Calculating Light Loss Factors for LED Street Lighting Systems

Rick Kauffman Kauffman Consulting, LLC

Contents

Why worry about LLF?Factors Affecting Light LossDiscussion of Major FactorsRecommended ProcedureMinimum RequirementsReferencesQ&A

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Global Deaths Caused by Injuries for 2008

3Source: World Health Organization

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Red is highest number

Blue is lowest number

Vehicle occupant deaths 1987-2003, FARS

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Red is highest number

Blue is lowest number

Pedestrian deaths 1987-2003, FARS

Why use LLF?

Luminaires age, and unlike fine wine they dont get better with time

RP-8 & AASHTO specify Minimum Maintained Average lighting levels

SafetySecurityCommercial interestsCommunity pride

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Factors Affecting LLF in SSL LuminairesLLF = LLD x LDD x LATF x HE x VF x BF x LCD

Maintenance Factors Equipment Factors

Lamp (Source) lumen depreciation LLD

Luminaire dirt depreciation LDD

System burnout or failureChanges in local conditions

Luminaire ambient temperature effects LATF

Luminaire heat extraction -- HEVoltage (Power, Transients) -- VFBallast (PSU) & Lamp (LED)

Factors -- BFLuminaire component

depreciation -- LCD8

Lumen Maintenance of Common Light Sources

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SOURCE HPS PSMH CMH INDUCTION

RATED LIFE (HRS) 24,000

15,000-20,000

15,000-20,000 100,000

LUMEN MAINTENANCE 75-80% 40-59% 70% 60%

MORTALITY RATE 35% 55% 50% 40%

LED Lumen Depreciation - LLDLumen Maintenance is a function of

Tj and Ta within the LED package.

L70 is commonly used to define the useful life of an LED system,use LLD= 0.7, OR

Define useful system life and determine LLD.

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LM-80 & TM-21 data relate to the LED package only but are useful.

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LM-80 Results for Cree XP-G

Note: TM-21 only projects life to 6x test period or 60,000 hours max., anything past that is based on trust.

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Lumen Maintenance Methods1. Define system life, calculate Lxx based on test

data

2. Use manufacturers recommended data

3. Use L70 as default at rated life

Luminaire Dirt Depreciation - LDD

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Luminaire CleaningGet recommended cleaning procedures from luminaire

manufacturers to get them to like new condition.Evaluate location types as to degree of depreciation.

i.e. residential, commercial, industrial, construction, etc.Perform an economic analysis by type of location to

determine optimum cleaning frequency.Establish cleaning schedules by location based on

conditions and economic analysis.Do periodic audits to verify the procedure is working.Caution: LED luminaires with exposed secondary optics

will experience light distribution pattern changes as theyget dirty, and may require special cleaning.

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LDD Field Determination

Remove existing luminaire in retrofit area of known length of service since last cleaning. Test, clean, and re-test to establish % LDD.

Testing can be performed by a lab or by using a homemade black box and light meter to establish relative difference.

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Changes in Local ConditionsPavement condition and future street pavingConstructionTrees and vegetationChanges in adjacent land useAtmospheric conditions, smog index, etc.

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LDD ~ 3-5% per year over 1-2 year period. More studies needed. Only 2 sites, very limited data & length of service.

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Luminaire Ambient Temperature Effects - LATFLM-79 and LM-80 testing performed in 25oC ambient

~0.224% per CLight reduction

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Drive Current vs. Forward Voltage

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Ambient Temperature Effects

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Luminaire Thermal Capacitance and T Effects

As Tj increases, Vf decreases and input power decreases

Internal and external ambient changes can affect Tj and Vf

Values in this chart are not significant. This data is only given to show that a relationship exists.

Courtesy of Mike Stevens, Georgia Power

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Small decrease in illumination (E) over 2-month period.One location had low output due to failing power supply.

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Ambient Temperature Effects

April avg. = 14CJune avg. = 26C T = 12 C E ~ 3%

www.weather.com24

Voltage (Power, Transient) EffectsMost PSUs operate between 120V and 277V, however

efficiency varies with input voltage, get the facts!480V is a more expensive option, 480V PSUs are more

expensive and less reliable. Step down transformers add cost and reduce system efficiency.

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Ballast (PSU) and Lamp (LED) FactorsGenerally this is 0.95 for HID systems and often assumed

to be 1.0Most drivers are constant current DC to the LEDSome drivers supply AC to the LED to increase system

efficiency (beware of flicker)Some drivers have fixed drive currents and some are

adjustableDimming circuits vary in design and operationManufacturing tolerances varyGet the variables from the manufacturer and determine

if they are significant for your environment

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LED Driver De-rating Curve

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Example LED Driver Efficiency vs. Load

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LED Driver Dimming Curve

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LED Driver Power Factor

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LED Driver Life vs. Tcase

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Luminaire Component Depreciation - LCDEnvironmental effects such as (acid) rain, exhaust fumes

(hydrocarbons), UV exposure, humidity, vibration, salt spray from ice and snow treatments, coastal conditions, etc. affect the surfaces of optical components, conductors, fasteners, etc. (DHMO is the worst)

Thermal cycling and aging affects plastic materials, rubber gaskets and wire insulation.

Require manufacturers to furnish test results to quantify these and determine if it is significant for the given system.

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LLF Calculation Procedure

Use the LLF checklist provided or develop your own.Determine if factors are significant or not. Document,

be prepared to defend your reasoning.All factors are 1.0. (The only exception to this is if

your maximum operating external ambient temperature is significantly below 25C or you have consistently heavy winds.)

Multiply them together to get LLF.Ex. LLF = LLD x LDD x LATF x LCD

Use this LLF in your lighting calculations and planning.

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Recommended Default Depreciation

LLD = 0.70, for L70 lifetime ratingLDD = 0.9, for very clean environments

LLF = 0.70 x 0.9 = 0.63

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ReferencesWorld Health OrganizationNational Highway Transportation Safety AdministrationRoadway Lighting Update-Research, Carl Andersen, FHWA

SALC 2007ANSI/IES RP-8-2000, American National Standard

Practice for Roadway LightingIESNA Lighting Handbook, 10th ed.Philips Lamp Data SheetsCree Data SheetsLEDs in the Real World, Bruce Kinzey, PNNL, DOE SSL

R&D Workshop 2010, Raleigh, NC

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Rick KauffmanKauffman Consulting, LLC

rick_kauffman@hotmail.comTel. (901) 219-6979

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Calculating Light Loss Factors for LED Street Lighting SystemsContentsGlobal Deaths Caused by Injuries for 2008Why use LLF?Factors Affecting LLF in SSL LuminairesLumen Maintenance of Common Light SourcesLED Lumen Depreciation -LLDGuide to Evaluating LED Lumen MaintenanceLM-80 Results for Cree XP-GLumen Maintenance MethodsLuminaire Dirt Depreciation -LDDLuminaire CleaningLDD Field DeterminationChanges in Local ConditionsSummary of Lab LDD MeasurementsLuminaire Ambient Temperature Effects -LATFDrive Current vs. Forward VoltageAmbient Temperature EffectsLuminaire Thermal Capacitance and T EffectsI-35 Illumination ReadingsAmbient Temperature EffectsVoltage (Power, Transient) EffectsBallast (PSU) and Lamp (LED) FactorsLED Driver De-rating CurveExample LED Driver Efficiency vs. LoadLED Driver Dimming CurveLED Driver Power FactorLED Driver Life vs. TcaseLuminaire Component Depreciation -LCDLLF Calculation ProcedureRecommended Default DepreciationReferences