Increasing brightness capability has seen coloured LEDs move into applications that were previously the domain of filtered incandescent light.

Perhaps the most obvious example is in car rear light clusters where red-emitting LEDs began to take over almost as soon as their efficiency allowed.

The advantage to car makers was that the rear light cluster could be made thinner, taking up less boot space, and the manufacturer could omit the paraphernalia which allows owners to change their own light bulbs.

Blue LEDs at last

The final LED colour to become available in a bright form was blue.

As soon as intense blue devices became available, consumer product designers incorporated a surfeit of them as indicators, display backlights and illuminators in a fit of fashion that has yet to end.

LED-based displays

The introduction of bright blue was also the last piece in the red-green-blue trilogy that allowed LED-based full colour displays to be produced.

For example, big video screens based on clusters of red, green and blue LEDs are a common sight at sporting events.

Colour coding LED info
	Driving into the blue
	Organic - grow your own
	Green chandeliers
	Into the black?
	Display Trilogy
	Headlight glare
	Phosphorous
	Sensing light
	Grey area
An alternative look at the hues and shades of LED technology

LEDs are also now used as backlights in LCD TVs and video monitors, either all on at the same time - when the LCD incorporates its own colour filters, or flickering rapidly in succession allowing a monochrome LCD to produce colour images.

This R-G-B-R-G-B- flickering approach is also the basis for lighting a number of near-to-eye microdisplays.

In a related development, the advent of blue laser diodes has allowed the storage capacity of optical discs to be increased beyond CDs (infra-red laser) and DVD (red lasers).

The explanation here is that the shorter wavelength of blue allows smaller data spots, which can be packed closer together, to be written and read,.

Within the industry, some doubt the move to ultraviolet lasers for another generation is worth it, as the expense of ultra-violet optical materials may outweigh the potential density gain.

LED light fittings

Red green and blue power LEDs are being combined in light fittings to produce multi-coloured architectural lighting.

RGB LED light fittings have better colour-rendering capability than so called 'white LEDs' which can emit no red and little green.

The light from separate RGB light sources is optically difficult, but possible, to combine without multi-coloured artefacts appearing in the resulting light, particularly visible at the edges of beams.

Many ways of controlling multi-coloured light fittings are covered by patents owned by Color Kinetics, which was taken over by Philips in June 2007.

LED materials

Two basic emissive semiconductor technologies have replaced virtually all other in commercial visible LEDs. 

AlInGaP devices handle red to yellow wavelengths, and InGaN (also see white LEDs) are used from green to violet, and on to ultra-violet.

Green LEDs emit less light than existing theories suggest they should, making them the subject of several research programmes, notably at the University of Cambridge in the UK.

Both the materials, but particularly InGaN, are physically robust and capable of handling high power densities with long life - although all LED have a brightness half-life which depends amongst other things on power density.

Both materials are also thermally-sensitive with light output dropping with increasing temperature.

Infra-red wavelengths are mostly handled by GaAs and GaAlAs emitters.

LED structure

In general, modern LEDs consist of a heterostructure emissive layer backed by a Bragg reflector (a stack of half-wavelength thick transparent layers) which throws otherwise wasted light forwards.

The only difference between this and a similar laser diode structure is that lasers have a Bragg reflector above as well as below the emissive layer.

An optical modifier may be included on top of the die that cuts back-reflections.

Die size is chosen depending on the amount of light required, with 0.25x0.25mm common in 5mm devices and 1x1mm more likely in 1W power LEDs.

LED packages

5mm and 3mm round LEDs are still extremely common, although surface-mount packages - some with extremely small die - are now popular.

Unusually shaped packages, including rectangles, arrows and triangles abound.

For high-power devices, packages containing copper, aluminium and ceramic thermal paths directly from the die have been developed to allow heat to be extracted effectively.

Lumileds screw-down 'Star' package is probable the most iconic and copied of these, although surface-mount packages with better thermal efficiency are rapidly growing in popularity. Cree, Lumileds, Nichia and Seoul Semiconductor all offer surface-mount power LEDs.

There are also medium-power devices (around 100-500mW), aimed at cars and outdoor signage.

Like 5mm LEDs, these use the leadframe as a heat path, but more leads - generally four - are provided to broaden the thermal path.

