LED Lighting Basics

What is LED?

A light-emitting diode (LED) is a two-lead semiconductor light source. It resembles a basic pn- junction diode, which emits light when activated.[7] When a fitting voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

How LEDs work?

A LED directly converts electricity into light. LEDs are made from joining together two semiconducting materials. On one side is a 'p-type' semiconducting material on the other side is a 'n-type' semiconducting material. P and N stand for positive and negative. These different materials are made by taking a pure semiconductor such as silicon, then mixing in small amounts of impurities such as Aluminium and Phosphorus, in a process known as doping. As electrons from the electrical current cross the boundary between the P and N materials the electrons move to a lower energy state and emit light of a certain colour. Different materials such as silicon carbide or gallium-nitride and different doping methods result in different coloured LEDs.

Because LEDs typically only emit monochromatic light, or light of a single colour, there are a couple of different methods to get white light (which is a mix of colours) from a LED. One method is to carefully mix up several different colored LEDs in a single light fixture to simulate white light. Another method uses a single blue or ultraviolet LED in combination with a phosphor, which absorbs the blue light and emits a broader spectrum of white light (similar to fluorescent lamps).

Pros and Cons of LED lighting

LED Benefits

  • Long life, if used in a suitable environment. Saving future maintenance & cost.
  • Dramatically higher efficiency & lower power usage compared to incandescent lights (up to 80% cheaper to run)
  • Lower heat output, conventional incandescent lights waste about 90% of their power on heat output.
  • Typically lower infra-red and ultraviolet output. This can also mean a lower fire risk, especially compared to halogen downlights
  • Typically faster switching time and less likely to be damaged by on off cycles
  • Faster warm up time (especially compared to fluorescent lights). However some LED drivers / transformers do have a short startup time of around 1 second
  • A direct retrofit solution is now available for almost all types of light fixtures
  • They are usually more rugged. There is normally no glass to break and no filament to damage via vibration.
  • They are more environmentally friendly. They last longer, don't result in the emissions of as much CO2 and normally don't contain any toxic materials, like the mercury vapor in CFL.
  • Some LEDs can be setup to dynamically change colour, but this is not the norm for residential lighting.

LED Disadvantages

  • Expensive to initially purchase globes
  • Not all LED bulbs are dimmable
  • Possible compatibility problems with existing dimmers when retrofitting. The larger brand name manufacturers do fairly extensive testing, but you might not be so lucky with generic LEDs
  • Possible compatibility problems with existing low voltage transformers when retrofitting
  • High powered LED downlights can be taller than halogen lights, which can be a problem if roof space is limited
  • May fail prematurely in high temperature conditions
  • Quality / colour of the light isn't always as good as incandescent or halogen lights, but is getting better.
  • Light output and colour quality can degrade over the life of the bulb (but other lights can also suffer this problem)
  • The need for a heatsink means some LEDs don't like being installed on their side as the flutes in the heatsink won't function efficiently.
  • They are a newer more complex product than an incandescent light, getting accurate specifications and advice can be difficult. Performance depends on good engineering, not over driving the chip, having a good heat sink and a stable power supply

Lumen output compared to incandescent

Products claiming to be direct replacements for an incandescent of a certain wattage can often produce light levels well below the typical performance of the claimed equivalent. Even when a manufacturer's light output claims are accurate, the number of lumens may not be consistent with the incandescent wattage it is recommended to replace. As a guide, the minimum lumen output for incandescent equivalence claims specified by ENERGY STAR® in their Program Requirements for Integral LED Lamps document is included below:

Nominal wattage of lamp to be replaced (watts)Minimum initial light output of LED lamp (lumens)
25 200
35 325
40 450
60 800
75 1,100
100 1,600
125 2,000
150 2,600

  

Colour Quality

The colour temperature and Colour Rendering Index of a light source determine the appearance of the light and the objects being illuminated. Incandescent bulbs generally generate a yellowish "warm white" colour with a CCT of around 2700K and a CRI of 100 (as incandescents are the reference light source used in the CRI metric). LED lights offer a much broader range of colour temperatures, ranging from warm white to natural (sometime's called 'pure' or 'daylight') white to a bluish cool white. Different colour temperatures may suit different spaces and tastes. In general, warm white may be a more appealing choice for spaces with lower levels of light while pure and cool white lighting tends to complement spaces with high levels of light or sunlight.