LEDs: leading the future of lighting

The Australian lighting industry may be facing a number of challenges, including complicated regulatory environment and counterfeit products, but there is little doubt that the efficiency of all types of lighting has improved over the last 20 or so years. In this article, Mike Smyth takes a light look at the past, present and future of different types of lighting technologies.

Lighting, in the home, in commercial and industrial situations and even on the streets, has come a long way in the last 70 years or so. Tungsten lights, sodium lights, cold cathode tubes, mercury vapour, halogen, neon, iodine, CFLs and fluorescent are all still with us but beginning to take a back row compared with the all-advancing, all-shining LED (light emitting diode) that threatens to take over any time soon.

Tungsten lights

Tungsten has become a dirty word, not only with electricians but especially with greenies who see them as needing more and more coal-fired stations to feed their hunger for power. Yes, they were/are demanding of volts and amps and the light they give out is that unnatural yellow colour that some call mellow. But perhaps their two greatest downsides are the heat they generate, meaning they cannot be used in small enclosed spaces except at the lowest wattages giving the meanest of light, and the high voltages they need to give worthwhile illumination. In today’s world they are not considered efficient.

Tungsten bulbs or globes have been with us not quite since Edison suspended carbon in a glass container but not many years afterwards. The carbon was quickly succeeded by metal and since those early days the metal has been improved to give the relatively long life we expected before their demise.

As a side issue it was, of course, found that these filaments not only gave off light but also a stream of electrons that were later harnessed by Fleming to create the thermionic valve or tube, and we all know how complex these became before the transistor made them as obsolete as the side button boot.

Nevertheless, despite their shortcomings, tungsten bulbs have stood us in good stead as our almost sole means of lighting for a good many years, and there are many among us who mourn their passing.

LEDs

LEDs are a quite different animal. They are solid state devices, a semiconductor that is so constructed that when a forward voltage is applied to it, it releases photons as light. On the whole, LEDs have taken a long time to come to fruition - at least as serious illumination. Initially used as indicator lamps on equipment such as televisions, hi-fi units and other domestic appliances, the little red glows become a feature of modern equipment.

Colour and LEDs was an initial problem. For a long time it was red only with gallium arsenide as the doping agent. Gradually other colours became available with various combinations of chemicals giving us green, yellow, blue and white that were very useful at making modern electronic devices look very pretty indeed. But the most useful colour for general lighting, white, was a long time coming and when it did it was only relatively slowly taken up by the industry and consumers. The breakthrough came with high-intensity white LEDs, especially when they were fitted into hand torches in banks of five or seven to give up to a watt of output. This intense bright white light, demanding low voltage and giving off no heat, was immediately attractive for use in confined spaces and it was only a natural progression from there to general household lighting.

LEDs are getting better and better. The 30 to 60 mW output types have been replaced with 1 W plus. There are now 5 W LEDs available as at 18 to 22 lumens per watt (lm/W), compared with a 60 to 100 W incandescent bulb that emits around 15 lm/W. A standard fluorescent light emits up to 100 lumens.

Six years ago a prototype LED with 131 lm/W at a forward current of 20 mA was demonstrated but this has since been superseded by a 150 lm/W version and last year (2011) 100 lm/W at full power of 10 W and 160 lm/W at about 2 W input was shown to be the shape of things to come. However, the current demand for these higher power devices has risen to 350 mA and beyond.

Of course, one of the great attractions of LEDs, apart from their low voltage and current requirements and their ability to operate almost cold, is their extended lifespan of anything up to 100,000 hours - well beyond the capability of tungsten. This makes them a highly efficient form of lighting and future development suggests that brighter and better are just around the corner.

Dramatic as this shift was, we are still not near using LEDs as the main source of artificial light within the average home. Yes, they are in use as table lamps and other single point sources of illumination but there is still some distance to go before they take over the world.

Fluorescent lamps

The fluorescent lamp is much more efficient than tungsten. It is a gas discharge device that excites mercury vapour with electricity. The excited mercury atoms generate ultraviolet light that influences a phosphor coating on the inside of the tube to produce light. Installation is more complicated than tungsten in that special fittings are needed and a ballast circuit is required to regulate the lamp’s current. These lamps are negative differential resistance devices, which means that as more current flows through them, the more the resistance falls. Without a ballast to restrict the current flow, the tubes would quickly enter a thermal runaway state and self-destruct.

The simplest ballast is a series inductor on a laminated magnetic core but they are all rated according to the lamp size and supply frequency. If the voltage is too low to strike an arc, a step-up transformer will bring the voltage to the strike level.

Halogen lights

Halogen lights have become extremely popular, especially by architects who are designing them into almost every domestic dwelling. While many householders like the idea of dimming these lights, perhaps for a mood change or perhaps in the mistaken belief that is saves electricity, halogen bulbs are not always the best choice for this. The lifespan may be considerably reduced if the wrong halogens are dimmed.

High temperature operation is essential to the operation of these lamps but because of this they do create a potential fire hazard and already some safety codes require a protective grid or grille to be incorporated into their design. A further safety feature is an ultraviolet-absorbing glass filter, designed to protect the user from a possible explosion of the device. Careful handling is also needed as surface contamination of the glass, especially from human fingers, can weaken the quartz and change it to a weaker crystalline structure that may create a bubble leading to its failure by explosion.

Halogen lamps are bright and efficient and are in wide use outside in stadium floodlights and motor vehicles. Within the home, 10 to 15 W outputs are the most common, giving light at 150 to 180 lumens.

A brighter future

There is little doubt that the efficiency of all types of lighting has improved over the last 20 or so years. The tungsten bulb has become a dinosaur to be replaced by low-voltage LEDs and halogen. However, their versatility is still not as great although it is growing.

Handling cautions with halogen plus their high operating temperature, the precise voltages and currents needed for LEDs and the mercury content of fluorescents cannot be seen as major problems - rather as challenges for the next decade.