Last week we discussed the evolution of traffic lights – and speaking of evolution, did you know that the different kinds of lights we are using today had also evolved over the course of time? Did you also know that humans started using light even in the ancient times? In those days, fire was the main source of light for humans. The discovery and control of fire was likely an invention of our ancestor Homo erectus, during the Early Stone Age. From that time onwards, there was no other source of light for humans until several inventors such as Thomas Edison invented the incandescent lamp. In 1878, Thomas Edison began a research in developing a practical incandescent lamp and continued to test several types of material for metal filaments to improve upon his original design. And on 1880 Thomas Edison and his team discovered that a carbonised bamboo filament could last over 1200 hours which gave birth to the first commercially practical incandescent bulb. Then a few decades ago, xenon-based flash lamps were invented. An on the latter end of the 19th century, the Light Emitting diode was discovered as another source of light. A few years more, other types of LEDs have also been discovered. People classify these lighting devices as solid state lighting. And this is going to be our subject for this article this week. As we go through, we will define what solid state lighting is, and what lighting devices have been classified in this area.
What is Solid State Lighting?
Solid state lighting is a type of lighting and technology in which light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) replaces traditional incandescent and fluorescent lamps for general lighting purposes. Solid state lighting devices produce visible light by means of electroluminescence rather than electrical filaments, plasma, or gas as sources for illumination. Electroluminescence is a phenomenon in which electric current passing through a specially formulated semiconductor diode causes the semiconductor material to glow. They produce visible light with reduced heat generation and less energy dissipation.
LEDs were originally used as indicator lamps in electronic devices such as radio transmitters and in some home appliances such as clock radios and television sets. As brighter LEDs were developed, solid state lighting lamps were eventually used in other applications such as traffic lighting, street and parking lot lights, train marker lights, building exteriors, electronic billboards, and headlamps for automobile vehicles. Today, we can also find them in flashlights, searchlights, cameras, indoor and outdoor lighting arrangements, and many other situations.
Some of the advantages of solid state lighting devices include the following: small size or mass which provides greater resistance to shock and vibration compared to filament wires and brittle glass tubes/bulbs, ease of control, unidirectional distribution, cool beam and colour rendering that can be very high and comparable to high fluorescent lamps with high colour rendering index (CRI), lower energy consumption, high performance in cold environments and longer operational life due to the elimination of filament evaporation, potentially increasing the life span of the illumination device.
Types of Solid State Lighting Devices
Light-Emitting Diodes
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current is passed through it. Light is produced when a suitable voltage is applied to the leads which allow the electrons to recombine with electron holes within the semiconductor material, releasing energy in the form of photons.
The colour of the light is determined by the energy band gap of the semiconductor. Inside the semiconductor material of the LED, the electrons and holes are contained within energy bands. The band gap determines the energy of photons that are produced by the LED. This photon energy determines the wavelength of the emitted light which is also the reason for its colour. Semiconductor materials with different band gaps produce different colours of light. The exact wavelength can be tuned by altering the composition of the light-emitting region.
Organic Light-Emitting Diodes
Organic light-emitting diode (OLED) is a flat light emitting technology, made by placing a series of organic thin films between two electrodes which generates bright light when applied with electrical current. In other words, an OLED light bulb is a thin film of material that emits light. OLEDs are usually used to create digital displays in devices such as television screens, computer monitors, and portable systems such as mobile phones, handheld game consoles and PDAs. OLED is the only technology that can create large area lighting panels. They can be used to make flexible and transparent panels, and can also be colour-tuneable. They are also capable of emitting beautiful soft diffused light. As of now, a major area of research is currently being done for the development of white OLED devices that can be used in other solid-state lighting applications.
Polymer Light-Emitting Diodes
Polymer LEDs or sometimes called light-emitting polymer or polyLED is a technology based on the use of polymer as the semiconductor material in LEDs and one of the two main families of OLED. Polymers are chemical substances that consist of large molecules that are made from many smaller and simpler molecules such as proteins and DNA which are naturally occurring polymers. Polymer LEDs have a number of essential qualities such as the capability to enable full-spectrum colour, produce high brightness at low drive voltages, glare-free viewing, and long operating life. The technology also offers a great deal of promise as a basis for cheaper and simpler lighting sources.
Solid state lighting has made significant advances in the lighting technology today. In lighting industry and in many other applications, standard incandescent tungsten-halogen lamps are being replaced by solid-state light lighting fixtures. Solid-state lighting is a new technology, and not simply a refinement of an existing one. New materials and technologies are not only offering substantial improvements in efficiency compared with the traditional incandescent lighting, but also opportunities to put light in new and sometimes startling places and modes that are only beginning to emerge in a rapidly changing field.