LED Terminology Reference Guide

LED Introduction

Contents

Standards page 4 Controls page 6Efficiency page 7Colour page 8Reliability page 9Hardware page 10

In any smart city design lighting plays a prominent role. At a time of significant technological change, the evolution of LED from an innovative to a mainstream and leading light source has impacted positively on energy use and carbon emissions.

LED sets new challenges for designers and users alike, introducing new legislation, terminology and sometimes misunderstanding.

At the heart of Thorn’s drive to help our customers understand these challenges we have introduced this guide to explain the common terms and their meaning.

LED Introduction

Standards

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

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

EN13032 A European standard defining how LED should be measured for electrical and photometric characteristics. Whilst there are parallels between LM79 and EN13032 there are also significant differences.

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.

IEC EN 62471 - Photobiological Safety (also PD IEC TR 62778) 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 are completely safe, RG2 is safe as long as nobody stares directly into the 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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Standards

Low Voltage Directive This demands that products are designed, manufactured and tested to give proof of electrical safety.

Electromagnetic Compatibility Directive This requires that products are designed and operate without interfering with or being interfered by other electrical products within defined limits.

RoHS Directive This places restrictions on the use of certain hazardous substances in electrical and electronic equipment.

Energy Related Products Directive The aim of this directive is to reduce consumption of natural resources and energy, and to minimise the environmental impact of a products across the whole of their life cycle.

CE Mark The CE mark shows that a product conforms to the required European directives for placing on the European market. It helps customs and market inspectors in facilitating the free trade and movement of product within Europe. Displaying the CE mark signifies compliance to four main European directives.to four mainEuropean directives

TM21 A North American technical memorandum on projecting long term lumen maintenance of LED light sources. It defines the number of units required for testing and how far the results may be safely extended to predict long term values. The maximum extension allowed is 6x the real test period, so for a 50,000 hour prediction the real test must have been for 8,333 hours. For a 100,000 hour prediction the real test must have been for 16,666 hours. (Generally accepted practice within standards is testing for 6,000 hours which according to TM21 only allows a prediction of up to 36,000 hours).

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ControlsBi-power Dimming (BP x) This is a method used by exterior lighting to reduce energy use during periods of low demand. The luminaire is automatically dimmed to 50% output around the middle of the night.

The dimming regime is indicated by the designation BPx where x is the number of hours the luminaire will be dimmed during the night. For example BP7 will dim the luminaire for 3 hours before the middle of the night and for 4 hours afterwards.

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

The middle of the night is automatically calculated as the midway point between switch-on and switch-off. For a luminaire fitted with a photocell to detect nightfall this automatically adjusts for the changing seasons throughout the calendar year.

CCR - Constant Current Reduction A method to dim LED. Unlike pulse width modulation the LED is constantly on and the drive current to 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 dimmer the LED appears.

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

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

Standards indicate that the quoted input power value should be within 10% 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 the efficacy value the more efficient the light source uses electricity. This may either be the:

lamp efficacy lm/W using lamp lumens, or luminaire efficacy L lm/W using lamp lumens multiplied by the luminaire light output ratio which indicates the losses within the luminaire.

Efficiency

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CCT - Colour Temperature LED may either be coloured, typically red/green/blue/amber, or white. However, similar to daylight, white can vary from a warm white with a higher red content to a cool white with a higher blue content. This is described by the colour temperature where 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 and 5000K

Ra - Colour Rendering Every light source is characterised according to how well it shows colours. This is described by the Ra number where the higher the number the more accurately colours are shown. For the majority of indoor workplaces a minimum of Ra80 is required. For general outdoor lighting Ra70 is more usual. Increasing the Ra number to 90 or above generally comes at the expense of a reduction in efficiency

Binning The LED manufacturing process is not accurate enough to ensure that every LED is identical. Therefore the LED are sorted according to specific characteristics such as colour, light output and electrical properties. This sorting is called binning.

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

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

Colour

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ReliabilityLED Driver (LED Control Gear) The reliability of a LED luminaire is also 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 life or is easily replaceable.Note that whilst the description LED Driver is commonly used the internationally recognised term is LED Control Gear

Ambient Temperature Ta When any testing is performed on a product it is for a defined surrounding air temperature. This is the ambient temperature and is designated as Ta. The standard Ta defined for testing is 25°C although testing at any other value is permissible as long 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 failures may be expected to decrease and the rated life may be expect to increase. However the size of these changes are technology dependant and there is no standard method to calculate these effects.

Lx - Rated life The time in hours during which the LED will produce more than a given percentage of their light output at new. x is the percentage of light output. e.g. L80 at 80,000 hours means that after 80,000 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 will have 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 the individual LED will have failed.

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

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

LED Driver A LED luminaire contains electronics to convert the mains electricity into the correct condition for the LED light source. This is called the driver.

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

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

LED Module One or more LED package with mechanical and optical components but no lamp cap

LED Lamp One or more LED package with mechanical and optical components and a lamp cap

LED Package A LED chip in a form 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 the spectrum (or rainbow) coloured LED can be similarly used to give the appearance of white light. The coloured LED blend on any surface they are lighting, giving white light.

Phosphor Conversion Blue LED are used to light a chemical phosphor, similar to that contained in a normal fluorescent tube, which converts the blue light into white light. This method will generally give superior colour rendering as the white light contains more colours within the spectrum.

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