In the following, we bring together resources from Electronics Weekly and UK and EU governmental bodies to provide detailed reference information about LED technology, specifically non-white LEDs - red LEDs, blue LEDs and green LEDs.

ELECTRONICS WEEKLY NEWS ON COLOURED LEDS

Most recent news

Previous news to note

Scottish firm Design LED begins lighting technology manufacturing
Design LED is expecting volume manufacture of products that include the firm’s LED printed light guide technology to begin this year.

Single material emits white light in Taiwan
What is being claimed as the world’s first single-material white phosphor has been made by researchers in Taiwan.

LED packaging gets clever
The widespread move to surface mount PCB assembly techniques over the last ten years has tested the ingenuity of LED suppliers.

Liion charge-pump needs 25mm²
Linear Technology has announced the LTC3206, an 800kHz, step-up/step-down fractional charge pump voltage converter.

Take a look at alphanumeric displays, they're changing
Two line alphanumeric displays are an enduring classic among components. Introduced 20 or so years ago, construction techniques and technologies have changed, but the basic product has endured.

Blue LED creator gets £103m
Blue LED and laser diode inventor Shuji Nakamura has been awarded £103m by a Japanese court, recognising his work at Nichia.

Microsemi demos next-gen LEDs
Microsemi has introduced its second family of blue LEDs and announced plans to develop a white LED.

Toshiba sees the white with latest LED device
Toshiba is moving into the white LED market, with a device that it claims can match incandescent lamps for colour and luminosity.

Philips carries the torch for LED home lighting
Philips Research in the US has set up a programme to determine how LEDs can be used fordomestic and commercial lighting.

Blue LED breakthrough with CVD
US firm Emcore says it has achieved a breakthrough in the production of commercial quality gallium nitride based blue light emitting diodes which could eventually lead to high resolution colour LED based displays and other applications.

Germans see white light at the end of the tunnel
The Fraunhofer Institute in Freiburg, Germany, has developed a white LED which does not use three different coloured LED chips in the same package.

Light me up
An emissive material has allowed light emitting diodes to shine brightly in the world. Steve Bush finds a new world opening up to LEDs

ELECTRONICS WEEKLY TRENDS & TECHNOLOGIES

LEDs - White LEDs

ELECTRONICS WEEKLY DESIGN IDEAS INVOLVING COLOURED LEDS

Drive a blue LED from a 3V battery
Using a blue LED can pose problems when available power-supply voltages don't meet or exceed the LED's 3V forward-voltage drop. This Design Idea shows how to drive a blue LED from a 3V battery or another power supply.

ELECTRONICS WEEKLY ANALYSIS OF COLOURED LEDS

Coloured LEDs: How are they programmed?
The colour output of LEDs is programmable which means a system of LEDs can be combined to create different colours.

Distributors see LED lighting opportunity
Talk of LED lighting applications was once guaranteed to send any self-respecting semiconductor distributor to sleep. Not any more.

Solid state lighting fires up chip firms
The technology of solid state lighting has its roots in the development of advanced LEDs. As a result it offers designers low voltage drive requirements, cool running, long lifetime reliability, small form-factor and energy saving potential.

Miniature OLED displays explained
Growth prospects for low power microdisplays continue to gain momentum - especially in mobile applications such as music/video players; viewfinders in cameras and video recorders; and in consumer video glasses.

Reducing laptop battery drain with LED backlights
Andrew Smith from Power Integrations looks at the benefits and techniques for switching from CCFL to LED backlighting on displays.

ELECTRONICS WEEKLY DESIGN IDEAS INVOLVING COLOURED LEDS

Drive a blue LED from a 3V battery
This entry details how to build an electric match controller that is intended for producing small-scale special effects like a confetti rainstorm.

Rainbow LED indicates voltage with color
Meters that indicate analog levels via a moving-pointer meter, a numeric display, or a column of LEDs typically occupy considerable panel area and require more than a casual glance to read.

ELECTRONICS WEEKLY BLOGS

From: Gadget Freak (Cool electronic gadgets for the electrical engineer)

Clive had an engagingly surreal stage presence
This entry details how to build an electric match controller that is intended for producing small-scale special effects like a confetti rainstorm.