AN EVOLUTIONARY HISTORY OF ARTIFCAL LIGHTING: THE AGE OF ELECTRIC LIGHTS [PART - 2]
Continuing where we left off, in the last century, the electric lighting industry achieved a hat-trick of phenomenal discoveries, viz., gas-discharge lamps, LEDs and Lasers. While gas-discharge and laser lights heralded in a new era of experimental and theoretical physics, LEDs, on the other hand, brought down the cost of lighting to a few cents a day, leaving behind room for future improvements.
Geissler Tubes: Electric discharge in gases was documented, as early as 1675, by the French astronomer Jean Picard while carrying a mercury barometer atop a mountain. Some thirty years later, Francis Hauksbee demonstrated the first gas discharge with a little amount of mercury placed inside a glass globe and charged with static electricity. To his exclamation, the light was bright enough for reading a book. Almost a century later, Michael Faraday and his contemporaries observed luminous gas discharge inside the rarefied atmosphere of the so-called electric egg, a prototypical gas discharge tube named so because of its oval shape.
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| Glow discharge of hydrogen gas inside a Plücker tube. The magenta color is a characteristic of the gas (or metal in the case of metal-halide lamps) used and is unique for all elements. Image Credits: Author's Camera |
In 1857, the German physicist, and an expert glassblower, Heinrich Geissler, invented a new mechanism for getting a high vacuum inside long glass tubes fitted with metal electrodes at both ends. When a suitable voltage was applied, the tubes glowed with a brilliant hue, characteristic of the gas enclosed (neon, argon, krypton). With the skills of glassblowing at hand, Geissler designed an intricate network of discharge tubes involving a series of twists, turns and hoops. Using uranium glass, he created aesthetically pleasing and alluring home decor items.
Types Of Gas Discharge Lamps: In terms of gas pressure, discharge lamps fall into two broad categories; Low-Pressure Discharge and High-Pressure Discharge. High-pressure ones can be further sub-classified into High-Intensity Discharge (HID) lamps. High-pressure discharge lamps, including the HIDs, produce intense illumination by sustaining an arc between a pair of tungsten electrodes housed within an envelope of fused quartz or alumina glass filled with noble gases and some specific metals. While the low-pressure discharge lamps operate at 0.3% of standard atmospheric pressure, the latter class operates at pressures greater than standard atmospheres or maybe less than that, but significantly higher. In terms of cathode temperature, discharge lamps can again be classified into hot cathodes and cold cathodes. While hot cathode lamps rely on the thermionic emission of electrons from the heated cathode element, cold cathodes do not require additional heating but accelerate more electrons (via secondary emission) than the former. Cold cathode devices include neon lights and cold cathode fluorescent lamps (CCFLs). Examples of hot cathode devices include ordinary fluorescent lamps (the long tubes) and mercury vapour lamps. As a side note, it is worth knowing, unlike incandescent or LED lights, gas discharge lamps (except CFLs as they are already equipped with an integrated ballast) cannot be directly plugged into the line voltage, which is too low to sustain the electric arc discharge. Hence these lamps require a ballast circuit that regulates the line voltage. Thus it requires some time, called the warm-up time, sometimes around 15 minutes, to achieve their full brightness.
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| Once upon a time, neon signs were quite popular, especially in New York and Las Vegas. Image Credits: Photo by pixabay |
Neon & Argon Tubes: When neon gas was discovered in 1898 by William Ramsay and Morris Travers, they were mesmerised by its characteristic, brilliant-red glow from a Geissler tube. Roundabout the same time, Daniel McFarlan Moore, improvising on Geissler's design, created the Moore Tubes or Moore Lamps. While using nitrogen and carbon dioxide instead of the expensive and rare neon, he obtained a bright illumination. By the 1930s, the French engineer Georges Claude introduced his neon tubes into the United States, which became the standard for decorative signs in major cities like Las Vegas and New York. Entranced by the orange glow of the neon gas, people took signboard making to a new genre of art and culture. Every shop owner from doughnut shops or lingerie competed hard for setting up the brightest, most eye-catching and resplendent signboards the world has ever seen.
Although neon is the generic term, in practice, other noble gases, like helium, argon, krypton, xenon and radon (which is radioactive), are mixed with mercury vapour. Ionised mercury shoots off a great deal of UV radiations, which are then obstructed in its path by the phosphor coating painted in the inner surface of the discharge tubes to create arrive at a wide range of available colours, viz., yellow, white, pinkish-white, green, blue, gold, etc.
