Patent Application
20060049762
1. Field of Invention
This invention relates most broadly to the field of arc discharge lamps and more specifically to alkali-gas high-pressure arc discharge lamps.
2. Description of Prior Art
Near-infrared night-vision systems often employ light sources to provide increased viewing range or additional lighting when the ambient light levels are too low. The most common sources are: gallium arsenide devices such as lasers and light emitting diodes, tungsten-halogen lamps, and xenon discharge lamps. The later two sources can be filtered to remove visible light from the beam in order to provide covert surveillance.
The three light sources discussed above suffer from significant disadvantages. Gallium arsenide devices are efficient but costly at high power levels. In addition, the lasers with coherent output near 0.9 nm present the possibility of unintended retinal damage. The lamps generate substantial incoherent light, and while eye-safe and less costly, suffer from poor efficiency in the near-infrared. Most of the light energy generated by the lamps is visible (not used). In the case of the xenon lamp significant energy is also lost to ultraviolet emissions (also not used.) Xe high-pressure short arc lamps are preferred over tungsten lamps when eye-safety is an issue due to their high brightness (radiant exitance), which allows the formation, with optics, of well collimated beams. There are currently two manufacturers of xenon short arc lamp systems for night-vision known to us, Xenonics (Carlsbad, Calif.) and Peakbeam (Edgemont, Pa.). The low efficiency of the lamps leads to large (heavy) batteries and limited lamp life due to high temperature operation and reactive plasma constituents. Clearly, a high-efficiency, eye-safe, high brightness, near-infrared lamp for night-vision applications is desirable.
The alkali elements L (lithium), Na (sodium), K (potassium), Rb (rubidium), and Cs (cesium) have certain properties that make them ideal candidates for night-vision discharge-lamps. Among these properties are: 1) low ionization potentials, 2) strong resonance transitions at reasonably long wavelengths, 3) very high double ionization potentials, 4) high vapor pressures at modest temperatures, 5) and low reactivity with certain practically significant dielectrics and metals.
High pressure Cs and Rb short-arc lamps were investigated for night-vision applications in the mid 1960s (H. S. Strauss et al, “Compact arc near infra-red radiation sources,” DITECH report # AD 821794 (1971)) and more recently by Peakbeam Corporation (private communication with W. Mcmanus CEO (2004)). In addition, K & C Technologies Inc. has conducted pulsed plasma experiments with low temperature cells with attached sapphire windows. To date, all experiments indicate that the alkali plasma will bloom radially out from the center-line between the cathode and anode and, although emitting copius near-IR radiation, the lamps will not have high brightness, especially at the cathode tip.
The alkali lamps described here for application as night-vision illuminators, by virtue of the physical properties of alkalis and selected lamp materials, will emit substantially more power into the near-infrared than either the xenon discharge or the tungsten halide lamps operated at the same electrical input power level. In addition, the lamps will operate with high radiant exitance. The advantages are that near-infrared illumination systems using alkali lamps will operate at lower temperatures with less power consumption and longer lamp life at any desired light level than systems using xenon or tungsten lamps.
The night-vision illuminator lamp is a hermetically sealed system comprised of a transparent dielectric container. Two metal electrodes in hermetic contact with the container are provided to allow electric current to pass from one electrode to the other through the dielectric container. The container is partially filled with an alkali mixture, and an inert gas, e.g., xenon. The alkali mixture may also be present in an easily dissociated mixture of high vapor pressure alkali salts. When voltage is applied to the electrodes the inert gas breaks down and current begins to flow causing the lamp to heat and the alkali metal or salt mixture to vaporize. Alternatively or in addition to the inert gas, an external pre-heater would provide enough alkali vapor pressure for lamp discharge ignition. The alkali vapor is ionized by electron impact and singly ionized alkali atoms become the dominant ionic species in the discharge. Due to the physical properties of the alkalis listed above the visible emissions from the lamp are suppressed in favor of near IR emissions as will be described in detail below. The proper selection of electrode materials inhibits the arc from blooming and the brightest radiance is located at the cathode tip.
These and other objects, features and advantages of the invention will be more readily apparent from the following detailed descriptions of the specific embodiments and physical characteristics of the invention.
A second embodiment depicted in
Cs, Rb, and K have ionization potentials of 3.9, 4.2, and 4.4 eV respectively. Each will be ionized when boiled off (atom surface collision) a hot tungsten surface (W work function=4.5 eV). One or more of these alkalis are considered constituents of an optimized near-infrared lamp. It is well known that in the high-pressure arc (from ˜20 Torr to several atmospheres) the electrode surfaces in nearest proximity to the positive column are at extremely high temperatures. Therefore previous work with tungsten electrodes produced in every case a large population of positive ions at the electrode-plasma interface. This is particularly problematic at the cathode, which is the source of electrons for maintenance of the discharge. Positive ion production changes the geometrical character of the discharge forcing it to bloom out radially reducing substantially the arc radiant exitance.
Cs will be used in the following discussion as representative of each element in the alkali group. A simplified and approximate energy level diagram for the Cs2 diatomic molecule and Cs atom is shown in
The energy pumped into the discharge leaves mainly by photo-emission and to a lesser extent as waste heat (conduction, convection). De-excitation of the ions is by various ion-electron recombination channels. Ion-electron recombination is a three-body process and favors either the direct emission of a photon or the formation of a short lived excited state molecule subsequent to a ion, electron, atom collision. The former process leads to various emission continua, but at pressures of interest the later process dominates the de-excitation. The aggregate effect of all the de-excitation reaction channels for the excited state molecules is to produce at least one (often two) first excited state atom (P state) per recombination event. These P state atoms may then radiate to the ground state and the de-excitation cycle is complete.
The majority species in the discharge is ground state atoms. A substantial population of first excited state atoms builds due to radiation trapping. In the trapping process a radiated photon is absorbed by a nearby ground state atom, re-radiated, reabsorbed and so forth 23. The effective lifetime of the first excited state (2P) in a rarefied alkali gas is on the order of 10 nanoseconds, while at pressures of a few Torr and above is effectively lengthened to milliseconds or longer. Trapping channels the radiative de-excitation of the P state atoms to molecular satellite bands 24 which exhibit broad near-infrared continua on the long wavelength side of the resonance line. A substantial fraction of the energy radiated by the discharge is therefore emitted in these continua. Thus the alkali discharge lamp is an ideal near-infrared source for night-vision illuminators.
Similar energy level diagrams and kinetic considerations represent molecules and plasmas composed of two different alkali atoms as would be present in an alkali mixture. The ratio of the mixture may be varied to enhance the radiation in a particular near-infrared spectral band. Only one alkali present represents one extreme of the mixture ratio.
