Low pressure sodium vapor (LPS) lights are a specific type of gas-discharge light (also known as a high intensity discharge, HID or arc light). The bulb principally contains solid sodium metal inside a borosilicate glass tube that vaporizes once the lamp is turned on. During start (while the sodium is still in solid form) the lamp emits a dim reddish/pink glow. Once the metal is vaporized the emissions become the characteristic bright yellow associated with sodium vapor lamps. The spectrum of visible emissions from an LPS light is actually very close together (589 and 589.6 nm, virtually monochromatic) resulting in the colors of illuminated objects being nearly indistinguishable.
Sodium vapor lighting has been around since the middle of the 20th century (in commercial production since the 1930s) and it generally represents a high efficiency way to provide lighting over a vast area. Sodium lights operate in a range where the human eye is very sensitive and so there is less power required to achieve the same lighting effect. For this reason they are very efficient. Additionally, despite their long warm-up period (5-10 minutes), low pressure sodium lamps will re-ignite immediately in the event of a power interruption. It is particularly useful for outdoor lighting where energy efficiency is at a premium (such as with municipalities lighting the streets or other common areas like parking lots.) LPS and HPS lights are much more efficient as well as longer lasting than incandescent bulbs, many fluorescent bulbs, and most high intensity discharge lamps in general. It is only recently with the advent of affordable and prevalent LED lighting that they are being consistently surpassed in terms of energy efficiency and lifespan.
How does the pressure inside a mercury vapor lamp affect its spectral distributi…
Spectral distribution of a black body at different temperatures More information about incandescent lamps. 2. Gas discharge lamps. When the lamp is turned on an electric discharge takes place and a current starts to flow through the vapor in the lamp; this causes the gas to heat up and it turns into a plasma (atoms lose a electron and the vapor becomes ionized). The plasma conducts the electric current, heats up and starts emitting light. Sometimes neon is used to startup the heating process in case of light sources based on metal vapors. A historic overview and explanation of how such arc lamps work is presented here, through this link. High or low pressure pressure mercury lamp In a mercury (Hg) lamp an arc goes through ionized mercury gas. At low pressure it shows the mercury atomic emission lines; at higher pressure the peaks become broader and a high pressure mercury lamp emits white (almost sun)light. Both high pressure and low pressure lamps are used in spectroscopy. For UV-Vis a high pressure lamp is needed because of the broad light spectrum it emits. Spectral distribution of low pressure (left) and high pressure Hg lamp This link from the Edison Techcenter shows more details including how the mercury lamp works plus its history. A video on the mercury lamp (7min):
Deuterium lampA deuterium lamp is a gas discharge lamp and is often used as a UV source. It emits in general somewhere between the 160-450nm: Spectral distribution of a Deuterium lamp Xenon lampThe xenon lamp is not often used in commercial UV/Vis-instruments.In a Xenon lamp an arc is made through ionized xenon gas under high pressure. The high pressure gives the lamp high efficiency. It has a high intensity and the color is close to that of sunlight. More information through this link. Spectral distribution of a xenon flash light Figure. 3. Light Emitting Diodes (LED) A LED is a semi conductor which can emit light. LEDs can be used as a light source in the spectral region of 375-1000nm. Most LED are narrow banded (20-50 nm) but LEDs are cheap, have a long lifetimes and a small environmental impact.Examples are: Gallium Aluminum Arsenide (900 nm) Gallium Aresenic Phosphide (650 nm) Gallium Nitride (465 nm) Indium Gallium Nitride (450nm) White LED: blue LED strikes phosphor (400-800 nm)
More information about LEDs can be found here. Additional informative and interesting material: A very nice website explaining how the different lamps work, their history and the components used in lamps can be found here. Mary Kate: ch310 chapter 7comp of optical instruments. Slides 1-4 introduction Skoog(8ed) Chapter 25A-2 Spectroscopic Sources A brief overview on light sources from the Colorado University. Comment: The wavelengths given for the light sources are not absolute. The light sources emit a distribution of different wavelengths. In short this means that light sources have an optimum wavelength and a region of wavelengths around the optimum for which they can be used. It does not mean that they do not emit other wavelengths. The optima and intensities of the sources can differ over their age.
