Fluorescent lamp with adjustable color temperature

Abstract: A fluorescent lamp having an adjustable color temperature comprising at least two elongated fluorescent discharge tubes (10, 20), one tube (10) having a larger diameter than the other (20). The tubes (10, 20) are assembled into a single unit. A groove (12) is disposed within the larger tube (10) and runs parallel to the longitudinal axis. The smaller diameter tub (20) is snugly nested within the groove (12) and in intimate contact with the larger diameter tube (10). The larger diameter tube (10) has a phosphor coating producing one color temperature and the smaller diameter tube (20) has a phosphor coating which produces a different color temperature. Preferably, the phosphor coating (18) of the larger tube emits a light of low color temperature of 3000K or below and the phosphor coating (28) of the smaller tube emits a light of high color temperature of 10000K or more. A control unit (50) is provided to divide the power to the two tubes such that a variable color temperature is produced at nearly constant total power.

Description: TECHNICAL FIELD This invention relates to a fluorescent lamp having color temperatures that can be adjusted to suit the lighting requirements in a particular space or time. More particularly, it relates to a fluorescent lamp in combination with a drive circuit which makes substantial use of existing technology. BACKGROUND ART Lamps for general illumination are designed to produce "white" light, i.e., their light emission have a color spectrum or mix of colors that appear "white." In incandescent lamps, the filament is heated to a temperature of about 2800K in order to produce white light. The incandescent lamp gives out a continuous color spectrum which blend together to give white light. White light may also be produced by mixing a few specific colors such as red, green and blue. One characteristic of color is the "correlated color temperature, " or more simply color temperature which is equivalent to the temperature of a black body source that matches that color. The color temperature of a white light source spans the range from about 2500K to 8000K; the preferred range is from 3000K to 6000K. The color temperature of a lamp is fixed at the time of manufacturing. In low pressure fluorescent lamps, the color temperature is determined by a phosphor coating on the tube. Typically a few discrete color temperature choices are available such as "warm white" (3000K), "neutral" (3500K), "cool white" (4100K) and "daylight" (5000K). The preference for a particular color temperature depends on a variety of psychological and evolutionary factors. People in northern latitudes favor warmer color temperatures, but tend towards the "cool white" for the work environment. Thus, in addition to human predisposition, color temperatures are kept different depending on the ambiance or mood of the living environment. A lighting system which allows the color temperature to be changed in a simple manner would allow the illumination needs of individuals to be met. The system would be flexible and will contribute to increased productivity and quality of life. There have been many attempts to realize a practical variable color temperature fluorescent lamp. None of these has become commercially successful since, in all cases, various schemes have not been economical, suitable for efficient manufacturing, or had performance limitations. The major schemes that have been proposed all use color mixing and can be divided into two categories; several individual lamps in a fixture or a single lamp that is pulse excited. The former method (see, for example, U.S. Patent No. 5,384,519) requires at least three special lamps, control circuits and a dedicated fixture. The power is partitioned between the lamps in order to produce the desired color temperature. The single lamp category invariably requires the use of a pulse excitation ballast circuit. In one device, neon is used as the fill gas. With suitable excitation, the red emission from neon mixes with the mercury/phosphor emissions to bring down the color temperature (U.S. Patent No. 5,410,216). In another disclosed lamp, mercury and xenon UV radiation is generated using a pulse drive. The emission from two different phosphors, each of which is sensitive to the mercury and xenon UV radiation, respectively, provide the color temperature variations (disclosed by M. Aono et al., the 7 th International Symposium on the Science & Technology of Light Sources). Yet another lamp with selective phosphors and pulse drive utilizes the UV radiation from mercury and argon to achieve color temperature variations (disclosed by S. Tanimizu et al, The 7 th International Symposium on the Science & Technology of Light Sources). While all of the above approaches describe color temperature change in a single lamp, there are still hurdles to overcome such as poor luminous efficacy, availability of special phosphors, the need for a complex and expensive ballast and poor lamp life due to the detrimental effect of pulsing on cathodes. DISCLOSURE OF THE INVENTION Therefore, it is a primary object of the present invention to furnish an adjustable color temperature lamp that will overcome the foregoing problems. Both the lamp and ballast circuit will be simple and inexpensive because it makes use of existing technology and does not require very special fixtures since the effect is achieved in a single lamp assembly. According to the present invention, this object is achieved by a fluorescent lamp assembly according to claim 1. Because of the geometry of the arrangement, the light of the two tubes is well mixed. Each discharge tube is driven by an appropriate dimming ballast and a controller ensures the partition of power between the two tubes so as to realize a desired color temperature. