GB2081497A - Fluorescent lamp construction utilizing a mixture of two phosphor materials - Google Patents

Abstract

A fluorescent lamp construction is described utilizing a blended phosphor mixture of two different phosphor materials to produce more efficient emission of white colored light in the daylight color spectral region. A first phosphor having a relatively narrow emission spectrum in a blue region of the visible spectrum is combined in predetermined proportions with a second phosphor having a relatively broad bimodal emission with blue-green color to achieve the desired white color point of lamp emission. The second phosphor is represented by the formula:- Ca10- omega -x-yCdwMnxSby (PO4)6F2-y-z ClzOy where w is 0 &cirf& 0 to 0 &cirf& 2; x is 0 &cirf& 03 to 0 &cirf& 025; z is 0 &cirf& 0 to 0 &cirf& 09 and y is 0 &cirf& 02 to 0 &cirf& 2. The first phosphor may be Sr10-zEuz(PO4)6Cl2 where z is 0 &cirf& 02 to 0 &cirf& 2, or it may be Ba2-z EuzMg2Al22O37 where z is 0 &cirf& 01 to 0 &cirf& 4. The blended phosphor mixture may contain 3-12 parts by weight of the first phosphor and 97-88 parts by weight of the second phosphor.

Description

SPECIFICATION Fluorescent lamp construction utilizing a mixture of two phosphor materials This invention relates generally to a low-pressure mercury vapour discharge fluorescent lamp having a phosphor coating to emit white light whose correlated color temperature is greater than the color temperature in the ANSI standard cool white oval when excited by the ultraviolet radiation generated from the mercury vapor discharge. It presently marketed lamps of this type two or more phosphors, one of which is calcium fluorophosphate activated only with antimony (so called "blue halo") and the others of which usually are calcium or strontium halophosphates coactivated with antimony and manganese, are blended to provide an overall white lamp color of desired color temperature.More particularly, the presently preferred type fluorescent lamp construction is intended for cjeneral illumination in the daylight color region as defined by well recognized ANSI color standards utilizing a two-component mixture of manganese and antimony activated calcium halophosphate blended with blue halo as the second phosphor both emitting broad band visible radiation. The known combination of haloapatite phosphors in a fluorescent lamp to generate white color light in the daylight color region yields a luminous efficacy of about 2600 lumens for a 40WT12 lamp size.

Improved luminous efficac.es near the cool white color region have been achieved with different phosphor combinations such as described in U.S. Patent No. 4,075,532. A phosphor blend is therein disclosed utilizing a first phosphor having a relatively narrow emission band peaking in a short visible wavelength (blue) region and a second phosphor having a relatively broad band emission peaking in the 570-600 nanometer (yellow) region of the visible spectrum to provide luminous efficacy greater than is obtained with a single conventional halapatite phosphor material at various ANSI defined standard white colors such as "cool white" and "white" when mixed in the proper proportions.As further therein defined, the required narrow blue emission peaking at approximately 450 nanometers wavelength is attributable to the blue phosphor corriponent is attained at a lesser power expenditure thereby permitting the excess power to be used in the yellow color region to enhance the overall luminosity of spectral power distribution in said type fluorescent lamps. The preferred yellow phosphor component in said two-phosphor system is a stoichiometric divalent manganese-activated calcium fluoroapatite compound further including limited antimony coactivation at proportions between said activator ions said to quench antimony emission in order to achieve the aforesaid overall spectral power distribution.Finally, said two-component phosphor system is said not only to increase the available luminosity but also to provide good fleshtome color rendition when compared with the standard cool white halo-phosphate phosphor material used in conventional cool white fluores cent lamps.

A preference for daylight color illumination exists in warmer climate regions as compared with cool white color illumination. The basis for such preference is believed to be the cooler appearance produced with daylight color illumination which provides more blue color emission than does cool white color illumination. Accordingly, it would be of significant benefit for this and other geographical regions to provide a phosphor material which produces such daylight color illumination with satisfa:tory color rendition and higher luminous efficacy than is achieved with the conventional phosphor mixture.As used herein, the term "daylight color region" signifies a white color lamp emission having C.I.E. color coordinates on or adjacent to the conventional black body locus with a color temperature greater than that of cool white and to include the color coordinates for the ANSI standard daylight color oval as well as non-standard colors such as "signwhite," "chroma 50" and "chroma 75" along with still further white colors not located within these color ovals. The desired phosphor material should thereby be capable of producing lamp emission of a white color within said daylight color region but which may produce other white color points as well.

