The lighting industry today is an incredibly dynamic and exciting place to be. Why? Because of the dramatic changes forced by increasingly tough energy saving and climate change legislation, but also by fast developing new lighting technologies. WireIN Editor James Hunt looks at the latest developments in LEDs for general lighting.

By far the most important lighting development of recent times is LEDs for general lighting. As a result, we are now experiencing a real lighting revolution – it is not often that we say that with real conviction, but this is one of those occasions.

Energy saving and climate change are part of the increasingly urgent drive to reduce energy consumption and our reliance on fossil fuel reserves, and to save money. With lighting representing up to 25% of a household’s electricity consumption, it contributes greatly to overall energy consumption and CO₂ emission.

In many parts of the world, the highly inefficient incandescent (GLS) lamp bulb is now effectively banned. The main modern energy saving lamp types that replace incandescents are linear fluorescent tubes, compact fluorescent lamps (CFLs), High intensity discharge (HID) lamps and LEDs. It is the latter that are providing a real revolution in general lighting.

LED manufacturing

LEDs are essentially microchips made not from silicone but from crystals derived from combinations of inorganic substances. They are ‘cooked’ as wafers in a reactor under high pressures and temperatures. These substances are arranged in layers such that light is emitted when an electric current is applied. This is ‘electro luminescence’.

LEDs produce instant bright, pure deeply saturated colours, depending upon the precise mix of materials. Red, green, blue, ultraviolet, orange and yellow colours are all attainable, as is white with different colour temperatures. Other colours are near ultraviolet (UV) and infrared.

The wavelength, colour and brightness of the emitted light varies according the combinations of substances used, and it is in the chemistry of these combinations that great progress has been made in recent years. There are two main methods of producing high intensity white-light LEDs:

The first is to use red, green and blue LEDs, mixing them to produce white light. However, the method requires electronic control of different colour blending and diffusion, so is relatively rarely used.

The second method is to use phosphors to change monochromatic light from a blue or UV to broad-spectrum white light. The technique means coating an LED of a single colour with phosphor of different colours to form white light. Such devices are phosphor-based white LEDs. Depending upon the colour of the original LED, phosphors of different colours can be used. If the LED is coated with several phosphor layers, the colour-rendering index is raised. This is the most widely used technique for manufacturing high intensity white LEDs.

A higher luminous efficiency will typically trade off with lower colour rendering, so white LEDs having the best luminous efficacy (typically 120 lm/W) have the lowest colour rendering capability, while those that possess very good colour rendering often show a poor luminous efficiency (perhaps less than 70 lm/W).

LED lighting benefits

There are many highly significant benefits of using white light LEDs for general lighting. These can be summarised as:

• LEDs can be waterproofed and are very robust and reliable.
• LEDs are easily digitally controlled.
• LEDs use Safety Extra Low Voltages (SELV).
• LEDs can be integrated into building materials and pathways.
• LEDs are ideal for sharply delineating architectural objects.
• LEDs can provide colourful dynamic lighting with minimal spill.
• LEDs present no irritating flicker and illumination is instant.
• LED lamps can be dimmed over a wide range.
• LEDs are very energy efficient.
• LED lighting has an extremely long life.

Note that LED lighting may not always be the most efficient available, because LEDs are usually clustered. The reason is that LEDs are extremely compact, and their light beams are highly directional, so to produce enough light for general lighting applications they have to be made in clusters – usually called ‘arrays’. Although LEDs are highly efficient, some heat is still produced, and in arrays, this needs to be conducted away. Even so, the energy consumption is typically reduced by around 50% or more compared with equivalent CFLs.

The long working life of LED lighting also needs some explanation. For example, modern LEDs will run 60,000 hours (or more), but that doesn’t mean that they fail after this time. What does happen is that the light output slowly decreases over life, so that after (say) 60,000 hours, only around 70% of the original light output remains. LEDs can last longer still with less light output. This is an amazingly lengthy working life, so LEDs are ideal where lamp replacement is difficult or expensive.

Note also that LED manufacturers sometimes quote life estimates based on the average life of a single LED measured under specific laboratory conditions (100,000 hours may be claimed). In reality, LEDs degrade significantly faster when packaged in a luminaire, which is one reason why dedicated LED luminaires are essential. However, this can provide significant benefits in other ways, as there are exciting possibilities open to lighting designers and architects, as well as for end-users.

Take care!

Among the main lighting industry designers and manufacturers of white LEDs for general lighting are Philips Lumileds, Cree, GE and Osram Opto Semiconductors. All provide latest generation high quality white LEDs, but there has been debate about certain other manufacturers claiming that their LED products are the brightest or the longest lasting, based upon test results.

The testing may have been carried out using only the LED sources, not their performance when installed in a luminaire. This can make a significant difference, so architects and lighting designers, as well as installers, should be aware of the possibility that some performance statements might be inaccurate in a real-life situation.

LED lighting applications

Traffic lights have been using LED lighting for some years, while LED road, street and amenity lighting applications are fast increasing. For example, OSRAM Opto Semiconductors’ streetlights with Golden Dragon Oval LEDs have special oval lenses, and just 12 LEDs for each streetlight are sufficient to illuminate footpaths.

