ELECTRIC LIGHT

An electric light is a device that produces light by the flow of electric current. It is the most common form of artificial lighting and is essential to modern society, providing interior lighting for buildings and exterior light for evening and nighttime activities. Most electric lighting is powered by centrally generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems. Battery-powered lights, usually called "flashlights" or "torches", are used for portability and as backups when the main lights fail. The two main families of electric light are incandescent lamps, which produce light by a filament heated white-hot by electric current, and gas-discharge lamps, which produce light by an electric arc through a gas. The energy efficiency of electric lighting has increased radically since the first demonstration of arc lamps and the incandescent light bulb of the 19th century. Modern electric light sources come in a profusion of types and sizes adapted to a myriad of applications.

Types of electric lighting include:

incandescent light bulbs arc lamps

gas-discharge lamps , e.g., fluorescent lights and compact fluorescent lamps, neon lamps, flood lamps, modern photographic flashes

lasers

light-emitting diodes , including OLEDs

sulfur lamps

The most efficient source of electric light is the low-pressure sodium lamp. It produces, for all practical purposes, a monochromatic orange/yellow light, which gives a similarly monochromatic perceprtion of any illuminated scene. For this reason, it is generally reserved for outdoor public lighting usages. Low-pressure sodium lights are favoured for public lighting by astronomers, since the light pollution that they generate can be easily filtered, contrary to broadband or continuous spectra.

Incandescent light bulb

Main article: Incandescent light bulb

The modern incandescent lightbulb, with a coiled filament of tungsten, was commercialized in the 1920s developed from the carbon filament lamp introduced in about 1880. As well as bulbs for normal illumination, there is a very wide range, including low voltage, low-power types often used as components in equipment, but now largely displaced by LEDs

There is currently interest in banning some types of filament lamp in some countries, such as Australia planning to ban standard incandescent light bulbs by 2010, because they are inefficient at converting electricity to light. Sri Lanka has already banned importing filament bulbs because of high use of electricity and less light. Less than 3% of the input energy is converted into usable light. Nearly all of the input energy ends up as heat that, in warm climates, must then be removed from the building by ventilation or air conditioning, often

resulting in more energy consumption. In colder climates where heating and lighting is required during the cold and dark winter months, the heat byproduct has at least some value.

Halogen lamp

Main article: Halogen lamp

Halogen lamps are usually much smaller than standard incandescents, because for successful operation a bulb temperature over 200 °C is generally necessary. For this reason, most have a bulb of fused silica (quartz), but sometimes aluminosilicate glass. This is often sealed inside an additional layer of glass. The outer glass is a safety precaution, reducing UV emission and because halogen bulbs can occasionally explode during operation. One reason is if the quartz bulb has oily residue from fingerprints. The risk of burns or fire is also greater with bare bulbs, leading to their prohibition in some places unless enclosed by the luminaire.

Those designed for 12 V or 24 V operation have compact filaments, useful for good optical control, also they have higher efficiencies (lumens per watt) and better lives than non halogen types. The light output remains almost constant throughout life.

Fluorescent lamp

Main article: Fluorescent lamp

Fluorescent lamps have much higher efficiency than filament lamps. For the same amount of light generated, they typically use around one-quarter to one-third the power of an incandescent.

Fluorescents were mostly limited to linear and a round 'Circline' lamp until the 1980s, with other shapes never gaining much popularity. The compact fluorescent lamp (CFL) was commercialized in the early 1980s.

Most CFLs have a built-in electrical ballast and fit into a standard screw or bayonet base. Some make use of a separate ballast so that the ballast and tube can be replaced separately.

Typical average lifetime ratings for linear fluorescent tubes are 10,000 and 20,000 hours, compared to 750 hours (110 V) and 1000 hours (240 V) for filament lamps.

Some types of fluorescent lamp ballast have difficulty starting lamps in very cold conditions, so lights used outdoors in cold climates need to be designed for outdoor use to work reliably.

Fluorescents come in a range of different color temperatures. In some countries cool white (CW) is most popular, while in some, warmer whites predominate.