Sodium Vapour: Sodium vapour lamps are of two types, i.e., low-pressure (LPS) and high-pressure (HPS). LPS lamps emit the characteristic monochromatic yellow light of sodium (d1-d2 resonance doublet, for those familiar with the term), and because of that, it has an extremely poor colour rendition index. Being a glow discharge lamp, the light from an LPS lamp is, however, less intense compared to the broader spectrum HPS lamps. However, apart from the LEDs, LPS lamps top in terms of power efficiency.
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| Start-up process of a typical Philips 35W U-shaped LPS lamp. The Penning mixture of neon and argon emits a pinkish light and is slowly replaced by the monochromatic yellow (averaging at 589 nm). Image Credits: CC BY-SA 4.0 , via Wikimedia Commons |
LPS lamps were invented in 1920 by none other than Arthur Holly Compton. The early lamps were essentially round in shape with two electrodes fitted at both ends, while a small quantity of solid sodium metal stayed at the bottom of the glass globe. When the electrodes became hot enough, they would vaporise the solid sodium metal (i.e., sublimation), and the globe would slowly start to glow yellow, corresponding to the emission lines of sodium. But early LPS lamps had one problem. The highly corrosive effects of sodium (being an alkali metal) attacked the glass, thereby reducing its overall lifetime. Later versions included a U-shaped or a straight tube made from borosilicate glass filled with neon and argon gas in a Penning mixture to act as a starter medium for activating (vaporising) the solid sodium metal. The discharge tube is thermally insulated by an outer glass envelope, with its insides coated with a reflecting layer of indium to stop the heat from radiating out.
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| Before the age of LEDs, HPS lamps were the most preferred means for street lighting, where perfect colour rendition is not strictly necessary. Image Credits: Author's Camera |
Thirty-five years after the invention of the LPS lamp, Robert L. Coble, a researcher working at the General Electric Company, developed a special-type of the arc tube, a ceramic of aluminium oxide, capable of withstanding the extremely high temperatures and pressures required for more luminous sodium light. In 1964, William Louden, Kurt Schmidt and Elmer Homonnay invented the HPS lamp. Unlike the LPS, HPS lamps consist of the alumina arc tube supported by a frame inside an evacuated glass bulb fitted with a getter to maintain the vacuum. The arc tube contains sodium-mercury amalgam and xenon as the starter gas. A high voltage pulse through the xenon produces the arc. The xenon and the mercury gives a bluish hue to the yellow sodium, thereby giving off a whitish sort of light in the process. The inside of the bulb is coated with a fluorescent substance in order to achieve further white light.
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| A mercury vapour lamp Image Credits: Wikimedia Commons |
Metal Halide (HID): Charles P. Steinmetz, in 1912, invented a distinct category of HID lamps involving mercury (i.e., a metal) mixed with halogen compounds (i.e., halides) in a fused quartz arc tube. Hence the name. Almost similar in construction to the HID mercury vapour lamps, metal halide lamps contains mercury (liquid at room temperatures) and halogens such as fluorine, chlorine, bromine and iodine, in addition to metals like sodium, potassium, lithium, rubidium, scandium, thallium, indium, dysprosium, lead, holmium, thulium, silver, and not to mention noble gases like argon or xenon. Different combinations of metal-halides are used to increase the range of available colour and also to stabilise the arc discharge. It is worth mentioning that the quality of the light obtained from Steinmetz's lamps were not that great and it was plagued by a lot of engineering difficulties as the high pressures and corrosive effects of the metal halides ruined the arc tube. In 1962, Robert Reiling solved these problems by using a quartz tube equipped with tungsten electrodes and sealed by molybdenum.
Two variations of the MH lamps that deserve mention are the Ceramic Discharge Metal-Halide (CDM or CMH) and the Hydrargyrum Medium Arc-Iodide (HMI) lamps which were invented around the 1980s. The CMH lamps use a sintered alumina arc tube, hence the name. Since both are HID lamps they have a very good colour rendition index and provide excellent illumination, they are most desired by filmmakers and production artists.
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| Space Shuttle Atlantis illuminated with xenon lights Image Credits: NASA/Kim Shiflett, Public domain, via Wikimedia Commons |
Xenon Short-Arc (HID): One of the brightest among the HID lamp types, the light emitted by a xenon arc lamp closely resembles natural sunlight. The typical arc discharge takes place between two tungsten electrodes separated by a very short distance, implying the name short-arc. Inside the xenon lamp bulb, pressures can be as high as 25 times that of standard atmospheric pressure, whereas temperatures can easily exceed 1000℃. As a result, it calls for complicated engineering to prevent the bulbs from exploding, and special care is taken not to touch the bulb with bare hands (obviously when it's not glowing) as a single minuscule drop of oil can cause uneven heating of the bulb surface, therefore resulting in a violent explosion, nothing short of a hand grenade.