LEDs’ SELV and low operating temperature make them ideal for hazardous areas. In offices, LEDs are now suitable for task lighting. Indeed, a couple of years ago, Philips lit the Generali-owned office building in Paris entirely by LEDs – a world first at the time. Now, Newcastle Building Society’s new flagship branch office in Newcastle city centre is using a fully integrated lighting scheme from Philips. The Luxspace, Spot LED III, and eW Cove products offer the Society a cost effective LED alternative combined with beauty and high design. This makes it one of the first organisations in the city to use all LED technology to illuminate the entire retail premises.

LED decorative lighting is increasingly being used in hotels, for decorative, bar / restaurant and wall lighting, plus bedside lighting and standing lamps. In supermarkets, LED shelf and retail freezer lighting is now common, as there is no toxic mercury to contaminate foodstuffs, as can happen with broken fluorescent lamps. In shops, LEDs are finding ever more applications in display lighting, décor, signage and logos.

LEDs are also now suitable for exit signs and other emergency lighting luminaires. Compared with equivalent fluorescents, good quality LED emergency luminaires and exit signs often show energy savings of 30% or more, resulting in significant cost savings over the installation’s life.

LEDs typically have a 60-degree beam, so when correctly installed at 3m mounting height at recommended spacings, emergency lighting luminaires should have no trouble achieving the required 1 lux at the floor, using just two 1W LEDs, plus LED control gear (driver), inverter and a three-hour non-maintained 3.6V battery.

Exit signs must be sufficiently well lit to be clear even at maximum viewing distances, according to application standard EN1838 and the luminaire construction standard EN60598-2-22, which define minimum luminance levels that signs must achieve. LEDs are most effective in ‘edge light’ signs. Maintained emergency lighting and exit signs provide best building safety, but maintenance costs will be higher than for a non-maintained installation because of (conventional) lamp aging. However, LED lamps have a much longer life in these applications and will significantly reduce maintenance costs and disruption to offices etc.

As a safeguard, purchasers could check that the proposed supplier is an Industry Committee for Emergency Lighting (ICEL – www.icel.co.uk) member.

Domestic lighting is different. Homeowners have only recently taken up CFLs, despite their significant benefits over the now banned incandescents. Most LED applications so far are mainly replacements for low-end halogen garden spotlights, or purely decorative. This is changing. A good early example is Thorn’s Base LED downlighter, while the author uses excellent Philips LED Econic ‘light bulbs’. Even so, CFLs will be domestic mainstays for some time to come.

LED developments

Current development in conventional white light LEDs is mainly concerned with optimising them to provide higher light output and operational temperatures, and luminous efficiencies are improving all the time. Some developments are aimed at optimisation whereby although more energy is converted into wasteful heat, the light produced will show better colour rendering. Other research aims to raise efficiency through better phosphors or more careful package design. This can result in a more homogeneous white light. Research is also being carried out into a third method of producing white light from LEDs that are not coated with any phosphors.

Summarising, current LED lighting technological developments include improving the following:

• LEDs in more complex packages for efficient heat dissipation
• LED light quality
• LED luminous efficiency
• LED driver solutions (controlgear etc)
• LED light source modularity
• LED optical design.

Organic light-emitting diodes

There is another type of LED entirely that promises much for the future. This is the organic light emitting diode (OLED). In terms of design, construction and application, OLEDs are very different from conventional white light LEDs. With this type, the electroluminescent material comprising the diode emissive layer is an electrically conductive organic compound.

OLED advantages include a low driving voltage, being able to be formed into very thin displays that provide a wide viewing angle with high contrast and colour range (as well as white). Note that polymer OLEDs can be printed or made flexible, so the already huge range of exciting lighting design possibilities provided by LEDs is further extended.

As a result, as Osram Opto Semiconductors says, ‘OLEDs are more than just light sources – they are design elements. Even when switched off they look very different from conventional light sources. They are very thin, very flat, very light and attractive. They can be transparent, diffused or mirrored, so offering a completely new look. Luminaire designers can, therefore, look forward to enormous freedom’.

OLEDs have been used to make visual displays for portable electronic devices and TVs, as well as for highly innovative lighting schemes. In fact, for lighting designers and architects, the available designs are really only limited by imagination. For example, Osram has developed a transparent white high-performance OLED. The large-scale prototype is transparent either ‘on’ or ‘off’. Under laboratory conditions it has achieved a luminous efficacy of more than 20 lm/W at a brightness of 1000 cd/m². This opens up possible applications such as partitions that are almost invisible by day and then provide a pleasant diffused light at night.

Current OLED lighting technological developments include improving the following:

• OLED luminous efficiency
• Phosphorescent OLEDs
• The optimisation of OLED device structure and materials
• White OLEDs for general lighting.

And finally…

Costs are still coming down fast, while LED performance is improving all the time, so there is no reason why electrical contractors and installers cannot benefit from the massive potential business opportunities presented, although suitable training may be required in some cases.

Most lighting engineers and designers have embraced LEDs for general lighting already, as have many architects – why not you too?