LED Lighting Overview

LED lighting 

LED lighting is increasingly used across commercial and domestic areas as a replacement to more traditional forms of lighting.

Despite initial limitations in the technology that meant that early LEDs were only capable of low-intensity red light, subsequent developments have seen visible light options burgeon to the point where they are now considered to be a viable alternative to resistance filament (incandescent) lights.

 

History of LEDs

Building on the observed phenomenon of electroluminescence (at various points during the twentieth century researchers and scientists had seen that semiconductor materials lit up when current was passed through them), the first “LED proper” was invented by a scientist whose name has been obscured by developments in the field that would happen several decades after he had died.

In mid-1920s, Oleg Losev recorded light emitting from zinc oxide and carbide crystal rectifiers as current passed through them. The resultant paper on the emission of silicon carbide diodes neatly outlined most of what is known about the behaviour of diodes today. Losev’s work also showed an acute understanding of the non-thermal nature of diode emission and anticipated later developments in measuring the current and voltage attributes of the diode.

Losev’s work would be lost in the fog of the mid-twentieth century’s upheaval, and he died a tragic death at the hands of the Nazis during the blockade of Leningrad.

In effect, it took another twenty years before the rest of the research community followed his lead. Gary Pittman and James Baird were working at Texas Instruments when they discovered that a popular semi-conductor material of its day – gallium arsenide – emitted light at infra-red wavelengths in the presence of applied current. Based on a P-N junction (allowing the flow of current in one direction but not the other) LED, and using a spaced cathode contact for the emission of infra-red light, they were able to lay the ground work for the first manufactured LED.

At this point, the non-visible nature of the diodes’ light emission meant that its use was limited beyond sensing and photoelectric applications. The visible light breakthrough came in 1962, when the General Electric Company’s Nick Holonyak Jr invented the first visible spectrum light emitting diode using gallium arsenide phosphide, an alloy semi-conductor material.

From here companies such as Monsanto and Hewlett-Packard were able to start rolling out LEDs on an industrial scale – although light output was still comparatively low compared with later developments.

For scientists and technicians addressing the replacement of incandescent lighting, it would be several decades later before major breakthroughs in producing “white” LEDs occurred. In 1996, Japanese firm Nichia demonstrated that white light diodes were possible by covering a blue diode in white phosphor. When the current is applied and the diode emits blue light, it hits the inner surface of the phosphor and changes to white light. Subsequent experiments with “mixing” RGB (or reflected) light have also been shown to be successful.

 

Different lighting types

LEDs are essentially semiconductors that exploit the partial current flow of semiconductor materials to produce light of various colours and intensity. Used in applications such as vehicle dashboards, traffic lights, home electrical appliances and light fittings, LEDs are known to be more energy efficient than traditional incandescent lighting as well as a longer optimum use life-cycle.

 

MR16 LED Lighting

MR 16 LED lighting is an alternative to halogen MR16 bulbs. Strictly defined, “MR” stands for a multi-faceted reflector and features a reflector with a number of faces that “collects” the light emanating from the filament and concentrates it in a beam. Halogen lamps are relatively inefficient when compared to LEDs, which require no interior reflector in their fittings as they emanate directional light by default.

 

LED downlights

LED downlights are designed to replace downlight fixtures and fittings already in situ in a room. Usually styled as replacement for halogen lighting, in reality they are a more efficient substitute for incandescent lighting too.

Most standard halogen downlights use around 55 watts. LED alternatives slash that figure down to between 4 and a half and 14 watts, with little noticeable loss of brightness.

 

LED Floodlights

Designed as an alternative to either the halogen or the incandescent floodlight, these lamps deliver a working life in excess of 50,000 hours. Wattage is usually specified in the range of 10 to 200, which gives equivalent luminosity to incandescent or halogen lamps in the 20-1260 watt range.

 

LED ceiling lights

LED ceiling lights are available either as recessed panels or as a variety of different styles of mounted lights, including pendant lighting, spot lighting and crystal lighting. Increasingly popular because of their native ability to “run cold” (produce little exothermic – or heat energy), LED ceiling lights have the added advantage of achieving optimum luminosity once the on switch is actuated.

 

LED torches

LED torches use white LEDs and operate as a replacement for torches using traditional incandescent bulbs. Known to provide more efficiency than technologies based on generating light through electrical resistance from a filament, the LED torch diodes produce roughly 100 “lumens” per watt (where lumens are defined as the total amount of visible spectrum light emitted by a device), as compared with between 8 – 10 lumens per watt from a torch using an incandescent lighting source. Additional benefits include increased battery longevity and a torch light-unit less susceptible to breakage.

 

Sylvania Lighting

Sylvania lighting is designed and produced by the Havells Sylvania, a company with more than one hundred years pedigree in the lighting industry. As well as manufacturing exterior and interior lights using technologies such a halogen, LED and incandescent lighting, Sylvania also specialise in downlighting, LED balls, LED tubes and recessed modular lighting options.

