If you believe the hype,the Light-Emitting Diode (LED) is going to save our lighting design souls and, in the process, the planet.

However, LEDS still account for only a tiny percentage of the lighting market. One of the main reasons for this is that they are still at the early stages of development. the GLS lamp (AKA the household bulb) took 70 years to develop and it was even longer before it became a truly usable product, so we can’t expect the LED to hit the market without a bedding-in period.

The technology is advancing rapidly, but here Jonathan Rush lists 10 things you need to know when designing with LED.

1 Light output

There is an expanding gulf between the R&D labs that keep developing higher- lumen-package LEDs and what actually comes on the market as a reliable product. For instance, one company recently announced it had developed a single-chip white LED with an output of 1050 lumens (equivalent to a standard household lamp) with an efficacy of 72 lumens per watt. Great, but will it work on-site?

In terms of tried-and-tested products available, you can only get about 50-60 lumens out of a “cool white” 1W LED. This means that you need to cluster the LEDs to get notable lumen packages.

2 Control

LEDs are regarded as the future of lighting because of the flexibility achieved through lighting control. Unlike many other light sources, they are fully dimmable from 0-100% and are almost infinitely switchable with instantaneous 100% output.

LED is best suited to a DMX (digital multiplexing) dimming protocol which has long been used in theatres. It allows control and scene-setting of hundreds of LED fittings from one central location.

3 Heat

Some like it hot, but not LEDs. A poorly considered LED luminaire will get hot and this can lead to output degradation, colour shift or even failure.

The light output of an LED is dependent upon input current and while at 1 amp you can get upwards of 180 lumens from some units, there would need to be a large heat sink to dissipate internal heat from the fitting.

4 Lamp life

Some manufacturers still quote an LED lamp life of more than 100,000 hours. However, as soon as that LED is encased in a fitting, the lamp life reduces significantly. In truth, 20,000 to 40,000 hours is more realistic when considering lumen maintenance values.

The lumen depreciation over time is not as good for high-output white LEDs and is very bad for older 5mm white LEDs, which can see lumen outputs reaching 50% of initial levels after only 6000 hours.

5 Colour – RGB

The use of theatre control is redefining architectural lighting by allowing excellent colour-mixing possibilities. More than 16.5 million colours can be achieved by combining red, green and blue (RGB), so a system can be tweaked to give a colour that everyone in the design team is happy with. The control and RGB facility also allows colour changing over time or in relation to external elements such as traffic flow or wind speed.

6 White Light

Composite RGB white is OK for feature lighting but lacks the depth of colour and colour rendering of a proper white light solution.

White-light LEDs are available but are quite new in terms of development. They work by taking a blue LED and using a phosphorus material to convert the blue light into white. White LEDs are available in a range of colour temperatures (2760K-10,000K) but fall short on colour rendering and lamp life.

7 Colour rendering

RGB white mixes colour from narrow wavelengths of light and lacks the depth of colour to provide a suitable colour rendering for anything other than feature lighting. White LEDs are also lacking, with a typical CRI (colour rendering index) of 60-70, although this is improving.

Many manufacturers are making luminaires that mix colour-temperature LEDs in order to obtain a suitable colour rendering for application. This isn’t an ideal solution – the LED should have the appropriate colour-rendering capabilities before it leaves the lab.

8 Photometric Performance

LEDs are being touted about by every manufacturer at the moment and many have simply replaced the compact fluorescent lamp with a cluster of LEDs in a standard product. Ask them how it performs and you’ll soon find most have not been photometered they should be.

9 Sustainability

LEDs are being hailed as the saviour of the planet in lighting terms but they’re not there yet. In terms of Part L of the Building Regulations, many still fail to meet the lumen per watt requirements.

Where LEDs have been a success in energy terms is in relation to feature lighting, which previously would have used high-wattage units. Designers are learning that, in the external environment, you can be bold, use less light and still get a good impression, especially when colour is used.

10 The future

Despite the drawbacks, LED is the most promising future light source. Recent developments are encouraging and, if they continue at the current pace, LEDs should provide a “green” alternative in the future.

For this to be assured, a few things need to happen:

White LED needs to be a viable, cost-effective alternative to tungsten halogen and, to a lesser extent, compact fluorescent. It will need to gain considerably greater light output, colour stability and colour rendering without resorting to higher wattages. Luminaire manufacturers need to incorporate the highest-efficacy LEDs and suitable reflectors or “secondary optics” within purpose-built LED luminaires. These must be designed to complement the light source, not just to fit a light source within an existing product.

The pace of innovation is unlikely to slow, so luminaire makers will not be able to rely on static technology as they have with previous lamp types. They will have to be LED experts, constantly checking for new developments and incorporating these into usable products. Don’t even ask me about “organic” LEDs – we’ll be here all week!