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Light Emitting Diodes (Leds)

Introduction
Light emitting diodes (LEDs) are a recent lighting technology that is sometimes referred to as solid state lighting (SSL). It is a rapidly developing and advancing technology with significant changes in performance occurring almost monthly. LEDs have also introduced a whole new vocabulary to lighting that is more aligned with the world of electronics; this guide seeks to define some of these as well as provide the reader with a basic knowledge of solid state lighting.

How it works
A Light Emitting Diode (LED) is a semiconductor device that emits light when an electric current is applied in its forward direction.

An ordinary diode is one of the basic components that have been used in electronic circuits for over 100 years. A diode allows an electric current to pass in one direction only; in effect a sort of one way valve. Diodes are key components in circuits used to rectify alternating currents (ac) to direct currents (dc). Their size varies according to the voltage and current values involved.

In a LED, when the correct forward current is applied to it, a form of electroluminescence occurs whereby incoherent and narrow-spectrum light is emitted from the p-n junction in the solid state material.  In effect, electricity is passed through a chemical compound (crystal) that is excited and generates light. The chosen compounds can alter the wavelength (and hence the colour) of the light emitted. The first LED to emit visible light was demonstrated in 1962 and its colour was red.

How are LEDs made?
The basis, or heart, of a LED is a very small die, or chip, that is made by depositing semi-conducting materials onto a substrate, generally known as a wafer. A wafer is a disc that may be 100mm or 150mm in diameter and each will produce a large number of individual LED chips. This is an established, proven manufacturing process producing most of the world’s semi-conductors, integrated circuits and similar electronic components.

After testing and selection, for colour and intensity, each chip is assembled into a package that makes it a practical component to be incorporated into a circuit and used to make light. Packages vary in construction and size depending on the intended application and choice of materials.

Light produced by LEDs
The light produced by LED chips can, in effect, cover the whole visible light spectrum plus infra-red and ultra violet. The emitted colour is determined by the choice of semi-conductor materials and each chip emits light in a comparatively narrow spectrum.

White light, however, is produced generally by either using a blue LED to shine through a yellow phosphor, or a number of coloured LEDs (comprising at least red, green and blue (RGB)) are combined to provide this broad spectrum light. The former method is used where only white light is required, while the latter offers the opportunity to vary the colour produced by managing the output of the component colours.

Heat produced by LEDs
LEDs dissipate their input energy in both luminous flux and heat. The heat is generated at the P-N junction of the device and the temperature here is critical to the performance of the LED.

Tj – the junction temperature
The junction temperature of a LED is an important operating parameter that must be maintained at an optimum value. Variations in the Tj can alter the efficacy, output spectrum and life of the LED. The characteristics of an individual LED chip are defined at a set value of the Tj, which is usually tested momentarily at 25oC as it comes from the production process. This is, however, not a real world measurement because most LEDs will be operating at much higher temperatures once they are incorporated into a luminaire.

Efficacy
The performance of LEDs is in constant development and their efficacy is improving all the time. LED suppliers highlight their progress with regular press releases claiming the latest ‘on the bench’ R&D lumens per Watt they have achieved. In the first quarter of 2013 one supplier claimed to have exceeded 250 lm/W, for a white light LED operating ‘on the bench’. However, these claims should not be confused with the real world performance to be expected from production LED chips. Typically, warm white (2700K – 3000K) LED chips are achieving around 120 lm/W and more, with improvements occurring almost continuously.

Once a chip has been incorporated in a LED luminaire the overall system efficacy will be reduced by the inclusion of the power consumed by the control gear (sometimes called a ‘driver’), as well as any light absorbed by the optics. Further loss of efficacy will occur if the luminaire forces the LED chip to operate outside its optimum operating parameters, especially with regard to its junction temperature (Tj). Nevertheless, some highly efficient control gears, optics and heat management solutions mean that some general lighting LED luminaires have already achieved in excess of 100 lm/W system efficacy.

Life

Introduction
The life of any light source can be measured in two ways; it’s actual survival or to that point when the light output has fallen below a useful level. Conventional lamp life is expressed as the time taken for 50% of the lamps to have failed. This is done because most lamps do not decline in output significantly during their rated life. This is not true for a LED source.