In the United States, fluorescents most often come in cool white (CW), with some home bulbs being a warm white (WW), which has a pinkish color. In between there is an "enhanced white" (EW), which is more neutral. There is also a very cold daylight white (DW). Compact fluorescent lamps are usually considered warm white, though many have a yellowish cast like an incandescent. "Warm" and "cool" are entirely relative terms and almost arbitrary so color temperature and the color rendering index (CRI) are used as absolute scales of color for fluorescents, and sometimes for other types of lighting.

LED lamp

Main article: Solid-state lighting

Solid state LEDs have been popular as indicator lights since the 1970s. In recent years, efficacy and output have risen to the point where LEDs are now being used in niche lighting applications.

Indicator LEDs are known for their extremely long life, up to 100,000 hours, but lighting LEDs are operated much less conservatively (due to high LED cost per watt), and consequently have much shorter lives.

Due to the relatively high cost per watt, LED lighting is most useful at very low powers, typically for lamp assemblies of under 10 W. LEDs are currently most useful and cost-effective in low power applications, such as nightlights and flashlights. Colored LEDs can also be used for accent lighting, such as for glass objects, and even in fake ice cubes for drinks at parties. They are also being increasingly used as holiday lighting.

LED efficiencies vary over a very wide range. Some have lower efficiency than filament lamps, and some significantly higher. LED performance in this respect is prone to being misinterpreted, as the inherent directionality of LEDs gives them a much higher light intensity in one direction per given total light output.

Single color LEDs are well developed technology, but white LEDs at time of writing still have some unresolved issues.

1. CRI is not particularly good, resulting in less than accurate color rendition.2. The light distribution from the phosphor does not fully match the distribution of light

from the LED die, so color temperature varies at differing angles.

3. Phosphor performance degrades over time, resulting in change of color temperature and falling output. With some LEDs degradation can be quite fast.

4. Limited heat tolerance means that the amount of power packable into a lamp assembly is a fraction of the power usable in a similarly sized incandescent lamp.

LED technology is useful for lighting designers because of its low power consumption, low heat generation, instantaneous on/off control, and in the case of single color LEDs, continuity of color throughout the life of the diode and relatively low cost of manufacture.

In the last few years, software has been developed to merge lighting and video by enabling lighting designers to stream video content to their LED fixtures, creating low resolution video walls.

For general domestic lighting, total cost of ownership of LED lighting is still much higher than for other well established lighting types.[dubious – discuss]

Carbon arc lamp

Main article: Arc lamp

Carbon arc lamps consist of two carbon rod electrodes in open air, supplied by a current-limiting ballast. The electric arc is struck by touching the rods then separating them. The ensuing arc heats the carbon tips to white heat. These lamps have higher efficiency than filament lamps, but the carbon rods are short lived and require constant adjustment in use. The lamps produce significant ultra-violet output, they require ventilation when used indoors, and due to their intensity they need protecting from direct sight.

Invented by Humphrey Davy around 1805, the carbon arc was the first practical electric light. They were used commercially beginning in the 1870s for large building and street lighting until they were superseded in the early 20th century by the incandescent light. Carbon arc lamps operate at high powers and produce high intensity white light. They also are a point source of light. They remained in use in limited applications that required these properties, such as movie projectors, stage lighting, and searchlights, until after World War 2.

Discharge lamp

A discharge lamp has a glass or silica envelope containing two metal electrodes separated by a gas. Gases used include, neon, argon, xenon, sodium, metal halide, and mercury.

The core operating principle is much the same as the carbon arc lamp, but the term 'arc lamp' is normally used to refer to carbon arc lamps, with more modern types of gas discharge lamp normally called discharge lamps.

With some discharge lamps, very high voltage is used to strike the arc. This requires an electrical circuit called an igniter, which is part of the ballast circuitry. After the arc is struck, the internal resistance of the lamp drops to a low level, and the ballast limits the current to the operating current. Without a ballast, excess current would flow, causing rapid destruction of the lamp.

Some lamp types contain a little neon, which permits striking at normal running voltage, with no external ignition circuitry. Low pressure sodium lamps operate this way.

The simplest ballasts are just an inductor, and are chosen where cost is the deciding factor, such as street lighting. More advanced electronic ballasts may be designed to maintain constant light output over the life of the lamp, may drive the lamp with a square wave to maintain completely flicker-free output, and shut down in the event of certain faults.