1. Field of the Invention
The present invention is directed to an electrodeless microwave excited bulbs which emit ultraviolet light and processes for generating ultraviolet light with a microwave excited electrodeless bulbs.
2. Description of the Prior Art
The use of forced air to cool microwave excited electrodeless bulbs which are rotated is known. See U.S. Pat. Nos. 4,485,332; 4,695,757; 4,947,080; 4,954,756; 5,021,704; 5,594,303; 5,866,990 and 5,997,724. The systems disclosed herein are generally for applications of rotating small bulbs cooled with air flow having a major dimension of the bulb envelope of less than {fraction (xcex/4)} or for larger bulbs having a major internal dimension greater than {fraction (xcex/2)} wherein xcex is the free space wavelength of the microwaves used for excitation which is 2.45 GHz.
Small bulb products with a major internal dimension less the {fraction (xcex/4)} of the foregoing type are used in special applications, such as photo lithography, used in semiconductor processing. These systems require a small bulb as a consequence of the optics associated with the photo lithography and are heavy, large, and expensive as a consequence of higher power magnetrons used therein and complicated forced air cooling systems associated therewith. High speed rotation of the bulb in them is required to prevent harmful thermal gradients in the wall temperatures of the bulb envelope. Moreover, while substantial forced air cooling in combination with high speed rotation of the electrodeless bulb provides stable operation, the internal temperatures of the walls of the envelope are too high to permit the use of known additives to the envelope to modify the spectrum from that produced by the primary emissive materials such as Hg.
Mercury based electrodeless lamps have been in use for many years. See Electric Discharge Lamps by Dr. John Waymout, MIT Press, 1971.
Metal halides in combination with halogen doping of electrodeless lamps has been known since the 1960""s. The use of Ba, Na, Ti, In and Cd iodides is disclosed in U.S. Pat. No. 3,234,431.
Lanthanides and rare earths are used as dopants in electrodeless bulbs to produce selected spectral emissions. U.S. Pat. No. 3,334,261 lists Y, La, Ce, Nd, Lu, Ho, Tn, Pr, Gd, Tb Dy and Er as dopants for electrodeless bulbs which produce visible light.
U.S. Pat. No. 3,947,714 discloses the use of Fel2 as an additive to the constituents of an electrodeless bulb.
U.S. Pat. No. 6,157,141 discloses the use of Ga as a dopant in an electrodeless bulb.
U.S. Pat. Nos. 5,837,484 and 4,945,290 disclose eximer electrodeless bulbs using noble gases and gas mixtures.
U.S. Pat. Nos. 5,504,391 and 5,686,793 disclose eximer electrodeless lamps which operate at high pressure.
Moreover, Fusion Systems, Inc. of Gaithersburg, Md., has used forced air cooling of a rotating bulb in their AEL/HIIQ products which are powered by magnetrons having a power of at least 0.85 kW to produce wavelengths in the visible range with major internal dimensions of 30 mm respectively. The angular speed of the bulb in the AEL/HIIQ is variable between 250-300 revolutions per minute with air impingement cooling being used. The angular speed of the 35 mm, visible light bulb in the Solar 1000/Light Drive 1000 products of the same company (and reorganized Fusion Lighting, Inc.) is 3250 revolutions per minute with no driven air cooling being used.
The present invention is an electrodeless bulb and a process of generating ultraviolet light with a microwave excited electrodeless bulb which utilizes a bulb with an envelope having a major dimension between ⅓ and xc2xdxcex to which a strong field of microwave excitation varying between 2.4 and 2.5 GHz is applied with the strong field being produced by a microwave source having a steady state rated power no greater than 0.85 kW while being cooled with a forced air stream and rotated at a rotational velocity of at least 20 rpm and which contains an ultraviolet emissive material emitting ultraviolet light in response to the microwave excitation.
The aforementioned parameters of operation provide an electrodeless bulb and a process of lower cost than the prior art bulbs and processes using small bulbs with a major internal dimension of xc2xcxcex or less. In a typical application of a rotating small electrodeless bulb used for photo lithography, the cost of the small bulb lamp may be between $15,000-$20,000, wherein the lamp of the present invention may cost $3,000-$5,000 which is useful for many applications not suited for small bulb lamp systems having major internal dimensions of the bulb less than xc2xcxcex.
Larger lamp systems having major bulb internal dimensions greater than xc2xdxcex typically are excited by microwave power greater than a kilowatt and therefore, produce too much ultraviolet light output and are too large and heavy for the applications of the present invention.
The intermediate size bulbs of the present invention having a major dimension between ⅓ and xc2xdxcex are excited with microwave sources of less than 0.85 kW at an excitation frequency of 2.4 and 2.5 GHz which permits selective ultraviolet emission to be produced in accordance with known microwave emissive materials which are stable thermally and chemically within the envelope as a result of the wall temperatures of the envelope being stabilized as a consequence of rotation and forced air cooling so as to avoid hot spots causing thermal degrading of the microwave emissive materials and additives at the walls of the envelope and further undesirable thermal gradients in the walls of the bulb envelope and/or attacking of the walls of the bulb envelope by the plasma.
In one embodiment, the envelope within the foregoing major internal dimensional range is filled with between 10 mg and 100 mg of mercury as the emissive material and at a pressure of 20-600 Torr at room temperature and includes at least one buffer gas selected from at least one of the group consisting essentially of Ar, Kr and Xe with the ultraviolet emission being in a range between 200-300 nm while directing a stream of air in contact with the bulb to provide cooling thereof. The first embodiment may utilize HgCl2 in an amount of not greater than 2 mg. in the envelope. The air stream may be laminar. The emission range between 200-300 nm may have a peak emission between 250-270 nm
In another embodiment the envelope within the foregoing major internal dimensional range is filled with one of Xe or Ar gas as the emissive material at a pressure range from 10-2500 Torr at room temperature and chlorine gas in a pressure range from 0.5-200 Torr at room temperature which excites microwave emission between 200-310 nm while directing a stream of air in contact with the bulb to provide cooling thereof. The envelope may be filled with a metal halide dopant. The stream of air may be a laminar flow. The range between 200-310 nm may have a maximum peak range between 300-310 nm.
In an additional embodiment the envelope within the foregoing major internal dimensional range is filled with between 10-100 mg of Hg as the emissive material at a pressure of 200-600 Torr at room temperature and includes at least one of the group consisting essentially of Ar, Kr and Xe as a buffer gas with ultraviolet emission in a range of between 320-400 nm while directing a stream of air in contact with the bulb to provide cooling thereof. The envelope may contain up to 50 mg of halide selected from the group consisting essentially of Fe, Co, Ni, Pb and Al. Additionally, the envelope may also contain at least one other group of elements consisting essentially of Mn, Mg, Mo, Be, Cd, Ge and Li to provide additional emission in the spectrum of the ultraviolet light. The stream of air may be laminar flow. The range between 320-400 nm may have a peak between 350-380 nm.
In an another embodiment the envelope within the foregoing internal dimensional range is filled between 10-100 mg of Hg as the emissive material and at a pressure of 20-600 Torr at room temperature including at least one buffer gas selected from at least one of the group consisting essentially of Ar, Kr and Xe and at least one salt selected from the group consisting essentially of Ga, Al and Pb in an amount between 1 and 50 mg with the ultraviolet emission being in the range between 390-450 nm while directing a stream of air in contact with the bulb to provide cooling thereof. The stream of air may be laminar flow. The range between 390-450 nm may have a peak emission between 395-420 nm.