General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
The Michelson interferometer is a common configuration for optical interferometry and was invented by Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms; each of those light beams is reflected back toward the beamsplitter which combines their amplitudes using the superposition principle. The resulting interference pattern, not directed back toward the source is directed to some type of photoelectric detector or camera. For different applications of the interferometer, the two light paths can be with different lengths or incorporate optical elements or materials under test; the Michelson interferometer is employed in many scientific experiments and became well known for its use by Albert Michelson and Edward Morley in the famous Michelson–Morley experiment in a configuration which would have detected the earth's motion through the supposed luminiferous aether that most physicists at the time believed was the medium in which light waves propagated. The null result of that experiment disproved the existence of such an aether, leading to the special theory of relativity and the revolution in physics at the beginning of the twentieth century.
In 2015, another application of the Michelson interferometer, LIGO, made the first direct observation of gravitational waves. That observation confirmed an important prediction of general relativity, validating the theory's prediction of space-time distortion in the context of large scale cosmic events. A Michelson interferometer consists minimally of mirrors M1 & M2 and a beam splitter M. In Fig 2, a source S emits light that hits the beam splitter surface M at point C. M is reflective, so part of the light is transmitted through to point B while some is reflected in the direction of A. Both beams recombine at point C' to produce an interference pattern incident on the detector at point E. If there is a slight angle between the two returning beams, for instance an imaging detector will record a sinusoidal fringe pattern as shown in Fig. 3b. If there is perfect spatial alignment between the returning beams there will not be any such pattern but rather a constant intensity over the beam dependent on the differential pathlength.
Fig. 2 shows use of a coherent source. Narrowband spectral light from a discharge or white light can be used, however to obtain significant interference contrast it is required that the differential pathlength is reduced below the coherence length of the light source; that can be only micrometers for white light. If a lossless beamsplitter is employed one can show that optical energy is conserved. At every point on the interference pattern, the power, not directed to the detector at E is rather present in a beam returning in the direction of the source; as shown in Fig. 3a and 3b, the observer has a direct view of mirror M1 seen through the beam splitter, sees a reflected image M'2 of mirror M2. The fringes can be interpreted as the result of interference between light coming from the two virtual images S'1 and S'2 of the original source S; the characteristics of the interference pattern depend on the nature of the light source and the precise orientation of the mirrors and beam splitter. In Fig. 3a, the optical elements are oriented so that S'1 and S'2 are in line with the observer, the resulting interference pattern consists of circles centered on the normal to M1 and M'2.
If, as in Fig. 3b, M1 and M'2 are tilted with respect to each other, the interference fringes will take the shape of conic sections, but if M1 and M'2 overlap, the fringes near the axis will be straight and spaced. If S is an extended source rather than a point source as illustrated, the fringes of Fig. 3a must be observed with a telescope set at infinity, while the fringes of Fig. 3b will be localized on the mirrors. White light is difficult to use in a Michelson interferometer. A narrowband spectral source requires careful attention to issues of chromatic dispersion when used to illuminate an interferometer; the two optical paths must be equal for all wavelengths present in the source. This requirement can be met if both light paths cross an equal thickness of glass of the same dispersion. In Fig. 4a, the horizontal beam crosses the beam splitter three times, while the vertical beam crosses the beam splitter once. To equalize the dispersion, a so-called compensating plate identical to the substrate of the beam splitter may be inserted into the path of the vertical beam.