LED life – actual and useful
An LED light source operating in ideal conditions has a very long life when compared to most previous lamp technologies. Typically, manufacturers are claiming a life of 50,000 hours plus, with some even predicting as much as 100,000 hours. Because LEDs offer such extreme life, one of the biggest challenges is how to achieve actual performance over life measurements i.e. ‘real life’ data. Testing LEDs for 24 hours a day, 365 days per year, would only provide 8,760 hours of data. So LEDs rated at 100,000 hours would require 11.5 years to produce ‘real life’ data. The current pace of technological development, and the need to commercialise LEDs, makes this approach impractical and unworkable.

Some manufacturers use a 6:1 ratio to ensure data can be gathered and extrapolated. So, 1,000 hours actual testing would provide the data for up to 6,000 hours and 2,000 hours actual for the 12,000 hours prediction.

The life of a solid state device like a LED also needs to be expressed in a different way compared with conventional lamp lifetimes. The life of individual electronic components usually conforms to a ‘bathtub’ mortality curve; a few die in infancy whilst the vast majority operate for their rated life and then failures accelerate thereafter.

In the case of a LED the light output is known to reduce over life, so this light source does have some common ground with previous lamp types. However, the light output does decline to such an extent that it will not be useful, even though the LED will continue to operate. Depending on the application, the life of the device can be expressed as the time it takes to fall to a specific percentage of its original output.

Lumen Maintenance – and maintenance factors
Historically, a conventional lamp’s life has been based on its mortality curve, which refers to the percentage of the lamps that fail over time. The most common mortality rating for these lamps is based on the time at which 50 per cent of lamps will have failed - see graph.

Most conventional lamps retain useful light output well beyond their rated life, which is not true for LEDs. They experience a gradual reduction in light output during operation and generally do not fail catastrophically. As a result the lighting industry has sought to develop alternative methods of defining the life of LEDs.

The lifespan, and light output, of a LED depends on its operational and environmental temperature as well as the quality of its power supply. These factors also determine the rate of degradation of the light output. It is therefore possible to define the useful life of a LED by stating how long it will take to reach a certain percentage of its original light output. However, the individual LEDs will arrive at this point after different elapsed times. The industry is working on providing a standard measure for LED life and the draft of IEC62717 suggests two life parameters; a Median Useful Life (Lx) and an Abrupt Failure Value (AFV).

The Median Useful Life (MUL) is the time for 50% of a batch of products to fade to x% of its original light output, and the AFV is the percentage of LED light sources, or luminaires, of the same type that no longer provide any light at the MUL. This might be expressed as L70 = 50,000 ABF = 6%. These are comparatively recent life definitions and all manufacturers may not have adopted them yet; it is important, therefore, to make sure that the life they do quote is fully explained.

Further reading on LED life can be found in the LIA document TS01 and the IES publications TM21, LM79 and LM80.

LED packages
A LED package may comprise just one chip or many and as a result there are many sizes, shapes and configurations available to the manufacturer of luminaires.

Beam Angle (Optics)
The package also provides protection for the semi-conductor crystal and also dictates the directionality of the light output. Usually this is constructed so that the light radiates in a near hemi-sphere of almost 180 degrees (the current maximum is about 160 degrees). Guidance of the light is thus easier than in filament or discharge lamps, which generally radiate light in all directions. There are various types of housings for LEDs of low, medium and high performance; whatever the choice, they should all give good mechanical stability.

The LED package may incorporate an optical component that helps to control the light output from the chip, whilst also offering mechanical protection.

LED modules
A LED package needs to be supported by additional components in order to manage its power and heat dissipation requirements. A number of LED packages may need to be brought together to form a single LED module, which is sometimes referred to as a LED light engine although this term is not used in International Standards (IEC).

The other essential component within a LED module is the provision of a suitable connection to its supply. This may also provide good thermal conduction to allow the generated heat to be taken away from the chip. This adds to the challenge of module construction, particularly with regard to heat density and the management the Tj of several chips.

Creating more diffuse light
The initial focus of LED development was based on the fact that the chips provided a very small, very intense light. This light source was suited to the creation of directional luminaires because an almost point source of light may be controlled very accurately. As the technology has developed it has become possible to increase the size of the light source by using many chips over a wider area. Delivering a more diffuse light from multi chip solutions has been done by using novel optics and the phenomenon of internal reflection as well as refraction.

White LEDs

Introduction
The majority of White LEDs used in general lighting applications are those using the combination of blue LEDs and phosphor coatings. This section looks at these types and considers their colour rendering properties, correlated colour temperature as well as the way they are constructed.

