CA1212984A - Energy efficient fluorescent lamp - Google Patents

Abstract

ENERGY EFFICIENT FLUORESCENT LAMP

ABSTRACT

A fluorescent lamp including a glass envelope having electrodes at its ends and having an inert gas fill in which a first phosphor layer is disposed on the inner surface of the envelope along with a second phosphor layer superimposed over the first layer. The inert gas is preferably substantially of a lighter weight inert gas than krypton gas and the envelope is of a diameter less than 1 1/2 inch diameter, preferably of one inch diameter. In a lamp fixture the lamp of this invention provides the same relative output as a prior art T12 lamp with substantially less energy consumption.

Description

- l -ENERGY EFF1C~ENT FLUORESCENT LAMP

TECHNICAL FIELD

The present invention relates to energy efficient fluorescent lamps, and more particularly pertains to a fluorescent lamp having significant improvements in both lamp and fixture efficiencies, along with an improved color rendering index.

BACKGROUND OF THE INVENTION

Because of the continuing increase in the cost of energy, there has been a substantial increase in the demand for fluorescent lamps with increased efficiency. This is clearly evident by the widespread acceptance of presently available, energy saving, fluoresent lamps and devices. The present energy saving fluorescent lamps employ a Tl2 (l l/2 inch diameter) glass tube. The tube typically contains krypton gas in a relatively high percentage along with another inert gas sucn as argon or neon. The heavier krypton gas is used to permit the lamp to operate at a lower lamp wattage.
However, there are certain disadvantages associated with the use of the heavier krypton inert gas. The high percentage of krypton increases the peak starting voltage of the lamp and, therefore, a starting aid is required to assure proper starting. This is accomplished by internally coating the glass tube with a conductive layer such as tin oxide (SnO2). Also, krypton is relatively expensive and~ occasionally, in short supply.
Using known manufacturing techniques, the Tl2 lamp is coated with a phosphor. This is deposited on the inner wall of the glass tube envelope and may be in the form of a single phosphor or a multiple phosphor layer. Double layer phosphors are shown in U.S.
patents 3,602,757~ 3,602,758, and 4,088,923. ~hen employing a single phosphor layer, this may comprise a calcium and/or strontium : , :

D-24,142 ` ~ Z~L~91~4 . -2-halophosphate activated by antimony and/or manganese. When a superimposed multiple layer is employed, a second phosphor layer may comprise, for example~ the tricomponent phosphor disclosed in the aforementioned U.S. patent 4,088,923.
Regardless of the phospnors employed, the spectral energy distribution (SED) curves of fluorescent lamps deviate considerably from the continuous spectrum of blackbody sources. This deviation changes the appearance of colored surfaces, and it is desirab1e to have a numerical rating for the quality of the light obtained from fluorescent lamps. Since it is customary to judge colors by their appearance in daylight or blackbody sources, the most acceptable method of rating is a comparison of the appearance of test colors under fluorescent light with their appearance when seen by light from a blackbody source of the same color temperature. This numerical rating is termed the color rendering index (CRI). In general, a CRI of 100 approaches a given daylight color temperature;
therefore, the closer the CRI of a fluorescent lamp approaches lOû, the more nearly a daylight quality of illumination is reached. For a more detailed account of the determination of CRI9 reference may be had to the ~.E.S. ~ighting Handbook, Fourth Edition, pgs. 5 14 through 5-17.
Typical T12 lamps such as those of single phosphor layer have color rendering indexes that range from, for example, a low of 48 for a Lite White (LW) lamp to a high index of on the order of 89 for a Cool White Deluxe (CWX) lamp. The COO1 White (CW) lamp is one that employs a single phosphor of calcium halophosphate activated by manganese and has a CRI of 62. The Lite ~hite lamp employs a combination of phosphors including calcium fluoroapatite activated by antimony and manganese; and strontium cloroapatite activated by europium. Under some lighting conditions, these existing lamps do not provide balanced color, particuarly in the red portion of the spectrum.
~loreover~ when the energy efficient or energy saving lamps are employed, there is a resultant decrease in either light output or the color rendering index when compared to the standard F40T12 lamp system.

, ., D-24~142 DISCL~SURE OF THE ~NVENTIO~' It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
~t i5 another object of the present invention to provide an improveo energy efficient and energy saving fluorescent lamp.
Another object of the present invention is to provide ar, improved energy efficient fluorescent lamp which provides a significant improvernent in both lamp and fixture efficiency.
A further object of the present invention is to provide an improved energy efficient fluorescent larnp characterized by a high color rendering index greater than 70 and preferably on the order of 75.
Another object of the present invention is to provide an energy saving fluorescent lamp characterized by enhanced thermal and optical mating with conventional fixtures.
Still another object of the present invention is to provide an improved fluorescent lamp employing a more efficient glass envelope, preferably a Tg, 1 inch diameter glass tube envelope.
Still a further object of the present invention is to provide an improved energy saving fluorescent lamp in accordance with the preceding object ana which enables more economic use of the more efficient and expensive rare earth phosphors forming a part of the phosphor layer means of the lamp of this invention.
Another object of the present invention is to provide an improved energy saving fluorescent lamp adapted for operation at substantially lower current levels which thus permits elimination of the discoloration shields around the coil of the lamp.
Another object of the present invention is to provide an improved energy saving fluorescent lamp in accordance with the preceding object and in which the lamp, due to its substantially lower current loading, operates cooler, generating less heat and thus operating more efficiently.
Still another object of the present invention is to provide an improved energy efficient fluorescent lamp characterized by improved larnp efficiency and lamp system efficiency. ~n this regard, the :,,.