 

Solar battery charger

Solar battery chargers use photovoltaic panels to harness the power of the sun and turn it into electrical energy. Light energy – in the form of the sun’s rays – is converted to DC (direct current) and passed through to the connected device that houses the battery. Many solar chargers also come equipped with USB connectivity, which means that the internal battery can be charged from a computer or via a mains connected USB adapter.

 

Technical elements of LED lighting

LED lights are built on our understanding of the known properties of semiconductors. In basic terms, an LED is a device that uses chemically “doped” polarised semiconductors to control and define electron flow between the two poles. The energy supplied by the electrons is converted into light as it passes through the device.

A simple example should suffice to explain the underlying principles: Incandescent lighting still uses the same basic design that was originally defined by Thomas Edison. Resistance along a filament builds up heat energy, which ultimately results in the production of visible light. Rather than using resistance to achieve luminosity, LEDs produce light at the junction point between two semiconductor materials. Light produced in this way can be said to be a function of the application of voltage across the junction.

The building block of the LED is the semiconductor material – primarily silicon – in the form of a chip. Semiconductors have properties intermediate between conductors – such as metal – and insulators such as rubber, and thus can be manipulated to control the flow of electrons. The chip is placed on a special cup called a reflector cup, which is then housed in a frame constructed of lead. Two wires are attached to the frame and the whole assembly is then encased in an epoxy lens. The semiconductor chip is divided into two regions – one of which carries a positive charge, and one of which carries a negative charge. These negative and positive charges are known as n-type and p-type respectively and give their name to the n-p junction that characterises the diode.

At the point at which current is applied to the chip, both the negative and the positive electrons combine, resulting in a reduction in energy level. Depending on the way that the semiconductor material has been “doped” or “altered” with small amounts of “foreign” materials designed to affect the behaviour of the chip, photons of light may be emitted that exhibit different properties relative to doping substance. Ultimately, the properties of the light that a diode emits is governed by the materials used to construct the chip, the semiconductor die (doping material) and the qualities of the reflector cup.

 

White light LEDs

Most people considering changing their incandescent or halogen lighting are looking for a like for like swap that produces similar, or better results than their current solution. In recent times, white light LEDs have become a viable alternative to traditional forms of luminescence, as the drive to produce more efficient lighting gathers momentum.

White light LEDs have arrived as a consequence of the red, green and blue LEDs that came to market in the years after the first diodes were mass-produced in the 1960s. Using a combination of all three colours at varying output intensities means that all colours in the visible light spectrum are possible.

In technical terms, photosensory cone cells that form part of the retina regulate the eye’s perception of light as being “white”. When they are stimulated at certain ratios, the information travelling down the optic nerve to the brain decodes these ratios as colours. In essence, the cones on the retina produce response signals. The most acute responses come after red, green and blue are mixed together in varying wavelengths, and hence perception of all visible light spectrum colours is possible.

One feature of the RGB technique is that maintaining the colour blend and the diffusion (important aspects of ensuring that the “white light” as perceived by the human eye stays constant) is usually a function of a controlling electronic circuit. For this reason other methods are more commonly used to produce white light.

Incorporating phosphor wavelength converters that change the colour of the light as luminescence from the diode hits the phosphor is one such method.

Phosphor based diodes suffer from the degradation of the phosphor coating over the life cycle of the light, but offer certain production advantages over the RGB technique. In general terms they are much easier to manufacture than diodes that require a form of electronic control and the phosphor application process can be relatively simply achieved with modern electronic construction techniques.

In terms of output efficiencies, phosphor coated diodes can achieve satisfying effects relative to the human eyes ability to perceive luminosity. Most diodes constructed using this technique will be blue, with a yellow phosphor coating. If current was applied to the original blue LED and the phosphor doped equivalent, the former will look up to 80 percent less luminous to the human eye, due to the way in which the human eye (and brain) perceive yellow relative to blue.

 

Manufacturing and research

Research is continuing into the most efficient way to produce white light LEDs as the market take up (and the interest in the low heat, low energy consumption features of solid state lighting) continues to grow.

One key area relates to the production of phosphors with higher levels of native efficiency that do not suffer from the Stokes shift phenomenon. This states that where there is a differential between the energy state of a photon leaving a system and the one that the system absorbs, an overall energy difference occurs. The current position sees the YAG (Yttrium aluminium garnet) phosphor most commonly used to mix blue and yellow to achieve white, but continual experimentation with this as a base is ongoing. Additional losses to reabsorption in the LED’s housing are also an issue, and time and energy continues to be invested in preparing phosphors with different properties as well as constant refinements to the diode’s design.

In terms of output, conventional sources of light usually offer a mean of 15 to 100 lumens per watt of energy. By way of comparison, forecasts for white LED lighting suggest that they may reach in excess of 300 lumens per watt – a huge advance on long-established watt/luminescence ratios.