Using phosphors to produce white light
For many years phosphors have been used to make white light fluorescent lamps. Over time these phosphors have become more complex as ever better colour rendering has been sought; co-incidentally the lamps have also become more efficient. Using phosphors to produce white light is therefore well known to the lighting industry. The major challenge, in general lighting use, is to produce a broad spectrum of light that does not over visibly emphasise any one particular colour.

With LEDs the use of phosphors is simplified because the original light source is visible blue light so the addition of a carefully chosen yellow phosphor will produce white light. It is for this reason that most white light LEDs appear to be yellow when switched OFF.

The combination of blue LEDs with suitable yellow phosphors can be done in different ways; the phosphor can be deposited directly onto the LED chip or onto a separate remote plate.

Phosphor coated dies
A phosphor coating can be deposited directly onto a blue LED die to produce white light. The quality of the consequent white light emitted depends on:

  • The quality of the phosphor
  • The consistency of the coating
  • The thickness of the coating

These factors also influence efficacy, which is why cool white LEDs are more efficient than warm white ones. The latter have a thicker layer of phosphor which reduces the light output.

Remote phosphor LEDs
An alternative approach uses a number of blue LEDs (sometimes known as ‘pumps’) to shine through a remote plate coated in an appropriate phosphor. This approach produces a lower glare light source when compared with individual LEDs. Additional benefits include better colour consistency over life and improved efficacy. The preferred LED colour choice for these applications is often referred to as Royal Blue.

The construction of a remote phosphor device comprises an array of LEDs enclosed within a mixing chamber that passes its light through the coated plate or substrate. These assemblies are often produced as LED modules which also may be designed to conform to an agreed standard physical package that allows the future replacement of this element within a LED luminaire.

Colour rendering of White LEDs
White LED light can be created by using either phosphor coated dies or a red, green, blue (RGB) combination. The latter may even be supplemented by a fourth colour, usually yellow or amber. The spectral power distributions of these two methods are very different, which means that the use of a calculated (measured) CRI ranking may not match a visual ranking. The current CRI calculation method was introduced in 1974 but there have been several research papers that have questioned it suitability for all types of light sources.  

A recent review by the CIE (Commission Internationale d’Eclairage) Technical Committee (Ref: CIE 177:2007) concluded that a supplementary method is required to better describe the performance of white LEDs. In the meantime it is worth noting that there are warm white LED sources with CRIs up to Ra98. Almost all white LEDs designed for general illumination are now exceeding Ra80. While the development of new metrics is being considered it is recommended that white LEDs are also subjected to visual assessments in the intended application if there is any doubt about the claimed CRI.

Colour consistency
Due to very small variations in manufacturing conditions individual LEDs will have different CRIs. The chips are therefore tested and selected, or binned, for colour consistency across a range of values. This permits customers to choose their LEDs for consistency of appearance and quality. Bins might be defined by one or two Macadam ellipses for high quality, professional applications; 3 -  4 ellipses for commercial use and even wider values where the consistency is less critical. See also EU Directive 1194/2012.  

Correlated colour temperature (CCT)
White LEDs can be supplied in a range of CCT values. Early white LEDs were often offered in a cool white with a CCT in the order of 5000K; this was largely driven by the fact that such LEDs were more efficient because less phosphor was required. It is still the case that a cool white LED is more efficient than a warm white version. 

Note on spectral power distribution (SPD) graphs
Most SPD graphs reproduced in colour show both the wavelength and a representation of the visual colour spectrum. Always check that the stated wavelength values correctly match the colour above them. In particular 550nm should be aligned with a nice bright green.

Coloured LEDs
Figure 3 has already shown that LEDs are available in a wide range of colours. These can be combined to extend the hues available to the lighting designer. Coloured LEDs can also be combined with white LEDs to alter the CCT, or to produce special effects related to the expected colour appearance when (e.g.) dimming a luminaire.

Coloured LEDs have a range of efficacies according to their colour but are much more efficient than using coloured filters with conventional white light sources.

Coloured LEDs are used in architectural lighting schemes and applications where particular wavelengths of light are beneficial. They are increasingly used in signalling applications including traffic lights.

Photobiology

Introduction
Light plays a very important role in our wellbeing and health but it can also be harmful if used incorrectly. Sunshine is good for us but if we expose ourselves to it for too long it can cause sunburn as well as more lasting harm to our skin. The same balance needs to be maintained when we are dealing with artificial light. In LEDs the current concerns relate to the use of blue light, which effectively covers most of the white sources available.

