Gas-Based Light Sources Incadescent and Halogen Bulbs Incandescent bulbs produce light by heating up a wire filament to a high temperature, by passing an electric current through the filament. As the wire heats up it begins to glow. The filament is protected from oxidation by placing it in a quartz or glass bulb that is evacuated of filled with an inert gas. These bulbs are essentially black-body emitters. A black body at a constant temperature emits electromagnetic radiation according to Planck’s law, as shown in the figure below.

"Black body" by Darth Kule - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Black_body.svg#/media/File:Black_body.svg

According to Planck’s Law, the spectral radiance (power emitted per unit area per unit solid angle) at wavelength λ of a black body at temperature T is given by

𝑅 𝜆,𝑇 =2ℎ𝑐!

𝜆!1

𝑒!!/!!!! − 1

where h is Planck’s constant, c is the speed of light and kB is Boltzmann’s constant. The figure also shows the classical Rayleigh–Jeans law and its ultraviolet catastrophe, which inspired the formulation of quantum mechanics. In a halogen lamp, a small amount of halogen gas such as iodine or bromine is added to the bulb. The halogen initiates a chemical process that re-deposits metal vapor onto the filament, extending its life. A halogen lamp can also be operated at a higher temperature than a regular incandescent lamp. What would you expect this to do to it’s appearance?

Gas-discharge lamps When a strong electrical potential is applied across a dilute gas, it is possible to produce an electrical discharge through the ionized gas. As the gas is ionized, free electrons are accelerated by the electrical field and collide with other gas atoms. Some of the bound electrons in the atom are excited to higher energy states during these collisions. When these excited atoms relax back to a lower energy state, they emit a photon of a characteristic wavelength, corresponding to this energy difference. If the emitted energy is in the ultraviolet, the lamp may contain a fluorescent coating that absorbs this ultraviolet radiation and emit light in the visible region. Gas discharge lamps typically use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases, and may also contain other elements such as mercury, sodium, and metal halides. Fluorescent bulbs are gas-discharge lamps. Solid-state Light Sources Light Emitting Diode (LED) light sources LED light sources typically use semiconductors or organic (polymer) materials and rely on the phenomena of electroluminescence, where the passage of an electrical current through a solid results in the emission of light. The energy of the light emission depends on the material properties of the semiconductor or polymer, in particular its band gap. The band gap of a material is the difference in energy between the conduction band (empty states) and the valence band (filled states) of the solid. The light emitted from a single LED is largely monochromatic (broadened by the spread in populations of the carriers at the band edges) and is produced by spontaneous emission.

Energy levels in a solid (from Solid State Physics: An Introduction, Philip Hoffmann, Wiley-VCH (2008))

 

In the early days of LED light sources, only infrared and red LEDs were available. However, improved materials science enabled the creations of LEDs from materials with increasingly larger band gaps. The 2014 Nobel Prize in Physics was awarded for the development of the blue LED, since it enabled the creation of true white light LED light sources. LED can also be placed inside bulbs coated in a phosphor that allows the absorption of light of one color and its re-emission as a different color to change the spectrum of the light output. Laser LEDs In some types of LEDs, if the injected current is increased even further, it is possible to invert the population of the energy levels such that the excited states are more highly populated than the ground state. This inversion enhances the rate of stimulated emission, which significantly increases the light output. The LED is also placed inside a cavity, which makes the beam monochromatic by only allowing the photons on resonance to interfere constructively.

Semiconductor Band Gap (from Introduction to Solid State Physics, Charles Kittel, Wiley (2005))