Led Terminology Reference Guide THORN

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LEDTerminology

ReferenceGuide





Contents

Standards page 4Controls page 6Efficiency page 7

Colour page 8Reliability page 9Hardware page 10

In any smart city design lighting plays a prominentrole. At a time of significant technological change,the evolution of LED from an innovative to amainstream and leading light source has impacted

positively on energy use and carbon emissions.LED sets new challenges for designers and usersalike, introducing new legislation, terminologyand sometimes misunderstanding.

At the heart of Thorn’s drive to help our customersunderstand these challenges we have introducedthis guide to explain the common terms andtheir meaning.

LED Introduction



Standards

LM79 A North American standard defining how LED should bemeasured for electrical and photometric characteristics.

LM80 A North American standard defining how LED should bemeasured for lumen maintenance characteristics.

EN13032 A European standard defining how LED should bemeasured for electrical and photometric characteristics. Whilstthere are parallels between LM79 and EN13032 there are alsosignificant differences.

The main hazard from LED is blue light which can cause damage

to the retina. Under normal lighting conditions almost all LED areRG0 or RG1.

IEC EN 62471 - Photobiological Safety (also PD IEC TR62778) Light is a form of energy and is capable of causing

physical harm. All light sources are classified by RISK GROUP(RG) which indicates how safe the source is. RG0 and RG1 arecompletely safe, RG2 is safe as long as nobody stares directly intothe light source (glancing briefly at it is safe), no light source for

general use should be RG3 (although technically the sun is RG3).The main hazard from LED is blue light which can cause damage

to the retina. Under normal lighting conditions almost all LED are

RG0 or RG1.

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Low Voltage DirectiveThis demands that products

are designed, manufacturedand tested to give proof ofelectrical safety.

ElectromagneticCompatibility Directive This requires that products aredesigned and operate withoutinterfering with or beinginterfered by other electricalproducts within defined limits.

RoHS Directive Thisplaces restrictions on theuse of certain hazardoussubstances in electrical andelectronic equipment.

Energy Related ProductsDirective The aim of this

directive is to reduceconsumption of naturalresources and energy, andto minimise the environmentalimpact of a products acrossthe whole of their life cycle.

CE Mark The CE mark showsthat a product conforms to therequired European directivesfor placing on the Europeanmarket. It helps customs andmarket inspectors in facilitatingthe free trade and movementof product within Europe.Displaying the CE mark signifies

compliance to four mainEuropean directives.to fourmainEuropean directives

TM21 A North American technical memorandum on projectinglong term lumen maintenance of LED light sources. It defines thenumber of units required for testing and how far the results maybe safely extended to predict long term values. The maximumextension allowed is 6x the real test period, so for a 50,000 hourprediction the real test must have been for 8,333 hours. For a

100,000 hour prediction the real test must have been for 16,666hours. (Generally accepted practice within standards is testing for6,000 hours which according to TM21 only allows a prediction of upto 36,000 hours).

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Controls

Bi-power Dimming (BP x) This is a method used by exteriorlighting to reduce energy use during periods of low demand. Theluminaire is automatically dimmed to 50% output around the middleof the night.

The dimming regime is indicated by the designation BPx wherex is the number of hours the luminaire will be dimmed during the

night. For example BP7 will dim the luminaire for 3 hours before themiddle of the night and for 4 hours afterwards.

For cases where dimming is to a level other than 50% this may beshown as BP x-y. For example BP 7-25 would indicate the luminaireis dimmed for 7 hours by 25% (i.e. running at 75% power).

The middle of the night is automatically calculated as the midwaypoint between switch-on and switch-off. For a luminaire fitted with

a photocell to detect nightfall this automatically adjusts for thechanging seasons throughout the calendar year.

CCR - Constant Current Reduction A method to dim LED. Unlikepulse width modulation the LED is constantly on and the drive currentto the LED is reduced to lower the brightness of the LED.

PWM - Pulse Width Modulation A method to dim LED.The LED is turned on and off at high frequency, invisible to the eye.The larger the amount of time off compared with on the dimmerthe LED appears.

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Lumen Output The lumen output is (approximately) the amount oflight produced by a product. This may be either related to the lampor the luminaire. The lumen output of a luminaire may be consideredto be the lumen output of the lamp reduced by the losses within theluminaire (the light output ratio). The luminaire losses are relatedto optical materials used (diffusers, reflectors, prisms) and also toany thermal losses due to the ambient temperature internal to the

luminaire.

Input Power The input power can vary considerably dependingupon the LED driver and the LED construction and configuration.