In Fig. 4b, we see using a cube beam splitter equalizes the pathlengths in glass. The requirement for dispersion equalization is eliminated by using narrowband light from a laser; the extent of the fringes depends on the coherence length of the source. In Fig. 3b, the yellow sodium light used for the fringe illustration consists of a pair of spaced lines, D1 and D2, implying that the interference pattern will blur after several hundred fringes. Single longitudinal mode lasers are coherent and can produce high contrast interference with differential pathlengths of millions or billions of wavelengths. On the other hand, using white light, the central fringe is sharp, but away from the central fringe the fringes are colored and become indistinct to the eye. Early experimentalists attempting to detect the earth's velocity relative to the supposed lumini
The interferometric visibility quantifies the contrast of interference in any system which has wave-like properties, such as optics, quantum mechanics, water waves, or electrical signals. Two or more waves are combined and as the phase difference between them varies, the power or intensity of the resulting wave oscillates, forming an interference pattern; the pattern may be visible all at once because the phase difference varies as a function of space, as in a 2-slit experiment. Alternately, the phase difference may be manually controlled by the operator, for example by adjusting a vernier knob in an interferometer; the ratio of the size or amplitude of these oscillations to the sum of the powers of the individual waves is defined as the visibility. The interferometric visibility gives a practical way to measure the coherence of two waves. A theoretical definition of the coherence is given by the degree of coherence, using the notion of correlation. In linear optical interferometers, interference manifests itself as intensity oscillations over time or space called fringes.
Under these circumstances, the interferometric visibility is known as the "Michelson visibility" or the "fringe visibility." For this type of interference, the sum of the intensities of the two interfering waves equals the average intensity over a given time or space domain. The visibility is written as: ν = A / I ¯, in terms of the amplitude envelope of the oscillating intensity and the average intensity: A = / 2, I ¯ = / 2. So it can be rewritten as: ν = I max − I min I max + I min, where Imax is the maximum intensity of the oscillations and Imin the minimum intensity of the oscillations. If the two optical fields are ideally monochromatic point sources of the same polarization the predicted visibility will be ν = 2 I 1 I 2 I 1 + I 2, where I 1 and I 2 indicate the intensity of the respective wave. Any dissimilarity between the optical fields will decrease the visibility from the ideal. In this sense, the visibility is a measure of the coherence between two optical fields. A theoretical definition for this is given by the degree of coherence.
This definition of interference directly applies to the interference of water waves and electric signals. Since the Schrödinger equation is a wave equation and all objects can be considered waves in quantum mechanics, interference is ubiquitous; some examples: Bose–Einstein condensates can exhibit interference fringes. Atomic populations show interference in a Ramsey interferometer. Photons, electrons and molecules have exhibited interference in double-slit interferometers. Degree of coherence Interferometry Optical interferometry List of types of interferometers Hong–Ou–Mandel effect Stedman Review of the Sagnac Effect
A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, neodymium, praseodymium and holmium. They are related to doped fiber amplifiers. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can provide gain and thus serve as gain media for a fiber laser; the advantages of fiber lasers over other types include: Light is coupled into a flexible fiber: The fact that the light is in a fiber allows it to be delivered to a movable focusing element. This is important for laser cutting and folding of metals and polymers. High output power: Fiber lasers can have active regions several kilometers long, so can provide high optical gain, they can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. High optical quality: The fiber's waveguiding properties reduce or eliminate thermal distortion of the optical path producing a diffraction-limited, high-quality optical beam.
Compact size: Fiber lasers are compact compared to rod or gas lasers of comparable power, because the fiber can be bent and coiled to save space. Reliability: Fiber lasers exhibit high temperature and vibrational stability, extended lifetime, maintenance-free turnkey operation. High peak power and nanosecond pulses enable effective engraving; the additional power and better beam quality provide faster cutting speeds. Lower cost of ownership. Fiber lasers are now being used to make high-performance surface-acoustic wave devices; these lasers raise throughput and lower cost of ownership in comparison to older solid-state laser technology. Fiber laser can refer to the machine tool that includes the fiber resonator. Applications of fiber lasers include material processing telecommunications, spectroscopy and directed energy weapons. Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber. Another type is the single longitudinal mode operation of ultra narrow distributed feedback lasers where a phase-shifted Bragg grating overlaps the gain medium.
Fiber lasers are pumped by other fiber lasers. Q-switched pulsed fiber lasers offer a compact, electrically efficient alternative to Nd:YAG technology. Many high-power fiber lasers are based on double-clad fiber; the gain medium forms the core of the fiber, surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer; the outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, allows the conversion of pump light with low brightness into a much higher-brightness signal; as a result, fiber lasers and amplifiers are referred to as "brightness converters." There is an important question about the shape of the double-clad fiber. The design should allow the core to be small enough to support only a few modes, it should provide sufficient cladding to confine the core and optical pump section over a short piece of the fiber.
Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in 2001 to >20 kW. Commercial single-mode lasers have reached 10 kW in CW power. In 2014 a combined beam fiber laser demonstrated power of 30 kW; when linearly polarized light is incident to a piece of weakly birefringent fiber, the polarization of the light will become elliptically polarized in the fiber. The orientation and ellipticity of the final light polarization is determined by the fiber length and its birefringence. However, if the intensity of the light is strong, the non-linear optical Kerr effect in the fiber must be considered, which introduces extra changes to the light polarization; as the polarization change introduced by the optical Kerr effect depends on the light intensity, if a polarizer is put behind the fiber, the light intensity transmission through the polarizer will become light intensity dependent.
Through appropriately selecting the orientation of the polarizer or the length of the fiber, an artificial saturable absorber effect with ultra-fast response could be achieved in such a system, where light of higher intensity experiences less absorption loss on the polarizer. The NPR technique makes use of this artificial saturable absorption to achieve the passive mode locking in a fiber laser. Once a mode-locked pulse is formed, the non-linearity of the fiber further shapes the pulse into an optical soliton and the ultrashort soliton operation is obtained in the laser. Soliton operation is a generic feature of the fiber lasers mode-locked by this technique and has been intensively investigated. Semiconductor saturable absorbers were used for laser mode-locking as early as 1974 when p-type germanium is used to mode lock a CO2 laser which generated pulses ~500 ps. Modern SESAMs are III-V semiconductor single quantum well or multiple quantum wells grown on semiconductor distr
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having this common property may be termed dispersive media. Sometimes the term chromatic dispersion is used for specificity. Although the term is used in the field of optics to describe light and other electromagnetic waves, dispersion in the same sense can apply to any sort of wave motion such as acoustic dispersion in the case of sound and seismic waves, in gravity waves, for telecommunication signals along transmission lines or optical fiber. In optics, one important and familiar consequence of dispersion is the change in the angle of refraction of different colors of light, as seen in the spectrum produced by a dispersive prism and in chromatic aberration of lenses. Design of compound achromatic lenses, in which chromatic aberration is cancelled, uses a quantification of a glass's dispersion given by its Abbe number V, where lower Abbe numbers correspond to greater dispersion over the visible spectrum.
In some applications such as telecommunications, the absolute phase of a wave is not important but only the propagation of wave packets or "pulses". The most familiar example of dispersion is a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths. However, dispersion has an effect in many other circumstances: for example, group velocity dispersion causes pulses to spread in optical fibers, degrading signals over long distances. Most chromatic dispersion refers to bulk material dispersion, that is, the change in refractive index with optical frequency. However, in a waveguide there is the phenomenon of waveguide dispersion, in which case a wave's phase velocity in a structure depends on its frequency due to the structure's geometry. More "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure, whether or not the waves are confined to some region. In a waveguide, both types of dispersion will be present, although they are not additive.
For example, in fiber optics the material and waveguide dispersion can cancel each other out to produce a zero-dispersion wavelength, important for fast fiber-optic communication. Material dispersion can be a desirable or undesirable effect in optical applications; the dispersion of light by glass prisms is used to construct spectrometers and spectroradiometers. Holographic gratings are used, as they allow more accurate discrimination of wavelengths. However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes and photographic objectives; the phase velocity, v, of a wave in a given uniform medium is given by v = c n where c is the speed of light in a vacuum and n is the refractive index of the medium. In general, the refractive index is some function of the frequency f of the light, thus n = n, or alternatively, with respect to the wave's wavelength n = n; the wavelength dependence of a material's refractive index is quantified by its Abbe number or its coefficients in an empirical formula such as the Cauchy or Sellmeier equations.