Blue light
The invention of the blue LED was the key to creating white light by using phosphors. This does mean that there can be a ‘blue’ peak in a white LED spectrum and it is therefore worth taking this into account for certain minority health groups. On balance blue light is more generally beneficial and it should always be remembered that it is present in sunlight. Lighting LEDs do not, however, produce heat on the skin, i.e. there is no forward infra-red radiation.     

A risk assessment of the blue light component of a white LED is, therefore, only necessary where there are likely to be subjects / patients with a known sensitivity condition, or the type of light involves illuminating a part of the body from a distance less than 20cm for several hours at a time. In the latter case it might involve an illuminated inspection lamp and the risk might be mitigated by selecting LEDs with reduced power in the blue area of the spectrum.

It is very important to note that these effects are limited to a very small number of people. The EU Directive 2006/25/EC addresses the issue of risks arising from optical radiation and the Non-binding guide to good practice for implementing Directive 2006/25/EC provides much useful additional reading. Much of this work covers the use of lasers, which should not be confused with LED lighting. Useful further reading on this subject can be found  by following these links: (Link: Human Centric Lighting: Beyond Energy Efficiency – Lighting Europe, July 2013; and Photobiological Safety in Lighting Products for use in Working Places – Lighting Europe, February 2013)

Powering LEDs
The LED itself is an electronic component, which will require a power supply to provide the correct voltage and current to suit its characteristics. The vast majority of LED chips and packages are very low voltage dc devices, although there are some that can be connected directly to the ac mains. The voltage and current required will determine the type of power supply, as will the need to control the light output. The performance and quality of an LED’s power supply will have a critical effect on its life, efficacy and light output.

Power supplies for LEDs are called ‘control gear’ (sometimes referred to as ‘drivers’) and these perform a similar function to their namesakes used with fluorescent and other discharge lighting sources. This equipment can be designed to power anything from a single LED to many hundreds. It is important to control the forward current to ensure the optimum performance is obtained.

Many current LEDs are being over-driven to maximise the lumen output rather than achieve their best efficacy. In effect the potential ‘lumens per Watt’ performance is compromised in favour of producing more lumens from a LED. It is for this reason that dimming some LEDs may increase their efficacy at certain lower outputs. (See ‘Controlling LEDs.)

Technical issues

Orientation
LEDs can be used in any orientation because they are solid state devices, have no moving parts and are not filled with gasses.

Run-up time
A LED will turn ON instantly at full brightness, or even at a pre-determined dimmed level. There is no significant, visible, warm-up period and no need to start at 100% output prior to dimming.

Re-strike time
There are no restrictions on either a ‘hot’ or ‘cold’ re-strike for a LED; it may be turned ON at any time and will produce instant light.

Supply voltage
The vast majority of LEDs operate at extra low voltage and require a direct current supply. As a result any fluctuations in the 230V ac mains supply will only affect the control gear powering the LED. Well designed control gear will isolate the LED from any such mains fluctuations and protect it from spikes and other disturbances.

Managing heat in LEDs
Maintaining a reasonable junction temperature in an LED is important for reasons of performance and life. Heat must be extracted from the LED as quickly and efficiently as possible so that the Tj does not exceed its design level. This means that there must be a very good thermal path from the LED to the luminaire and out to the surrounding air. It is for this reason that most LED luminaires have obvious heatsinks, as well as clear advice about their location and local ambient temperature requirements.

In circumstances where it is not possible to optimise the heatsink (e.g. in some replacement lamp solutions, or when using replaceable LED modules) the stated life may be lower.

Controlling LEDs

Introduction
LEDs are very well suited to being controlled because they can be turned ON instantly, and their output can be varied readily from 0% - 100% with the correct control gear circuits. Neither the switching cycle nor the dimming process has any impact on an LED’s life; further demonstrating their suitability for being controlled. However, dimming and switching may affect the life of control gear, depending on its design and components.

Dimming
The available techniques for dimming LEDs are determined by whether it is a LED lamp (i.e. an integrated LED and control gear intended to replace an existing conventional 230V lamp) or a LED luminaire.  A LED lamp is only connected to the mains supply and can only be dimmed by altering that supply using either phase-cut or sine wave dimming. On the other hand a LED luminaire, which will have an associated control gear, can be dimmed electronically; either by constant current reduction or pulse width modulation (PWM).

