Geometry enhanced optical output for RF excited fluorescent lights

Abstract: A fluorescent lighting structure (10) has an inner glass envelope (11) and an outer glass envelope (13) surrounding the inner glass envelope (11). An ionizable gas is contained within the volume (21) between the inner (11) and the outer (13) glass envelopes. An electrode structure (15) is disposed on the inside surface of the inner glass envelope (11). A phosphor coating (17) is disposed on the outside surface of the inner glass envelope (11). An ultraviolet reflective coating (19) is disposed on the inside surface of the outer glass envelope (13). Excitation of the electrode structure (15) causes discharge of the ionizable gas that produces ultraviolet radiation, which in turn excites the phosphor coating to emit visible light.

Description: The disclosed invention is directed generally to fluorescent light structures, and is directed more particularly to a fluorescent light structure that is configured to reduce the light attenuating effects of the phosphor coating which produces the visible light. The prior art consists of conventional fluorescent light tubes. These use a glow discharge to generate ultraviolet (UV) light from a low pressure gas. As shown in FIG. 1, the gas is contained in a sealed tube whose interior surface is coated with a phosphor. The UV light excites the phosphor atoms which then emit visible light as they return to lower energy states. Although the phosphor is thin, it attenuates the optical output from the phosphor atoms except those at the interior surface of the tube. It also attenuates the UV which energizes the phosphor. The result is that the light intensity is highest on the inside of the tube where it is useless with the light reaching the outside heavily attenuated. From EP-A-0 030 593 a fluorescent lighting structure according to the preamble of claim 1 has become known which includes an envelope which is typically pear-shaped with a re-entrant cavity. The lamp envelope encloses a fill material which forms during discharge a plasma which emits ultraviolet radiation and has an effective electrical impedance. The re-entrant cavity is enclosed by a glass container having a conductive layer on its inside surface and a phosphor coating on its outside surface. A phosphor coating is also applied to the inside surface of the lamp envelope. Also this known structure suffers from the drawback that the phosphor coating on the inside surface of the lamp envelope attenuates the optical output from the phosphor atoms except those at the interior surface of the tube. It also attenuates the UV which energizes the phosphor. Reference is also made to US-A-4 240 010 which discloses a similar fluorescent lighting structure utilizing a phosphor coating on the inner surface of the lamp envelope. Finally, a similar structure according to US-A-4 983 881 includes a discharge space bounded by dielectrics and filled with a noble gas or gas mixture. Adjacent to the dielectrics luminescent coatings are provided. On the outside surface of the inner glas container an UV reflective coating may be provided, while on the inside surface of the outer glass container, functioning as the glass envelope, a luminescent phosphor coating is provided. Also this structure suffers from the drawback that the phosphor coating attenuates the optical output of the lamp. It is therefore an object of the invention to provide a fluorescent lighting structure emitting visible light having a significantly increased efficiency with a higher light output to electrical input ratio. This object is achieved by a fluorescent lighting structure according to claim 1. The phosphor coating provided in conventional structures on the inside surface of the lamp envelope is deleted according to the invention. Instead, a phosphor coating is provided only on the outside surface of the inner glass container. The total efficiency improvement achieved thereby, may be as high as a factor of five. The reduced electrical power requirements require a smaller, lower cost ballast. Further, since much less electrical power is utilized, the effects on electrical power factor and total harmonic distortion are reduced, making it easier to meet increasingly stringent governmental regulations. The foregoing and other advantages are provided by the invention in a fluorescent lighting structure that includes an inner glass container, an outer glass container that encloses the inner glass container, an ionizable gas contained in the volume between the inner and outer glass containers, an electrode structure disposed on the inside surface of the inner glass container, and a phosphor coating disposed on the outside surface of the inner glass container. Excitation of the electrode structure causes discharge of the ionizable gas that produces ultraviolet (UV) radiation, which in turn excites the phosphor coating to emit visible light. The lighting structure can further include a UV reflective coating