Fluorescent Lamps: Fluorescent lamps are low-pressure gas discharge tubes filled with mercury vapour and other gases that emit most of their energy in the form of short-wave UV radiations converted into visible wavelengths through the phosphor coating on the inner lining of the discharge tube. Although fluorescence was known for a long time, the first practical and commercially viable mode of fluorescent lighting was not invented until the 1930s. In 1926, Edmund Germer created a prototype of the modern fluorescent lights from a mercury vapour discharge, but due to the green glow from his phosphor coating, it served no practical purpose. By 1934, a team of inventors tackled most of the engineering problems and the first white (fluorescent) lamp was developed by George Inman and others. In 1976, Edward Hammer developed the compact fluorescent lamp (CFL) which in a few years time, became the most used type of lamp before the advent of LEDs.
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| A Compact Fluorescent Light (CFL) Image Credits: Photo by pxhere.com/en/photo |
Before LEDs, fluorescent lamps and CFLs were the most power-efficient gas discharge lighting in the whole industry, apart from possessing a very good colour rendition index compared to other gas discharge lamps. The phosphor coating can be prepared according to the colour desired, such as white, green, blue, yellow, white with a bit of a pinkish or yellowish tinge, using various rare-earth elements such as europium, terbium, lanthanum, yttrium, cerium, etc. They come in various shapes and sizes, with some containing twisted tubes, circular tubes, U-tubes, T-shaped and similar others. Fluorescent lamps can be further subdivided into black-light, which converts the short-wave UV to long-wave UV and induces fluorescence to achieve artistic effects; germicidal lamps that are used in hospitals to kill germs, medical lamps for the treatment of certain skin diseases, grow lights to promote photosynthesis in absence of the sun, tanning lamps; infrared lamps; and electrodeless lamps.
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| Image Credits: https://commons.wikimedia.org/wiki/File |
Electrodeless (Induction): An induction lamp or electrodeless induction lamp is a special type of gas discharge lamp that uses an external electric or magnetic field to energise the gas atoms inside the discharge tube. As the name suggests, these lights do not require traditional electrodes and are lit externally via electromagnetic induction. A crude version of the induction lamp was conceived as early as the 1880s by a group of physicists, namely, Johann Wilhelm Hittorf, J.J. Thomson, Nikola Tesla and Peter Cooper Hewitt. In 1967, John Anderson invented the first electrodeless lamps.
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| Image Credits: https://commons.wikimedia.org/wiki/File |
Induction lamps are of three types - plasma lamps of which the Sulfur Lamp is a notable example, External Closed Core Induction Lamp, and Internal Inductor Lamp. The sulfur lamp, which is in essence a plasma lamp, was developed in 1990 by Michael Ury and Charles Wood. In sulfur lamps, a mixture of sulfur powder and noble gases like argon is excited via high-frequency microwave radiation. In High-Efficiency Plasma Lamps, metal halide salts are used. The external closed core induction lamp and the internal inductor lamp are quite similar to fluorescent lamps. In the first case, a magnet core (like a really thick ring) is wrapped at places around the discharge tube whose inner walls are coated with a phosphor and is filled with mercury amalgam. A time-varying magnetic field induces an electric current inside the discharge tube, thereby exciting the mercury atoms, and what happens after is already known. In the latter case, the magnet core goes inside the bulb, and instead of the round piece of the magnet as in the former case, is a cylindrical magnet core configured like a solenoid. Since electrodeless lamps do not contain electrodes, they last for tens of thousands of hours, i.e., more than a decade.
LED: A LED, i.e., Light Emitting Diode, is a semiconductor (solid-state) light emitting device. As we all know, a semiconductor is a type of material whose electrical conductivity lies in-between that of a conductor like copper and an insulator like glass. Due to their peculiar electrical behaviour, they offer a tactical advantage over ordinary metallic conductors. Their electrical properties are modified by a process known as doping, and therefore, can be programmed to behave in all sorts of ways, such as being sensitive to heat and light or responding only to the passage of electricity in one direction or can be used for signal processing.