D-2~,142 ~ .2~

lamp preferably does not use tne more expensive ~rypton gas but re1ies subtantially on an argon fill.
~o accomplish the foregoing and other objects of this invention, there is provided, in one aspect of the invention, a fluorescent lamp comprising a glass envelope with associated electrodes at its ends, filled with an inert gas and having a first phosphor layer disposed on the inner envelope surface and a second phosphor layer superimposed on the first layer. The lamp diameter is the more efficient T8, l inch diameter glass tube instead of the Tl2, l l/2 inch diameter tube envelope. The reduction in tube size permits economic use of the more efficient rare earth phosphors of the aforementioned second layer. Although these phosphors are more expensive, by using the T8 tube, l/3 less phospnor is required to coat the inside glass surface. In spi-te of the reduction in diameter, it has been found that an increase in overall fixture efficiency is attained by virtue of the aforementioned thermal and optical rnating of the lamp with the fixture. Furthermore, the lamp of the present invention preFerably uses a substantially lO0% argon fill rather than a krypton or partial krypton fill. Although the argon fill lamp, per se, has a sightly lower lamp efficiency in comparison with the krypton lamp, the overall system efficiency is higher.
In accordance with a particular ernbodiment the first layer, which is the more inexpensive layer of phosphor, may comprise a calcium and/or strontium halophosphate activated by antimony and/or manganese. The inner or second phosphor layer may comprise the tricornponent phosphor disclosed in U.S. patent 4,088,923, namely, a blend of red-emittiny yttrium oxide activated by trivalent europium:
green-emitting cerium magnesium aluminate activated by trivalent terbium: and blue-emitting barium magnesium aluminate activated by divalent europium. In accord with a preferred construction of this invention, the second layer pGwder weiyht is in the range of 20-30 of the total power weight of the lamp phosphor.

~2~2~84 FIG. l shows a fluorescent lamp partly in section construction in accordan~e with the present invention;
FIG~ 2 shows a bank of such lamps in a lamp fixture; and FIG. 3 is a graph of percent second layer powder weight to tot~l powder weight versus light output.

BEST MO~E FOR CARRYlN6 OUT THE INVENTION

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction wi$h the above-described drawings.
Referring now to the drawings with greater particularity, there is shown in FI6. l a lamp which is comprised of a sealed glass envelope l having electrodes 2 at each end. Envelope l contains the usual discharge sustainlng filling including an inert gas. The nature of the inert gas is discussed in detail hereinafter. A first or outer layer 3 of phosphor is disposed on the inner wall of the envelope l.
A second~ or inner, phosphor layer 4 is superimposed on layer 3.
Examples of dual phosphor layers are shown in U.S. paten~s 3,602,757;
3,602,758; and 4,088,923. Another dual phosphor layer construction is shown in U.S. Patent 4,431,941 and assi~ned to the present assignee.
In accordance with this dual phosphor layer technique, such as illustrated in U.S. patent 3,602,758, a thin coat of the more expensive rare earth phosphor is placed over the first or suter coat of the less expensive halophosphate phosphor. The first layer~ namely layer 3 in FIG. l, may comprise a calcium andfor strontium halophosphate activated by antimony and/or manganese. The inner or second phosphor layer 4 may comprise the tricomponent phosphor disclosed ~n, for example, U.S. patent 4~088,923, which is a blend of red-emitting yttrium oxide activated by trivalent europium;
green-emitting cerium magnesium aluminate activated by trivalent .
~,;,;~