Controlling LED Lamps
When existing, conventional, 230V ac lamps (e.g. incandescent GLS or tungsten halogen spots) are replaced by LED lamps and use the existing lamp holders, control options may be limited. Switching ON and OFF will always be possible but dimming may not be. The compatibility (or otherwise) between a phase cut dimmer and the LED lamps is dependent on a number factors. Many dimmers rely on the load to be above a minimum value, which is often higher than the total load presented by the new LED lamps. Other compatibility issues relate to whether the dimmer is a trailing or leading edge design, which might be a legacy of the original load being either magnetic or electronic.

Before carrying out a major LED lamp replacement programme it is well worth while carrying out tests to see if the LEDs are compatible with the existing controls. Incompatibility can be shown by the LEDs flickering, the dimming range being limited or even the generation of audible noise.

Controlling LED luminaires
Although it is possible that a new LED luminaire might be used to replace a luminaire that is already controlled by a dimmer it is more likely that it will be installed with appropriate new controls. A new LED luminaire will be supplied with control gear, which will be either ‘switching only’ or fully dimmable. If it is the latter then the dimming control commands may be analogue or digital; full information will be provided on the control gear.

Dimming control commands
A dimming control gear may be controlled by 1-10V dc, DALI, DSI or DMX signals; indeed some may offer the option of all three. There are also control gears that can be directly connected to a local area network (LAN) and able to respond to TCP/IP instructions. This will be the case when using ‘Power over Ethernet’ systems or wi-fi based controls. Whatever the command protocol used, the control gear will usually offer a full dimming response from 0% - 100%.

Dimming operation
The light output from a LED is a function of the forward current passing through the device. It is therefore possible to dim a LED by reducing the forward current. However, many LEDs will experience significant colour shift at lower forward current levels. The forward current level also affects the Tj and the efficacy of the device. This is the analogue approach to dimming LEDs.

An alternative approach to reducing the apparent light output is to use pulse width modulation (PWM), where the LED is – in effect – turned ON and OFF very rapidly. The duty cycle (ON period) then dictates the apparent brightness of the LED.

In order to avoid any possible flicker problems – either visible or invisible – the frequency of the PWM used must exceed 3000Hz. The benefit of using PWM is that the colour appearance of the LED is constant and the dimming curve is virtually linear.

If colour consistency is important in a given application then the supplier of the LED fixture should be consulted about the most appropriate control method.

Additional comment re flicker
Another source of flicker is the 50Hz mains frequency, which may not be entirely eliminated by the LED’s control gear. If the sinusoidal ripple is not sufficiently damped then it is possible for strobe effects to occur and/or perceptible flicker. Partial damping may still give rise to an imperceptible flicker that is still registered by the brain.

Advantages
LED lighting is proving to be a highly versatile and very efficient new light source. It offers the benefits of:

  • A very long service life
  • Instant start and re-start
  • Physically robust and largely shock proof
  • Reduced environmental impact ; both in manufacture and use
  • Wide choice of colours
  • Good colour rendering from high quality white LEDs
  • Highly controllable; switching cycles have little or no impact on device life
  • Very compact sources allowing highly efficient optics and less wasted light (spill)
  • No forward UV or IR possible, unless specifically required.

Initially offered as single dies (or chips) providing an almost point source of light, the LED has rapidly evolved from a device thought to be only appropriate to directional and effect (coloured) lighting. LEDs are now available in a wide range of form factors and are challenging virtually all the lighting applications covered by other lamp technologies. The only barrier to wider adoption is the higher original cost of most LED luminaires when compared to established solutions.

Disadvantages
Aside from the question of cost there are very few real disadvantages in using LED sources. There are stories of LED traffic lights generating insufficient heat to clear away obscuring snow but this can be overcome by design. The intensity of an individual high power LED can be uncomfortable but proper optical design and glare control can be applied. Their use in high ambient operating temperatures can be a problem and here more conventional light source may hold their own for longer.

Standards and other relevant publications
There are a number of standards and other publications that are relevant to the production, specification and use of LEDs. Many of these are those that have been applied to conventional lighting products but there are several that have been developed specifically for LEDs.

Current standards and guidance
Please refer to LIA Technical Statement TS01, which is kept up to date.

WEEE and RoHS
LEDs are electronic components and are subject to legislation regarding the use of hazardous substances in manufacture as well as disposal at the end of life.

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