Because of the Kramers–Kronig relations, the wavelength dependence of the real part of the refractive index is related to the material absorption, described by the imaginary part of the refractive index. In particular, for non-magnetic materials, the susceptibility χ that appears in the Kramers–Kronig relations is the electric susceptibility χe = n2 − 1; the most seen consequence of dispersion in optics is the separation of white light into a color spectrum by a prism. From Snell's law it can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material. Since that refractive index varies with wavelength, it follows that the angle that the light is refracted by will vary with wavelength, causing an angular separation of the colors known as angular dispersion. For visible light, refraction indices n of most transparent materials decrease with increasing wavelength λ: 1 < n < n < n, or alternatively: d n d λ < 0. In this case, the medium is said to have normal dispersion.
Whereas, if the index increases with increasing wavelength, the medium is said to have anomalous dispersion. At the interface of such a material with air or vacuum, Snell's law predicts that light incident at an angle θ to the normal will be refracted at an angle arcsin. Thus, blue light, with a higher refractive index, will be bent more than red light, resulting in the well-known rainbow pattern. Another consequence of dispersion manifests itself as a temporal effect; the formula v = c/n calculates the
In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves, for example, radio, surface water waves, gravity waves, or matter waves; the resulting images or graphs are called interferograms. The principle of superposition of waves states that when two or more propagating waves of same type are incident on the same point, the resultant amplitude at that point is equal to the vector sum of the amplitudes of the individual waves. If a crest of a wave meets a crest of another wave of the same frequency at the same point the amplitude is the sum of the individual amplitudes—this is constructive interference. If a crest of one wave meets a trough of another wave the amplitude is equal to the difference in the individual amplitudes—this is known as destructive interference.
Constructive interference occurs when the phase difference between the waves is an multiple of π, whereas destructive interference occurs when the difference is an odd multiple of π. If the difference between the phases is intermediate between these two extremes the magnitude of the displacement of the summed waves lies between the minimum and maximum values. Consider, for example, what happens when two identical stones are dropped into a still pool of water at different locations; each stone generates a circular wave propagating outwards from the point where the stone was dropped. When the two waves overlap, the net displacement at a particular point is the sum of the displacements of the individual waves. At some points, these will be in phase, will produce a maximum displacement. In other places, the waves will be in anti-phase, there will be no net displacement at these points. Thus, parts of the surface will be stationary—these are seen in the figure above and to the right as stationary blue-green lines radiating from the centre.
Interference of light is a common phenomenon that can be explained classically by the superposition of waves, however a deeper understanding of light interference requires knowledge of wave-particle duality of light, due to quantum mechanics. Prime examples of light interference are the famous double-slit experiment, laser speckle, anti-reflective coatings and interferometers. Traditionally the classical wave model is taught as a basis for understanding optical interference, based on the Huygens–Fresnel principle; the above can be demonstrated in one dimension by deriving the formula for the sum of two waves. The equation for the amplitude of a sinusoidal wave traveling to the right along the x-axis is W 1 = A cos where A is the peak amplitude, k = 2 π / λ is the wavenumber and ω = 2 π f is the angular frequency of the wave. Suppose a second wave of the same frequency and amplitude but with a different phase is traveling to the right W 2 = A cos where φ is the phase difference between the waves in radians.
The two waves will superpose and add: the sum of the two waves is W 1 + W 2 = A. Using the trigonometric identity for the sum of two cosines: cos a + cos b = 2 cos cos , this can be written W 1 + W 2 = 2 A cos cos ; this represents a wave at the original frequency, traveling to the right like the components, whose amplitude is proportional to the cosine of φ / 2. Constructive interference: If the phase difference is an multiple of π: φ = …, − 4 π, − 2 π, 0, 2 π, 4 π, …
A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb; the outer bulb may be clear or coated with a phosphor. Mercury vapor lamps are more energy efficient than incandescent and most fluorescent lights, with luminous efficacies of 35 to 65 lumens/watt, their other advantages are a long bulb lifetime in the range of 24,000 hours and a high intensity, clear white light output. For these reasons, they are used for large area overhead lighting, such as in factories and sports arenas as well as for streetlights. Clear mercury lamps produce white light with a bluish-green tint due to mercury's combination of spectral lines; this is not flattering to human skin color, so such lamps are not used in retail stores. "Color corrected" mercury bulbs overcome this problem with a phosphor on the inside of the outer bulb that emits white light, offering better color rendition.