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a simplified side cross-sectional view of the fluorescent lamp assembled from two discharge tubes, shown without a phosphor coating, in accordance with a preferred embodiment of the present invention; Figure 2 is a cross-sectional view of the lamp shown in Figure 1, taken on the line A-A'; Figure 3a and 3b are cross-sectional views showing two alternative embodiments of the present invention which can enhance color mixing from the two discharge tubes; Figure 4 is a schematic block diagram showing lamp drive ballasts and a control unit for the above lamp; and Figure 5a and 5b are cross-sectional views showing two additional embodiments of adjustable color temperature lamps of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A fluorescent lamp in accordance with a preferred embodiment of the present invention comprises two discharge tubes of different diameters, as shown in Figures 1 and 2. The envelope material for the tubes is glass. A larger diameter tube 10 has a groove 12 running along its back, parallel to its longitudinal axis. The smaller diameter tube 20 , which is circular in cross-section, is located in the groove of the larger tube and is attached in place. Both tubes contain filler gases 14 and 24 , of mercury and rare gas, typically argon, and are phosphor-coated on their inner walls for conversion of the mercury ultra violet radiation to visible light. The discharge tubes also have conventional electrodes 16 and 26 at each end. The two discharge tubes together thus form a single lamp assembly. The groove 12 in the surface of the larger tube 10 does not extend all the way to the ends, since a circular cross-section at the ends facilitates the sealing of stems which support the electrodes and lead-in wires. The length of the smaller tube 20 should be such that it approximates the larger diameter tube so that observable color difference of the two tubes is minimized. A cross-section of the lamp assembly in the middle (section A-A' of Figure 1) is shown in Figure 2. The groove 12 has a radius of curvature that is slightly larger than the outside radius of the smaller tube 20 . Further, the depth of the groove is such that the smaller tube 20 sinks in the groove at least to its diameter. In fact, it is more advantageous if the smaller tube is submerged completely inside the groove. Besides the aesthetic appearance of a near round cross-section for the envelope of the lamp assembly, another desirable feature is that more radiation from the smaller tube is injected into the larger tube. The variable color temperature feature of this lamp is achieved by color mixing of the light from the two discharge tubes. Accordingly, the phosphor blends in the two tubes are different. In Figure 2, the larger tube has a phosphor coating 18 that converts the UV radiation to a "warm" color light of low color temperature of 3000K or less, preferably, 2700K. For example, a blend of red and green phosphors such as a composition available under the trade name of "NICHIA NP92" from NICHIA KAGAKU KOGYO, JAPAN might be used for this purpose. The other discharge tube then has to emit light of very high color temperature of 10000K or more. In this embodiment, a phosphor coating 28 of the smaller tube is a blend of blue and green phosphors, approximately in the proportion 70/30. The phosphor blends are chosen so that the emitted light lies substantially on the black body locus for all color temperatures. It should be apparent that the sizes and geometries of the two discharge tubes shown should be chosen such that good color mixing is possible and the lamp assembly is easy to fabricate. Except for the groove in the larger tube, all other steps involved in the lamp-making process are very similar or identical to those used in conventional fluorescent lamp manufacturing. Small variations may be introduced to realize better lamp performance, such as not coating a wall of the groove with phosphor, leaving a transparent strip or strips, or coating the tube with a very thin layer on the wall of the groove to reduce the scattering of the light going from the smaller tube into the larger tube. The particular configuration of the coating is primarily determined by manufacturing ease and cost. Further, the bluish-green light emanating from the exposed top surface of the small tube can be redirected into the larger tube in order to realize a wider range of color temperature and more uniform appearance. This may be done in a special fixture. If, however, a standard fixture is to be used, then a reflecting surface may be incorporated in the top of the lamp assembly. These embodiments are shown in Figure 3a where the phosphor coating 18a is very thin or not present in the groove area. Portions of a curved surface not within the groove have a highly reflecting surface, i.e., an external reflector 30 that also improves the lamp appearance by hiding the smaller tube 20 . Alternately, the light reflection from the top surface of the small tube 20 may be accomplished by having an internal reflective coating 32 covering the upper half of the small discharge tube 20 , as shown in Figure 3b. The diameters of the two discharge tubes and the depth and shape of the groove are chosen such that the smaller tube is almost completely surrounded by the larger tube. An external reflector, if needed, should then be considerably smaller in size. A preferred example is a 20 W/ 2 foot lamp as follows: Large discharge tube 2.54 or 3.17 cm diameter and 61 cm long. (1.0 or 1.25 inches diameter and 24 inches long) Small discharge tube 1.27 cm diameter and 58 cm long (0.5 inches diameter and 23 inches long) Color temperature range 2700K - 5500K The two-tube assembly lamp also will provide a better control of the cold spot temperature and, hence, to a great extent, ambient temperature insensitivity since the lamp can be always operated at its rated power, i.e., at a constant total power supplied to the two tubes, and the two discharge tubes are in good thermal contact with each other. In the system of the prior art which uses several separate tubes in a fixture to effect color temperature change, when some tubes are not operated at their individual rated powers, their cold spot temperatures can be much lower than optimal. It should be pointed out that there are fluorescent lamps commercially available or described in the art that have a grooved pattern on top of the cylindrical envelope. The configurations shown have either