# Patent

## Title
Blue-enriched incandescent lamp

## Abstract

A blue-enriched incandescent lamp having on the interior surface of its light transmissive glass envelope a coating in accordance with an aspect of the invention. The coating contains a phosphor that is energized by the ultraviolet/violet emission (<420 nm) from the hot filament causing it to emit radiation in the range of 420 to 490 nm. In a preferred embodiment of the invention, the coating contains a europium-activated barium magnesium aluminate phosphor and a blue pigment.

## Background
TECHNICAL FIELD
This invention relates to incandescent lamps and more particularly to such lamps having enhanced emission in the blue region of the spectrum.
BACKGROUND OF THE INVENTION
The incandescent lamp has been the mainstay of illumination for well over one hundred years. It is simple to construct, relatively inexpensive to manufacture, easily transportable, and has a strong emission in the yellow-green portion of the spectrum. Numerous techniques have been employed over the years to modify the black-body emission of the hot filament. For example, U.S. Pat. No. 2,405,261 suggests adding a layer of a chrome green enamel to a clear enamel to modify the light output in the 500 to 560 nm range. U.S. Pat. No. 2,759,119 suggests adding a phosphor material to the interior of the lamp envelope, such phosphor material to be energized by the electron emission and the ultraviolet radiation from the tungsten filament to produce luminescence in the green portion of the spectrum. U.S. Pat. No. 5,118,985 suggests the inclusion of a phosphor material that absorbs radiation below 500 nm and emits radiation above 500 nm to provide an improved bugfoiler lamp.
Recently, incandescent lamps having an enriched emission in the blue region of the spectrum have been introduced for use in household applications, so-called “daylight” incandescent lamps. One of the methods for achieving this relative enhancement in the blue region has been to provide a blue-tinted glass for the envelope, e.g., a glass employing an additive of neodymium in the form of Nd2O3. Another technique, disclosed in U.S. Pat. No. 6,670,768, employs a coating employing a blue cobalt aluminate, CoAl2O4, pigment. Both of these techniques increase the relative proportion of blue in the emission spectrum of the lamp by absorbing a significant portion of the yellow light emitted by the filament
DISCLOSURE OF INVENTION
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enrich the blue emission of an incandescent lamp.
These and other objects are accomplished, in one aspect of the invention, by the provision of a blue-enriched incandescent lamp comprising: a light transmissive envelope; a filament within said envelope, the filament when operating producing at least an emission having a wavelength less than 420 nm; and a coating on the interior surface of said envelope, said coating comprising a phosphor that emits radiation in the range of 420 to 490 nm when stimulated by the emission from the filament. In a preferred embodiment of the invention, the coating contains a europium-activated barium magnesium aluminate phosphor and may contain a cobalt aluminate pigment. Other phosphors that may be used in this invention include: XMgAl10O17:Eu and XMgAl10O17:Eu,Mn (X=Sr and/or Ba), XAl2O4:Eu and X4Al14O25:Eu, where X=Ca, Sr, and/or Ba, as well as Sr6B(PO4)5:Eu, Sr2P2O7:Eu, X5Cl(PO4)3:Eu (X=Ca,Sr,Ba), and Y2O3:Bi. Since the incandescent lamps do not contain mercury, many sulfide, silicate, halide, oxyhalide and oxysulfide phosphors, e.g., ZnS, ZnS:Ag, and SrS:Eu, that are not used in Hg-discharge lamps could be employed for the present purpose. Blue-emitting phosphors developed for LED applications will also be suitable for this purpose, e.g., BaGa2S4:Eu.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view, partially in section, of an embodiment of the invention;
FIG. 2 is a graph of the reflectance of a coating comprising a blue-emitting phosphor and a blue pigment;
FIG. 3 is a graph of normalized emission curves of incandescent lamps having a clear envelope, an envelope coated with a blue-emitting phosphor, and an envelope coated with a mixture of the blue-emitting phosphor and a cobalt aluminate pigment; and
FIG. 4 is graph of the normalized emissions of incandescent lamps having a phosphor-coated envelope and a clear envelope.



DETAILED DESCRIPTION OF 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 with the above-described drawings.
Referring now to the drawings with greater particularity, there is shown in FIG. 1 an A-line incandescent type lamp 10 having on the interior surface of its light transmissive glass envelope 12 a coating 14 in accordance with an aspect of the invention. A filament 16 of, for example, tungsten, is electrically connected to and supported by lead-ins 18a and 18b, which extend through the seal of the lamp and are attached, as is known, to a screw base 20. Other forms of bases can be employed, such as the type known in the art as a bayonet base. Additionally, support wires 17 can provide another support for the filament 16. Coating 14 can be applied via a slurry or, alternatively, the coating can be applied electrostatically by means well known to those skilled in the art. See, for example, U.S. Pat. Nos. 2,995,463; 3,125,457; 3,320,460 and 4,633,127.
Coating 14 contains a blue-emitting phosphor that is energized by the ultraviolet/violet emission (<420 nm) from the hot tungsten filament and as a result emits radiation in the range of 420 to 490 nm. In a preferred embodiment of the invention the phosphor comprises a europium-activated barium magnesium aluminate phosphor. Preferably, the phosphor has a general formula of BaMgAl10O17:Eu (hereinafter, and in the drawing figures, BAM). The coating can be enhanced even further by the addition of a blue pigment such as cobalt aluminate, CoAl2O4.
Referring now to FIG. 2 there is shown the reflectance curve of a coating according to this invention. The coating comprises a mixture of BaMgAl10O17:Eu (OSRAM SYLVANIA Type 2467) and 2 weight percent CoAl2O4. This coating provides strong absorption in the yellow part of the spectrum, thus providing a blue body color.
FIG. 3 demonstrates the emission comparison of incandescent lamps having a clear envelope (the solid line), a phosphor-coated envelope (the dashed line) and an envelope coated with a phosphor and CoAl2O4. In each of these instances the phosphor is BaMgAl10O17:Eu.
FIG. 4 provides a detailed comparison of the normalized emission curves from a BaMgAl10O17:Eu coated incandescent lamp and a clear incandescent lamp. Both lamps were operated at 150 watts. The absorption of the short wavelength emission from the filament from about 350 nm to about 410 nm together with the enhanced emission at about 450 nm can be clearly seen.
While there have been shown and described what are present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

## Claims

1. A blue-enriched incandescent lamp comprising:
a light transmissive envelope;
a filament within said envelope, the filament producing at least an emission having a wavelength less than 420 nm when operating; and
a coating on the interior surface of said envelope, said coating containing a phosphor having a general formula BaMgAl10O17: Eu that emits radiation in the range of 420 to 490 nm when stimulated by the emission from the filament.

2. The blue-enriched incandescent lamp of claim 1 wherein said coating further contains a blue pigment.

3. The blue-enriched incandescent lamp of claim 1 wherein said coating includes about 2 weight percent cobalt aluminate.

4. The blue-enriched incandescent lamp of claim 1 wherein said emitted radiation from the phosphor has a maximum at about 450 nm.