on the inside surface of the outer glass container. By way of specific examples, the inner and outer glass containers comprise concentric glass tubes or glass bulbs. The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein: FIG. 1 is a schematic sectional illustration of a typical prior art fluorescent lighting structure. FIGS. 2 and 3 are schematic sectional illustrations of a fluorescent lighting structure in accordance with the invention. FIGS. 4 and 5 are schematic sectional illustrations a further fluorescent lighting structure in accordance with the invention. In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. The desired mode of operation for a fluorescent light is to have the same surface of the phosphor that is exposed to the ultraviolet (UV) radiation from the discharge also be the one that is directly exposed to the outside environment (i.e., the area to be lighted). This invention produces this condition by utilizing internal electrodes in conjunction with an inside-out geometric structure. Fluorescent lights come in a variety of sizes and shapes. The invention is described for implementation in one of the most common applications, a tube structure such as could be used in 4 or 8 foot applications. However, the principles and structure relationships can be achieved in almost any lamp overall geometry. Referring now to FIGS. 2 and 3, schematically depicted therein by way of illustrative example is a fluorescent lighting structure 10 which includes an inner cylindrical glass tube 11 and an outer cylindrical glass tube 13 which is concentric with and surrounds the inner glass tube 11. An electrode structure 15 is disposed on the inside surface of the inner glass tube 11, and a phosphor layer 17 is disposed as a coating on the outer surface of the inner glass tube 11. An ultraviolet (UV) reflective coating 19 that is transparent to visible light is disposed on the inside of the outer glass tube 13, and an optically transparent conductive coating 23 is disposed on the outside of the outer glass tube 13. For considerations such as simplification of manufacture and cost reduction, the UV reflection coating 19 may be omitted. The ends of the glass tubes 11, 13 are appropriately sealed so as to seal a region 21 between the cylinder glass tubes 11, 13 which forms a discharge region 21 and contains a low pressure gas. Preferably, the electrode structure 15 and connections thereto are outside the discharge region 21 and the ends of the glass tubes 11, 13 are sealed by a glass to glass process, so as to minimize leakage and maximize lamp life. The volume of the discharge region 21 is made as small as practicable consistent with electrode and overall light output requirements, which allows the phosphor area to be only slightly smaller than conventional fluorescent tubes for the same outer lamp diameter. The electrode structure 15 is driven with an RF source and produces an electric field which penetrates the inner glass tube 11 and the phosphor layer 17 to induce a controlled breakdown and discharge of the gas in the discharge region 21, with the highest intensity being directly adjacent the phosphor layer 17. Depending upon the particular implementation, the RF source as well as other appropriate RF circuits can be located inside the inner glass tube 11. The UV reflection coating 19 reflects UV light emitted away from the phosphor layer 17 back towards the phosphor layer 17. This increases the electrical to UV efficiency by a factor of about two. The outer glass tube 13 is preferably transparent to visible light but opaque to UV to minimize UV emissions. The optically transparent electrically conductive coating 23 provides shielding to minimize RF radiation and resulting EMI, and is preferably configured to be an effective attenuator of RF radiation from the fundamental operating frequency of the RF source out through the 7th harmonic at a minimum. The outer glass tube 13 of the lamp could perform this function instead of the coating 23 if the glass is configured to have the electrical/RF characteristics for performing the shielding function. Referring now to FIGS. 4 and 5, schematically depicted therein by way of illustrative example is a fluorescent lighting structure 100 which includes an inner bulb-shaped glass envelope 111 and an outer bulb-shaped glass envelope 113 which is shaped similarly to the inner glass envelope 111 and surrounds the inner glass envelope 111. Electrode structures 115 distributed on the inside surface of the inner glass envelope 111, and a phosphor layer 117 is disposed on the outer surface of the inner glass envelope 111. An ultraviolet (UV) reflective coating 119 that is optically transparent to visible light is disposed on the inside surface of the outer glass envelope 113, and an optically transparent conductive coating 123 is disposed on the outside surface of the outer glass envelope 113. A glass seal 112 is located in the stem portions of the bulb-shaped glass envelopes 111, 113 to