continuous or a plurality of separate grooves of various cross-sections (See, for example, U.S. Design Patent No. 198,268; U.S. Patents Nos. 2,915,664; 2,950,410; 2,973,447; 3,098,945; 3,560,786; 3,988,633; 4,825,125; 5,498,924). The purpose of the grooves is mainly to raise the lamp voltage with an aim to increase the lamp luminous efficacy, or to make the lamp operable on a ballast designed for another lamp geometry, or to provide better control of the mercury vapor pressure. The grooves cause an increase of the lamp voltage due to one or all of these reasons: lengthening the path of the arc between the electrodes, increasing the wall recombination rate of the plasma ions with the phosphor and constriction of the plasma discharge. While the grooved lamp of the present invention will also have a slightly higher voltage compared to a circular cross-section lamp of the same envelope diameter, the effect is incidental. Further, from a manufacturing point of view, the longitudinal groove parallel to the lamp axis in the present lamp is simpler in design and easier to fabricate than the groove patterns shown in the references cited before. As explained earlier, the presence of the groove allows a smaller diameter discharge tube to be nestled inside the large tube and thereby makes possible good color mixing of the light from the two tubes. For color temperature variation, in addition to the lamp assembly as described above, a lamp power control is required. Each discharge tube is driven by a variable power (dimming) ballast 51 , 52 . As an example, for the preferred embodiment detailed earlier, the larger tube 10 may be operated from 20 W to 8 W, while the smaller tube 20 is operated over the range from 0 W to 12 W. The desired color temperature is set by a control unit 50 that adjusts the power from the individual ballasts 51 and 52 such that the total power to the lamp assembly is constant (20 W). A block diagram schematic of the lamp drive and control is shown in Figure 4. Again, the drive system for the two discharge tubes can use existing technology with only the addition of the proportioning control unit 50 for the color temperature control unit. The power division between the two tubes gives rise to the color temperature variation. This invention essentially discloses a color temperature variable fluorescent lamp that comprises two externally-assembled discharge tubes, one of which produces a "warm" color radiation and the other a "cool" color. It is also possible to reverse the "warm" and "cool" phosphor coatings on the two discharge tubes or to have different phosphor blends. Some of the many configurations possible are shown in Figures 5a and 5b. In Figure 5a, a small size tube 20A of rather crescent section is placed on a complementary tube 10A of a larger size to give a unitary lamp assembly of a circular cross section. Phosphor coatings 18A and 28A of different color temperatures are provided on the inner wall surfaces of the tubes 10A and 20A. In Figure 5b, an additional tube 40B of a larger diameter with a like groove 42B is placed over a like smaller diameter tube 30B to completely surround the tube 20B between the two larger diameter tubes 10B and 40B. Phosphor coatings 18B, 28B, and 48B of different color temperatures are provided on the inner wall surface of the individual tubes 10B, 20B, and 40B. The phosphor coatings of the two larger tubes 10B and 40B may have the same color temperature.

Claims: A fluorescent lamp assembly having an adjustable color temperature comprising: at least two elongated cylindrical fluorescent discharge tubes (10, 20), each tube forming a lamp, one tube (10) having a larger diameter than the other tube (20), said tubes being assembled into a single lamp assembly; a groove (12) disposed within the larger diameter tube, said groove running parallel to a longitudinal axis of the larger tube; the smaller diameter tube being snugly nested within said groove and in intimate contact with said larger diameter tube; said larger diameter tube having a phosphor coating (18) which produces one color temperature and said smaller diameter tube having a phosphor coating (28) which produces a different color temperature. The lamp assembly according to claim 1 wherein the phosphor coating (18) of said larger diameter tube (10) emits a low color temperature of 3000K or below and the phosphor coating (28) of said smaller tube (20) emits a higher color temperature of 10.000K or more. The lamp assembly according to claim 1 wherein the depth of said groove (12) is such that said smaller diameter lamp (20) sinks in said groove at least to its diameter. The lamp assembly according to claim 1 wherein each discharge tube (10, 20) in said lamp assembly is driven by a variable power ballast (51, 52) and further including a control unit (50) for setting the operating points of the individual ballasts such that a desired color temperature is obtained at a nearly constant total power. The lamp assembly according to claim 1 wherein said phosphor coating (18) of said larger diameter tube (10) is not present at said groove (12) or thinner at said groove (12) than at the outer surface in the opposite side in order to enhance color mixing of the light from the smaller diameter tube with the light from the larger diameter tube. The lamp assembly according to claim 1 wherein said smaller diameter tube (20) has an exposed surface not in contact with said larger diameter tube, said exposed surface being provided with either an internal or external reflective coating (32, 30) which directs light emanating from the exposed surface into said larger diameter tube for enhanced color mixing. The lamp assembly according to claim 1, wherein the length of the smaller diameter tube (20) is substantially the length of the larger diameter tube (20) whereby the emission color of the lamp does not change significantly over the length of the lamp.

Inventors: ANANDAN MUNISAMY, CONNOLLY JOSEPH, RAVI JAGANNATHAN, SHEA MICHAEL J, ANANDAN, MUNISAMY, CONNOLLY, JOSEPH, RAVI, JAGANNATHAN, SHEA, MICHAEL J.
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1997
Main classification: H01J61
IPC codes: H01J61, H05B41