It has now been discovered that an increased luminous efficacy in the white color region of high color temperature may be obtained by replacing the above-mentioned "blue halo" phosphor with a blue phosphor exhibiting a narrower emission bandwidth and by replacing the rernainder of the conventional phosphor blend with a second phosphor of blue-green hue which is an antimony and manganese activated calcium halophosphate activated with antimony and the proper concentration of manganese to obtain a blend line passing through the color point of .the said conventional phosphor, said second phosphor being Calo-w-x-yCdwMnXsby(po4)6F2-y-zClzOy where w is 00-02, x is 003-025, z is in the 0 0-0 09, and y is 002-02. The spectral 'power distribution of the phosphor herein disclosed is such that the color rendition index is lowered somewhat but nevertheless remains in the acceptable range and provides a pleasing color rendition of flesh tones. Said phosphor combination thereby represents a modification of the two-phosphor system commonly employed in daylight fluorescent lamps wherein the manganese and antimony activator levels and the halide content of the white phosphor component have been adjusted to provide appropriate blend lines in the daylight color region of the visible spectrum and which now appears blue-green in hue.As will be hereinafter described, the blue-green (modified white) phosphor component in the present two-phosphor system includes substantial antimony emission, however, in order to achieve lamp emission color points residing generally in the daylight color region. It will be apparent from the hereinafter provided detailed description that the desired white color point of lamp emission can be adjusted by variation in the weight proportions of these two-phosphor components in the blended mixture.

The color coordinates for said lamp emission can thereby be located not only within the ANSI standard daylight color oval but at white color points adjacent thereto while providing a 5-10% or greater gain in luminous efficacy at the same color point relative to a conventional phosphor mixture of high color temperature.

Useful blue-emitting phosphor materials in the present phosphor combination exhibit a relatively narrow emission band when excited by ultraviolet radiation from the mercury discharge at a peak wavelength of approximately 450 nanometer wavelength along with a quantum efficiency of at least 80%. An important function of said blue phosphor component is to pull the C.l.E. trichromaticity coordinates of the lamp emission to a point near the black body locus at the desired color point in the daylight color spectral region. A narrow emission band near the peak of the z tristimulus function efficiently performs said function while enabling a greater proportion of the visible emission from the lamp to be located at a higher wavelength region for increased luminous efficiency.A stoichiometric strontium europium chloroapatite (SECA) phosphor having the chemical formula Sr,O zEuz(PO4)6Cl2 where 0 02'z~0 02 can be selected to provide such narrow banded blue color emission and said phosphor material exhibits a color point having trichromatic coordinates x = 0-152, u = 0-027. Alternatively, a stoichiometric europium-activated barium magnesium aluminate having the chemical formula Ba22Mg2 EuA122037 where 0-1 z0-04 and exhibiting approximately the same trichromatic coordinates as the SECA compound, may be used.

As above pointed out, most of the ultraviolet excitation energy in the present improved lamp is utilized to excite the broader emission banded blue-green phosphor component. Useful phosphor materials exhibiting the desired broad band blue-green emission can be characterized as demonstrating a quantum efficiency within 10% of the phosphor blends presently used in conventional daylight color type fluorescent lamps and can be selected from the class of antimony and manganese coactivated halophosphate phosphors exhibiting both antimony and manganese emission in this host matrix. Specifically, the second phosphor component in the present two-phosphor system exhibits bimodal emission with one principal peak in the 550-600 nanometer region attributable to manganese emission and a second broader principal peak in the 450-510 nanometer spectral region attributable to antimony emission.The chemical composition of useful phosphor materials exhibiting such desired emission characteristics is Ca,O w x yCdwMnxSby(PO4)6F2 y zCIzOy where w is in the approximate range 0-0-0-2, x is in the approximate range 0-03-0-25, z is in the approximate range 0-0-0-09, and y is in the approximate range 0-02-0-2. Although the preferred embodiment of this phosphor material has a value of z = O to provide the maximum theoretical luminous efficacy, non-zero value of z may be desired to improve the quantum efficiency or to reduce the overall expense of manufacturing the phosphor blend.

The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view partially broken away of a fluorescent lamp construction in accordance with the present invention; Figure 2 depicts the emission spectra of each component of the phosphor blend most commonly used in present daylight fluorescent lamps as well as those of that phosphor blend described in this invention which can produce the same color in a 40-watt fluorescent lamp; and Figure 3 is a C.l.E. (x,y) chromaticity diagram including the black body locus line as well as the cool white ANSI oval with representative white color ovals in the daylight color region and further illustrating the operating principles of the present invention.