seal the region 121 between the bulb-shaped glass envelopes 111, 113 which forms a discharge region 121 and contains a low pressure ionizable gas. The electrode structure 115 and connections thereto are outside the discharge region 121, which minimizes leakage and maximizes lamp life. The volume of the discharge region 121 is made as small as practicable consistent with electrode and overall light output requirements. Each of the electrode structures 115 includes elongate interconnected outer ground electrodes 115a and an elongate central power electrode 115b which generally extend in parallel from the upper portion to the lower portion of the bulb-shaped envelope 111. The electrode structures 115 are appropriately driven by respective matching networks (not shown) responsive to respective outputs of a splitter circuit connected to an RF source. The electrode structures 115 produce respective electric fields which penetrate the inner glass envelope 111 and the phosphor coating 117 to induce a controlled breakdown and discharge of the gas in the discharge region 121, with the highest intensity being directly adjacent the phosphor layer 117. Depending upon the particular implementation, the RF source, splitter circuit, and matching networks can be located inside the inner glass envelope 111. The UV reflection coating 119 reflects UV light emitted away from the phosphor layer 117 back towards the phosphor layer 117, which increases the electrical to UV efficiency. The outer glass envelope 113 is preferably transparent to visible light but opaque to UV to minimize UV emissions. The optically transparent electrically conductive coating 121 provides shielding to minimize RF radiation and resulting EMI, and is preferably configured to be an effective attenuator of RF radiation from the fundamental operating frequency of the RF source out through the 7th harmonic at a minimum. The outer glass envelope 113 of the lamp could perform this function instead of the coating 121 if the glass is configured to have the electrical/RF characteristics for performing the shielding function. It should be appreciated that in accordance with the invention, a bulb-shaped outer glass envelope can be utilized with a cylindrical inner glass tube similar to the inner glass tube 11 of the lighting structure shown in FIGS. 2 and 3, which would provide for a simpler electrode structure. Further, it is important to notice that the invention is intended to be incorporated into fluorescent lighting products. Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: A fluorescent lighting structure, comprising: a first glass container (11; 111) having an outer surface and an inner surface; a phosphor coating (17; 117) disposed on said outer surface; a second glass container (13; 113) being arranged to enclose said first glass container (11; 111) leaving a volume (21; 121) therebetween; an ionizable gas contained in said volume (21; 121) between said first (11; 111) and said second (13; 113) glass containers; and electrode means (15; 115) arranged on said inner surface of said first glass container (11; 111) for exciting the ionizable gas to cause discharge thereof producing ultraviolet radiation, which, in turn, excites the phosphor coating (17; 117) to emit visible light, characterized in that said second glass container (13; 113) does not comprise any phosphor coating. The fluorescent lighting structure of claim 1, characterized in that the first and second glass containers (11, 13; 111, 113) comprise first and second concentric glass tubes. The fluorescent lighting structure of claim 1, characterized in that the first and second glass containers (111, 113) comprise bulb-shaped glass envelopes. The fluorescent lighting structure of claim 1, characterized in that the first glass container comprises a glass tube and the second glass container comprises a bulb-shaped glass envelope. The fluorescent lighting structure of claim 1, characterized in that the first glass container (111) comprises a bulb-shaped glass envelope and that the electrode structure (115) comprises elongate electrodes (115a, 115b) extending from an upper portion to a lower portion of the bulb-shaped glass envelope. The fluorescent lighting structure of any of claims 1 - 5, characterized in that an ultraviolet reflection coating (19; 119) is disposed on the inside surface of the second glass container (13; 113). The fluorescent lighting structure of any of claims 1 - 6, characterized in that the electrode structure (15; 115) comprises interconnected outer ground electrodes (115a) and a central power electrode (115b). The fluorescent lighting structure of any of claims 1 - 7, characterized in that the electrode structure (15; 115) comprises elongate electrodes (115a, 115b).

Inventors: HEAD DAVID A, MCCLANAHAN ROBERT F, WASHBURN ROBERT D, HEAD, DAVID A., MCCLANAHAN, ROBERT F., WASHBURN, ROBERT D.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1992
Main classification: H01J65
IPC codes: H01J65