOTOMETERENDCAP
Atube‐photometerendcapisrequiredtoholdthelightmetersensorinplaceandtorestrictambientlightfromenteringintothetubefromoneend.Theendcapispreferablymadeofarigidmaterial(typicallywood),butanequally
acceptableendcapcanbefashionedfromcardboardandadhesivetapeifprecisionwoodworkingtoolsarenotavailable.Alatheistypicallyrequiredtofabricateawoodendcap.Thecardboardendcapcanbecraftedusingautilityknifeandadhesivetape.Forbothtypesofendcap,greatcaremustbetakentocutthecaptotheprecise
measurementsofthelightmetersensordiameterandtubediametertoensuretightfits.Photosofawoodandcardboardendcapforthetube‐photometerareincludedbelowasareference.
APPENDIXB:Tube‐PhotometerConstructionLumenMaintenanceTestingOff‐gridLightingProducts
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