Trade Resources Industry Knowledge Increasing Brightness Capability Has Seen Coloured LEDs Move Into Applications

Increasing Brightness Capability Has Seen Coloured LEDs Move Into Applications

Tags: LED

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

LED Technology - Coloured LEDs

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.

LED Technology - Coloured LEDs_1

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.

LEDs are also now used as backlights inLCD 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 seewhite 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.

Related Links:

LEDs

LED technology - White LEDs

 

Source: http://www.electronicsweekly.com/Articles/14/08/2009/41951/LED-technology-Coloured-LEDs.htm
Contribute Copyright Policy
LED Technology - Coloured LEDs
Topics: Lighting