Presented in Figure 3 are spectral distribution curves demonstrating the relative amounts of energy versus wavelength for several different sources of white light (comprised of a mixture containing all or most of the colors in the visible spectrum). The red curve represents the relative energy of tungsten light over the entire visible spectrum. As is apparent from examining the figure, the energy of tungsten light increases as wavelength increases. This effect dramatically influences the average color temperature of the resultant light, especially when it is compared to that of natural sunlight and fluorescent light (the mercury vapor lamp). The spectrum represented by a yellow curve profiles the visible light distribution from the natural sunlight spectrum sampled at noon. Under normal circumstances, sunlight contains the greatest amount of energy, but the curves illustrated in Figure 3 have all been normalized to the tungsten spectrum in order to ease comparison. The dark blue spectral curve is characteristic of a mercury arc lamp, and exhibits some notable differences from the tungsten and natural sunlight spectra. Several energy peaks are present in the discharge arc lamp spectrum that occur a result of superposed individual line spectra originating from the mercury vapor.
Tungsten lamps tend to suffer several drawbacks, such as a decreased intensity with age and a blackening of the inside envelope surface as evaporated tungsten is slowly deposited onto the glass. The color temperature and luminance of tungsten lamps vary with the applied voltage, but average values for color temperature range from about 2200 K to 3400 K. The surface temperature of an active tungsten filament is very high, typically averaging 2,550 degrees Celsius for a standard 100-watt commercial light bulb. In some cases, tungsten bulb envelopes are filled with the Noble gases krypton or xenon (inert fill gas) as an alternative to creating a vacuum in order to protect the hot tungsten filament. These gases improve the efficiency of incandescent lamps because they reduce the amount of evaporated tungsten that is deposited on the interior of the surrounding glass vessel.
The glass tube of a common fluorescent lamp is coated with phosphor on the inside surface of the glass, and the tube is filled with mercury vapor at very low pressure (see Figure 5). An electric current is applied between the electrodes at the ends of the tube, producing a stream of electrons that flow from one electrode to the other. When electrons from the stream collide with mercury atoms, they excite electrons within the atoms to a higher energy state. This energy is released in the form of ultraviolet radiation when electrons in the mercury atoms return to the ground state. The ultraviolet radiation subsequently energizes the internal phosphor coating, causing it to emit the bright white light that we observe from fluorescent lights. Fluorescent lamps are about two to four times more efficient at emitting visible light, produce less waste heat, and typically last ten to twenty times longer than incandescent lamps.
A unique feature of fluorescent light sources is that they generate a series of wavelengths that are often concentrated into narrow bands termed line spectra. As a consequence, these sources do not produce the continuous spectrum of illumination that is characteristic of incandescent sources. A good example of a (almost exclusively) single wavelength source of non-incandescent visible light is the sodium-vapor lamps commonly employed in street lighting. These lamps emit a very intense yellow light, with over 95 percent of the emission being composed of 589-nanometer light and virtually no other wavelengths present in the output. It is possible to design gas-discharge lamps that will emit a nearly continuous spectrum in addition to the line spectra inherent in most of these lamps. The most common technique is to coat the inside surface of the tube with phosphor particles, which will absorb radiation emitted by the glowing gas and convert it into a broad spectrum of visible light ranging from blue to red.
Arc discharge lamps, filled with gases such as mercury vapor and xenon, are favored sources of illumination for some specialized forms of fluorescence microscopy. A typical arc lamp is 10-100 times brighter than tungsten-based counterparts and can provide brilliant monochromatic illumination when combined with specially coated dichromatic interference filters. Unlike tungsten and tungsten-halogen lamps, arc lamps do not contain a filament, but rather, depend on ionization of the gaseous vapor though a high-energy arc discharge between two electrodes to produce their intense light. In general, arc lamps have an average lifetime of about 100-200 hours, and most external power supplies are equipped with a timer that enables the microscopist to monitor how much time has elapsed. Mercury arc lamps (often referred to as burners; see the mercury and xenon lamps illustrated in Figure 6) range in power from 50 to 200 watts and usually consist of two electrodes sealed under high mercury vapor pressure in a quartz glass envelope.
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