They operate at an internal pressure of around one atmosphere and require special fixtures, as well as an electrical ballast. They require a warm-up period of 4 – 7 minutes to reach full light output. Mercury vapor lamps are becoming obsolete due to the higher efficiency and better color balance of metal halide lamps. Charles Wheatstone observed the spectrum of an electric discharge in mercury vapor in 1835, noted the ultraviolet lines in that spectrum. In 1860, John Thomas Way used arc lamps operated in a mixture of air and mercury vapor at atmospheric pressure for lighting; the German physicist Leo Arons studied mercury discharges in 1892 and developed a lamp based on a mercury arc. In February 1896 Herbert John Dowsing and H. S. Keating of England patented a mercury vapour lamp, considered by some to be the first true mercury vapour lamp; the first mercury vapor lamp to achieve widespread success was invented in 1901 by American engineer Peter Cooper Hewitt. Hewitt was issued U. S. Patent 682,692 on September 17, 1901.
In 1903, Hewitt created an improved version that possessed higher color qualities which found widespread industrial use. The ultraviolet light from mercury vapor lamps was applied to water treatment by 1910; the Hewitt lamps used a large amount of mercury. In the 1930s, improved lamps of the modern form, developed by the Osram-GEC company, General Electric company and others led to widespread use of mercury vapor lamps for general lighting; the mercury in the tube is a liquid at normal temperatures. It needs to be ionized before the lamp can produce its full light output. To facilitate starting of the lamp, a third electrode is mounted near one of the main electrodes and connected through a resistor to the other main electrode. In addition to the mercury, the tube is filled with argon gas at low pressure; when power is applied, there is sufficient voltage to ionize the argon and strike a small arc between the starting electrode and the adjacent main electrode. When ions and free electrons have been introduced into the arc tube, an arc initiates between the two main electrodes.
The heat from this arc vaporizes the liquid mercury inside the lamp which radiates green, yellow and ultraviolet emission lines when ionized. Continued vaporization of the liquid mercury increases the arc tube pressure to between 2 and 18 bar, depending on lamp size; the increase in pressure results in further brightening of the lamp. The entire warm-up process takes 4 to 7 minutes; some bulbs include a thermal switch which shorts the starting electrode to the adjacent main electrode, extinguishing the starting arc once the main arc strikes. The mercury vapor lamp is a negative resistance device; this means its resistance decreases as the current through the tube increases. So if the lamp is connected directly to a constant-voltage source like the power lines, the current through it will increase until it destroys itself. Therefore, it requires a ballast to limit the current through it. Mercury vapor lamp ballasts are similar to the ballasts used with fluorescent lamps. In fact, the first British fluorescent lamps were designed to operate from 80-watt mercury vapor ballasts.
There are self-ballasted mercury vapor lamps available. These lamps use a tungsten filament in series with the arc tube both to act as a resistive ballast and add full spectrum light to that of the arc tube. Self-ballasted mercury vapor lamps can be screwed into a standard incandescent light socket supplied with the proper voltage. A closely related lamp design called the metal halide lamp uses various compounds in an amalgam with the mercury. Sodium iodide and scandium iodide are in use; these lamps can produce much better quality light without resorting to phosphors. If they use a starting electrode, there is always a thermal shorting switch to eliminate any electrical potential between the main electrode and the starting electrode once the lamp is lit.. More modern metal halide systems do not use a separate starting electrode. Self-ballasted lamps are mercury vapor lamps with a filament inside connected in series with the arc tube that functions as an electrical ballast; this is the only kind of mercury vapor lamp that can be connected directly to the mains without an external ballast.
These lamps have only the same or higher efficiency than incandescent lamps of similar size