D-24,142 8~
~, terbium; and blue-emitting barium magnesium aluminate activated D~
divalent europium.
In accordance with the present invention it has been found that the second layer powder weight-is to be maintained within a range of 20-30% of the total powder weight of both phosphor layers. In this regard refer to FIG. 3 which is a plot of percentage of the second layer powder weight versus light output. In FIG. 3 there is a slight curved knee at 6 which comes at about a 25% powder weight.
There is some increase in light output up to about 50% powder weight, but the cost is too great to use that high a percenta~e.
Due to the increased ~ain achieved by proper optical and thermal mating, it has been found that a powder weight of the expensive phosphor layer in the range of 20-30% to tha-t of the total powder weight of the two phosphors provides proper results.
lS As mentioned previously, when the lamp of this invention is operated in a conventional fixture~ the efficiency of the fixture is increased as a result of certain optical and thermal effects now to be discussed. In this connection reference is made to FIG. 2 ~hich shows a lamp fixture 8 for holding a plurality of fluorescent lamps 10 of the construction shown in F~G. 1. In FIG. 2 four such lamps are employed.
It has been found that with the smaller T8~ 1 inch diameter fluorescent tube, optical losses caused by the absorption of light by the fixture walls and the companion lamps themselves are reduced. Also, the slimmer diameter fluorescent lamp allows the fixture manufacturer to optionally design a shallower fixture cutting material costs and making it possible to make a less expensive fixture. Thus, overall improved efficiency has been found as far as the lamp of this invention is concerned when employed in a fixture such as illustrated in FIG. 2.
Moreover, the lamps of this invention are constructed to operate at a lamp current that is relatively low, on the order of 220 mA.
This low current operation allows for the elimination of discoloraton shields around the coils of the lamp. The present F40T12 lamps and energy-saving T12 larnp types operate on conventional ballasts that run at approximately 430 mA an~ 480 mA, respectively. However~ the lamp of the present invention operates L2~8~

at about 1~2 the prior current rating. This means there i5 less heat generated by both the lamp and the ballast. This reduction in heat not only allows the lamp to operate more efficiently, but also reduces the air conditioning requirements of buildings in which the lamps are used.
A suitable, and preferred ballast having the features required for the lamp of t~is invention is shown and described in U.S. patent 4,fllO,836 by Wîlliam 3. Roche and assigned to the assignee of the present invention.
As illustrated in FIG. 2, the improved lamp of this invention may be used in a four lamp enclosed recessed troffer fixture. When used in such a fixture the photometric and electrical data shown in Table I, below9 are attached.

TABL I

Relative Relative 8010r 8allast Light System System Rendering Lamp T~e Output Watts Efficacy Index Standard F40T12 ~CW) Standard 100% 174100% 6Z
Standard FflOT12 ~CW~ Energy Saving 100%158 110%
Energy Saving T12 (LW/LWX) Standard 97% 156 108%48/57 Energy Saving T12 ~LWfLWX) Energy Saving 95% 138120X
T8 Energy Saving 100% 132132% 75 The data in Table I indicate that the T8 lamp of the present invention, when used in a multiple lamp fixture, produces the same ' D-24~142 ~2~
~, , amount of light as the standard F40Tl2/CW lamp array ~Jhile consumin5 24% less energy. An attendant 32~ improvement in system efficiency is also achieved. As noted in Table I, the system efficiency of the present invention is 10-22Yo higher than that achieved with an energy-saving T12 lamp. It is also noted that the color rendering index shown in the last column of Table I is substantially higher than the index found in the prior energy-saving lamps.
A fixture efficiency of 74% is attained in the fixture illustrated in FI~. 2~ This compares with an efficiency of 68% for a standard F40 system.
There is thus provided an efficient fluorescent lamp with improved optics and which provides a higher lamp fixture efficiency.
The lamp is constructed for adaptation both thermally and optically to conventional lamp fixtures. The improved fluorescent lamp of this invention is also characterized by a relatively high color rendering index on the order of 75 achieved to a great extent by the use of rare earth phosphors.
The lamp differs from presently existing energy-saving fluorescent lamps by employing a T8, l inch diameter glass tube rather than the Tl23 1 1/2 inch diameter tube of the presently existing energy-saving lamps. This decrease in diameter allows for the economic use of the more efficient but more expensive rare earth phosphors. Although these phosphors are more expensive, with the use of the smaller diameter lamp, l/3 less phosphor is required to coat the inside glass surface.
Having described a limited number of embodiments of the present invention, it should now be apparent to those skilled in the art that numerous other embodiments and modifications thereof are contemplated as falling within the scope of claims.

Claims (7)

1. A fluorescent lamp operable at a lamp current on the order of 220 mA comprising: a glass envelope having electrodes at its ends and containing a discharge-sustaining and ultraviolet-radiation-emitting filling including an inert gas, a first layer of a halophosphate phosphor disposed on the inner envelope surface and a second layer of a rare earth phosphor superimposed on the first layer, said inert gas filling being substantially of a lighting weight inert gas than krypton and said envelope being of a diameter less than one and one-half inch diameter so as to require less of the second phosphor layer, the second layer powder weight being in the range on the order of 20% - 30% of the total phosphor powder weight.

2. A fluorescent lamp as set forth in Claim 1 wherein said inert gas comprises argon.

3. A fluorescent lamp as set forth in Claim 2 comprising substantially only argon as the inert gas.

4. A fluorescent lamp as set forth in Claim 1 wherein said envelope is of one inch diameter.

5. A fluorescent lamp as set forth in Claim 1 including ballast means for operating the lamp; at a current on the order of 220 milliamps.

6. A fluorescent lamp as set forth in Claim 1 wherein the color rendering index is on the order of 75.

7. A fluorescent lamp as set forth in Claim 1 wherein the relative system efficiency is on the order of 132% for a four lamp fixture, compared to the standard F40T12/CW system.