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| Parts of a conventional led Image Credits: Public Domain, Wikimedia Commons |
An LED is one type of semiconductor diode that responds to the passage of electricity and emit visible light, where the colour of the light depends intrinsically on the semiconductor material. LED technology is based upon electroluminescence, i.e., the emission of visible radiation from certain solids such as silicon carbide (SiC), zinc sulphide (ZnS), boron carbide (B₄C), gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium phosphide (GaP) and similar others in presence of an electric passage. Electroluminescence in solids was first discovered in 1907 by Henry Joseph Round, who was an engineer at the Marconi lab and also a personal assistant to Guglielmo Marconi. While experimenting with a crystal of silicon carbide (carborundum) he observed that upon applying a small voltage, certain parts of the crystal gave a yellowish light, and sometimes even green, blue, and even orange. At that time Round's discovery was not of much importance as no one actually know what to do with the new discovery. In 1927, the Russian inventor, Oleg Lasov invented the first solid-state semiconductor light emitting diode that gave off a faint greenish light. In 1961, Robert Biard, and Gary Pittman invented the first infrared led (using a GaAs diode), which was already proposed by Rubin Braunstein in 1955.
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| Collection of red and blue light-emitting diodes Image Credits: pixabay |
A year later, in 1962, Nick Holonyak, an engineer from General Electric, invented the first visible-spectrum LED which gave off a red light, which has earned him the title of 'the father of the light-emitting diode'. Ten years later, another engineer, M. George Craford invented the first yellow LED. However, widescale application of LED was restricted due to the unavailability of the blue LED, which was necessary to complete the RGB spectrum and hence arrive at universal white illumination. After much effort, the blue LED was finally invented in 1995 by the Japanese engineer, Shuji Nakamura, for which he has been awarded the 2014 Nobel Prize in Physics, jointly with Isamu Akasaki and Hiroshi Amano who fabricated the most needed and extremely difficult gallium nitride (GaN) diode.
LASER: An acronym for Light Amplification by the Stimulated Emission of Radiation, a LASER is a device capable of delivering a highly monochromatic, unidirectional and coherent beam of light, having a high power-density. The concept of stimulated emission was first predicted by Albert Einstein in 1917 from a re-derivation of Planck's law of blackbody radiation. In a typical black-body chamber at a constant temperature, there exists an equilibrium between matter and radiation from the spontaneous absorption and emission of photons by atoms. Einstein observed that the said equilibrium can not be explained via spontaneous emission and argued about the possibility of something called stimulated emission. Unfortunately, the concept of stimulated emission is beyond the scope of the present article. At present, without any loss of generality, it is best described as a process of producing a photon from a previously excited atom as it transitions to its ground state.
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| That is some really powerful laser beam protruding from ESO's VLT. The telescope sees it as an artificial star and calibrates itself with atmospheric turbulence Image Credits: ESO/G. Hüdepohl (www.atacamaphoto.com) |
Before lasers, stimulated emission was used for microwave amplification. Hence the name MASER or Microwave Amplification by the Stimulated Emission of Radiation. It was independently discovered by Joseph Weber, C.H. Townes, James P. Gordon, Herbert J. Zeiger, Nikolay Basov, and Aleksandr Prokhorov. Soon they realised that an optical maser, i.e., a maser, operating in the visible frequencies is indeed possible. So, the first functioning LASER was developed in 1960 by Theodore H. Maiman, which opened up previously impossible branches of physics, such as quantum optics, quantum electronics, laser cooling, inertial confinement fusion, quantum chemistry, quantum computing, including spectrometry, interferometry, radar and what not.
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| Lasers emit an intense beam of visible radiation which travel in a straight line. The beam remains invisible unless it is obstructed by some material surface or if it falls directly on our eyes. In this photograph, the beam is visible due to Rayleigh scattering Image Credits: Photo by unsplash.com |
Based on their construction and mechanism of action, there is a wide range of laser types. These include gas lasers, chemical lasers, dye lasers, metal vapour lasers, solid-state lasers, fibre lasers, free-electron lasers and some other types. Lasers operate through a wide range of the electromagnetic spectrum, from x rays to far-infrared. Other than the one shown in the above photograph, there are red, green, orange, yellow, blue, etc.
Future Of Lighting: As per recent technological advancements, it seems that within a few years time, LED will lead the global market. We don't use incandescent (filament-type) light bulbs anymore than traditional gas discharge lamps except for specific purposes. Evolution never runs backwards, and neither does science and technology. We might discover something more exotic than what our forefathers could ever conceive in their wildest dreams. If we are willing to venture out further, we could try using some of the non-conventional sources such as bioluminescent mushrooms (maybe a whole bioluminescent tree via genetic engineering), fireflies and marine planktons, chemiluminescent reactions or redefine architecture to reduce the need for artificial lights, at least during the daytime.
Author's Note: The story of electricity with all the eureka moments and every swear word uttered when a bulb went out is too vast to condense into a single book, let alone my mere two thousand word articles. However, in good faith, I suppose my article would be enough for procuring a general idea about the evolution of artificial lights. Those who are hungry for more can browse through the list of books and online sources at the end of every article, stuff that I personally think is worth giving a read.
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