Electric Lighting

We all recognise the need for artificial lighting in our homes when day-lighting is not adequate. However, it is estimated that British households spend around £1.2 billion per year on power for lighting, and as only 2% of our electricity comes from renewables, this has huge consequences for Global Warming and the environment in general. Thus, the main worry for environmentally conscious homeowners is how to get the best out of our lighting without consuming too much electricity.
Prior to the late 19th Century, all artificial lighting was provided by some means of fire, be it candles, gas or oil lamps, or some other means. For many centuries the most prised lamp oil was provided by the whaling industry, nearly driving these huge mammals to extinction.
Candles were extremely inefficient, providing only 12 lumens of light for the consumption of 80 watts of chemical energy, equating to 0.15 lumens per watt (lm/W). This was improved upon in 1879 by Thomas Edison’s first light bulb, and within a few years 60W incandescent lamps were producing 180 lumens of light (3 lm/W). By 1938, fluorescent lamps had been introduced, offering a major leap in efficiency and longevity, though at the expense of a reduction in the quality of the light (colour rendering index – CRI, on a scale of 1 to 100, indicates how realistically colours are represented, daylight having a CRI of 100), and distracting ballast noise. These early fluorescents had efficiencies around 35 lm/W. Around the same time, high-intensity discharge lamps began to appear, mainly in outdoor applications, offering longer life and higher efficiencies than fluorescents. The first HID’s were mercury vapour lamps, followed in the 1960’s by high-pressure sodium and metal halide, both offering further efficiency improvements.

Incandescent Lamps
Also known as tungsten lamps, these are closely related to the lamp originally invented by Thomas Edison. Electricity burns a tungsten filament in argon gas, giving off a considerable amount of heat along with the required light, making these lamps very inefficient – 85% of the energy is given off as heat. The metal filament burns away, leading to a fairly short life of around 1000 hours. So-called long life incandescents are even less efficient than the normal ones.
On the positive side, tungsten bulbs have excellent colour rendition, giving a warm yellowish glow similar to afternoon sun.
An advance on standard incandescents was made when halogen bulbs were introduced. The filament is encapsulated with a halogen gas (bromine or iodine), allowing the filament to burn hotter and brighter. The gas captures vapourised tungsten and redeposits it on the filament, thereby extending the life of the lamp. Special quartz glass is used due to the high filament temperature. The light quality is excellent, and usually directed as required enabling a lower wattage to be adequate for the task. The bulbs are easily obtainable in 12-volt form, making them ideal for use with renewables without going through an inverter.

Fluorescent Lamps
The emergence of fluorescent lamps dramatically reduced the costs and maintenance needs of lighting in commercial buildings in the second half of the 20th Century. They operate by passing a current between electrodes at either end of a tube filled with mercury vapour. Energised mercury atoms emit UV radiation, which energises the phosphor coating on the inside of the glass tube, thereby emitting visible light. Durability, efficiency, light quality and controllability have all increased considerably since they were first introduced. Improved phosphor coatings, smaller tube diameters and electronic ballasts are some of the innovations, which have brought about the improvements.
The biggest revolution in household lighting has been brought about by the introduction of compact fluorescents (CFL’s). They are four to five times more efficient than tungsten lamps, because once the gas has been energised, very little power is needed to keep it glowing. Very little heat is given off so combustible materials can be used for shading, and the bulbs have a life of up to 10000 hours. These lamps are more expensive to buy, but pay for themselves many times over in saved electricity. The technology is continually improving, and early problems such as slow start-up, and short life when switched frequently, have been largely eliminated.
The only problem with fluorescents is their mercury content. This is not a problem in use, but could lead to mercury leaching if too many used bulbs are landfilled. Some local authorities have set up recycling schemes to reduce the number of these bulbs that end up in landfill.

Other types of lamp
As mentioned in the introduction, there are many other types of lighting, most of which have exterior or commercial applications. A summary of these lamps is given in the Table below, along with those already described. A technology to watch out for in the future is the evolution of solid-state lighting, i.e. light emitting diodes. The main application for LEDs until now has been indicator lights on electronic equipment, and the red numbers on the older types of electronic calculator and watch. The emergence of white LEDs with improved efficiency has opened up many possibilities for the future.