Standards indicate that the quoted input power value should be within10% of the worst case situation (e.g. maximum input power).

Efficacy How efficiently a light source converts electricity into light

is described by efficacy, with units lumens per Watt. The higher theefficacy value the more efficient the light source uses electricity. Thismay either be the:

lamp efficacy lm/W using lamp lumens, or luminaire efficacyL lm/W using lamp lumens multiplied by the luminaire light outputratio which indicates the losses within the luminaire.

Efficiency

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CCT - Colour Temperature LED may either be coloured, typicallyred/green/blue/amber, or white. However, similar to daylight, whitecan vary from a warm white with a higher red content to a cool whitewith a higher blue content. This is described by the colour temperaturewhere warm white has a lower value (in Kelvin) than cool white.Typically warm white could be considered as 3000K or less, cool

white as 5000K or above, and intermediate as between 3000K and5000K

Ra - Colour Rendering Every light source is characterisedaccording to how well it shows colours. This is described by the Ranumber where the higher the number the more accurately colours areshown. For the majority of indoor workplaces a minimum of Ra80 is

required. For general outdoor lighting Ra70 is more usual. Increasingthe Ra number to 90 or above generally comes at the expense of areduction in efficiency

Binning The LED manufacturing process is not accurate enoughto ensure that every LED is identical. Therefore the LED are sortedaccording to specific characteristics such as colour, light output and

electrical properties. This sorting is called binning.

MacAdam Elipse One MacAdam elipse contains the variation incolour possible without this variation being visible.

Colour Shift As white LED age the colour temperature will slowlychange. The colour shift is normally given as the maximum number ofMacAdam elipse the colour will change by.

Colour

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Reliability

LED Driver (LED Control Gear) The reliability of a LED luminaire isalso dependant upon the driver. If this fails the luminaire will also fail.Therefore it is important to make sure the driver has a suitable rated lifeor is easily replaceable.

Note that whilst the description LED Driver is commonly used the internationally recognised termis LED Control Gear

Ambient Temperature Ta When any testing is performed on aproduct it is for a defined surrounding air temperature. This is theambient temperature and is designated as Ta. The standard Ta definedfor testing is 25°C although testing at any other value is permissible aslong as the temperature is declared.

Dimming and temperature When dimming is used or products are

used at a lower ambient temperature than normal the number of failuresmay be expected to decrease and the rated life may be expect toincrease. However the size of these changes are technology dependantand there is no standard method to calculate these effects.

Lx - Rated life The time in hours during which the LED will producemore than a given percentage of their light output at new. x is thepercentage of light output. e.g. L80 at 80,000 hours means that after80,000 hours the light output from the LED will have dropped to 80%of the light output when new

Fy - failure fraction The percentage of individual LED which willhave failed within a given time in hours. y is the percentage of failures.e.g. F10 at 50,000 hours means that after 50,000 hours 10% of theindividual LED will have failed.

Fy is a combination of two components, By and Cy By - describing gradual loss in light outputCy - describing total loss in light output

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Hardware

Junction Temperature Inside an LED chip is a junction between twomaterials, one positively charged and one negatively charged. Lightis emitted from this junctions by the exchange of electrons betweenthe two materials, and as a side-effect heat is also generated at thejunction. The temperature of the junction needs to be controlled toensure that the light output and LED lifetime fulfill the requirements for

a given application.

LED Driver A LED luminaire contains electronics to convert the mainselectricity into the correct condition for the LED light source. This iscalled the driver.

Drive Current This is the electrical current supplied through thedriver to operate the LED. Over-driving the LED (giving a higher thanrecommended current) will result in high light output but a reducedlifetime. The drive current should provide the correct balance to ensurethe performance and lifetime claims are met.

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LED Luminaire Luminairecontaining an LED light source,either a LED module or a LED lamp

LED Module One or more LEDpackage with mechanical andoptical components but no

lamp cap

LED Lamp One or more LEDpackage with mechanical andoptical components and alamp cap

LED Package A LED chip in aform allowing simple electrical

connection

White Light For LED white light is generated in one or two ways:

RGB or RGBY As white light is a combination of the colours in thespectrum (or rainbow) coloured LED can be similarly used to give theappearance of white light. The coloured LED blend on any surface theyare lighting, giving white light.

Phosphor Conversion Blue LED are used to light a chemicalphosphor, similar to that contained in a normal fluorescent tube, whichconverts the blue light into white light. This method will generally givesuperior colour rendering as the white light contains more colourswithin the spectrum.

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Led Terminology Reference Guide THORN