Incandescent technology was the first commercially successful electric lighting. Prior to its development gas lighting was the only viable alternative with many people still using candles. After its invention, for nearly 100 years, incandescent light bulbs were the only available technology and competing firms developed many shape variations.
Today we still use incandescent lighting in many forms but its inefficiency at converting electricity to light means there is growing international pressure to reduce and even remove it entirely.
The word incandescent means “A body emitting electromagnetic radiation in the visible spectrum due to the elevation of its temperature” or, simply, a material that glows when hot. An incandescent light bulb uses a thin wire element usually made of tungsten that gets so hot it glows white when connected to a source of electricity. When the wire gets this hot the oxygen in the air reacts with the metal causing it to oxidise and break almost instantly. To solve this problem the wire is surrounded by a glass bulb filled with argon and / or nitrogen gas removing the oxygen from inside the bulb.
When this technology was first invented glass making technology was very limited and the bulbs were all hand blown. The simplest shape to make was an oval or rugby ball shape but over time this turned into the familiar pear shape, now called GLS, used extensively all over the world. As glass manufacturing improved the shapes available increased to help in specific applications and to change their style..
Early incandescent bulbs had a life of less than 100 hours so early development efforts went into extending their life reaching around 1500 hours today. To make any incandescent bulb brighter you just needed to fit a slightly thicker wire and pass more electricity through it.
Halogen bulbs are an incandecsent bulb that uses that halogen cycle to improve efficience and increase life. The halogen cycle is described well here but it needs to run at high temperatures and elevated pressures. For this reason the halogen bulbs are always encapsulated in a stronger quarts silica type glass capsule.
Fluorescence is the emission of visible light from a substance that has absorbed electromagnetic radiation of a different wavelength. First observed in nature, animals such as fireflies and some minerals appeared to glow without the use of electricity. Scientists in the 18th century discovered how to replicate these phenomena in glass tubes but it was not until the Second World War when the demand for illumination in the factories stimulated General Electric to commercialise their research which created the fluorescent tubes we have today.
Inside a glass tube is a partial vacuum and a small amount of mercury. An electric arc, a very small and contained version of lightning, in the tube causes the mercury atoms to emit ultraviolet light. The tube is lined with a coating of a fluorescent material called phosphor which absorbs the ultraviolet light and re-emits the energy as visible light.
Fluorescent lamps are negative differential resistance devices or in other words as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing even more current to flow. Connected directly to the mains a fluorescent lamp would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent lamps must use an auxiliary device, called a ballast, to regulate their current flow.
A major variation of the fluorescent lamp is the compact fluorescent lamp (CFL) this uses the same technology but the tubes are twisted into loops and spirals reducing the overall size as much as possible. CFL bulbs come in two versions, the integrated type includes the ballast and starter in the base of the bulb and the non integrated type uses external ballast and starting systems.
Neon lights are another variation to the fluorescent tube. They use identical technology to standard fluorescent tubes but with different phosphors and neon gas to alter the visible colours.
Fluorescent bulbs are always a sealed tube but can be different diameters and twisted into just about any shape. The tube must be coated in an opaque powder, white for standard bulbs and coloured for neon.
Integrated CFLs replace existing incandescent bulbs such as screw and bayonet fittings. Non integrated CFLs use special fittings that are not interchangeable with any existing types so the user doesn't inadvertantly run a fluorescent tube without a ballast system..
High Intensity Discharge Lighting
A high-intensity discharge (HID) lamp produces light by means of an electric arc, similar to a fluorescent tube but created between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina tube. This tube is filled with both gas and metal salts; the gas facilitates the arc's initial strike and once started the arc heats and evaporates the metal salts forming a plasma. The plasma greatly increases the intensity of light produced by the arc and reduces its power consumption.
HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or light intensity are desired. These areas include gymnasiums, large public areas, warehouses, movie theatres, football stadiums, outdoor activity areas, roads and pathways. More recently, HID lamps have been used in retail and residential environments to spotlight features.
High-intensity discharge lamps work by intensifying an arc. Various combinations of metal salts are used in HID lamps, depending on the desired characteristics of the light. Varieties of HID lamp include:
- Metal halide (MH) lamps
- Ceramic MH lamps
- Sodium vapour lamps
Metal halide and ceramic metal halide lamps can be made to give off neutral white light useful for applications where a controlled white colour is critical, such as movie projection or product display but sodium lamps are used where maximum efficiency is required at the expense of an orange coloured light.
Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury vapour lamps and some metal halide lamps are usually started using a third electrode near one of the main electrodes while other lamp styles are usually started using pulses of high voltage.
The most common variation in retail and light commercial use is as a spot light to display products or highlight specific areas. They are sealed glass bulbs but they are not designed for use in common domestic fittings. All versions require an external control module, or ballast, and they will be dangerously hot during operation.
Light Emitting Diodes
With the development of high efficiency and high power LEDs it has become possible to use them for general lighting purposes. The first commercial LEDs were commonly used as indicator or warning lamps; as the materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level
Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers, electrons and holes, flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and therefore its colour, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.
Initially LEDs were designed to replace existing fittings and lighting systems but as the technology has developed bespoke fixtures and fittings are now available.