There is illustrated in Fig. 1 a typical fluorescent lamp 1 comprising an elongated soda-lime glass bulb 2 with circular cross section. The discharge assembly in said lamp is the usual electrode structure 3 at each end supported by lead-in wires 4 and 5 which extend through a glass press seal 6 in a mount stem 7 to the contacts of a base 8 affixed at opposite ends of the lamp. The discharge sustaining filling in the sealed glass tube is an inert gas such as argon or mixture of argon and other gases at a low pressure in combination with a small quantity of mercury to provide the low vapor pressure manner of lamp operation. The inner surface of the glass bulb is provided with a phosphor coating 9 of the present phosphor combination which is applied extending substantially the full length of the bulb and around the bulb circumferential inner wall.

In a computer study utilizing experimentally measured spectra of pertinent phosphors, a preferred embodiment consisting of a blend of SECA and a blue-green calcium fluorapatite with a manganese concentration appropriate for the attainment of a color point equal to the specification of the standard daylight fluorescent lamp presently on the market was compared to that lamp on a lumen output basis assuming equal power input and equal quantum efficiency.

The lamp with the new phosphor was determined to operate with an improved luminous efficacy df nine percent. The particular stoichiometric fluorapatite phosphor selected provides approxi fnately 99% of the total luminous output in said phosphor combination and with the mol fraction of manganese ion in said phosphor being varied to achieve the desired color point of lamp emission in the daylight color region.

In Fig. 2 are shown in proper relative magnitude the spectral power distribution of each phosphor blend component such that the addition of the blue halo emission 1 and the white calcium halophosphate emission 2 together with the visible light emitted directly by the mercury discharge produce the emission spectrum in the present embodiment of a daylight fluorescent lamp. The components of the improved blend are SECA 3 and a calcium fluorapatite containing 0'1 5 mol fraction of Mn 4 which together with the same visible Hg emission will result in a lamp with an identical color point. To be noted particularly in the relative width of the blue component of the established blend 1 and the much narrower width of the blue component of the proposed blend 3.The smaller average wavelength of the blue-green component of the proposed blend 4 when compared to the yellow component of the established blend 2 is necessary to attain the same color point for the two systems.

To more fully illustrate the improvement obtained in emission behavior for the above type lamp construction utilizing the present phosphor combination as a blended mixture, various 40WT12 size lamps were constructed for comparison with the conventional daylight fluorescent lamp. The test lamps were constructed in the conventional manner except for substitution of the present phosphor mixture and utilizing a binary admixture of the blue-green phosphor material with europium-activated chloroapatite (SECA) phosphor as the narrow band emitting blue phosphor component, all in the weight proportions specified in Table I below.The broad band blue-green emitting phosphor component in said test lamps consisted of antimony and manganese coactivated fluoroapatite phosphors exhibiting both antimony and manganese emission in this host matrix as above described with a constant mol fraction content of antimony activation at approximately 0 10 and with the manganese activation level being varied at the mol fractions specified in said Table. The SECA phosphor weight proportions were also varied as specified in said Table at a weight ratio range of approximately 3-12 weight percent of said phosphor in the particular admixture employed at a constant 0'2 mol fraction of divalent europium activation in said phosphor material.The conventional 100-hour lumen values achieved with said test lamps together with the lamp emission color coordinates are also reported in said Table along with the color coordinates of lamps with the blue-green phosphor component alone.

TABLE I-IN LAMP COLOR COORDINATES Blue-Green Phosphor Blue Phosphor Lamp Values Sample (Mn Mol Fraction) X Y (Weight Percent) X Y Lumens CR1 Values A 06 281 345 9 271 306 2492 76 B 12 327 377 8 311 377 2859 68 C '18 363 398 7 347 368 3096 62 The lamp color points and lumen values reported in the above Table for said test lamps can best be understood by reference to Fig. 3 in the drawings. Such reference demonstrates that the color points for all lamp samples A-C generally reside in the daylight color region of the visible spectrum and with the color point for the sample B lamp residing within the standard daylight color oval.Valid comparison of the lumen value reported in the above Table for the sample B lamp can also be made with the 2600 lumens obtained by conventional daylight fluorescent lamps of the same size by reason of the general correspondence in color points found. Such further comparison finds a 10% lumen gain being derived in accordance with the present invention. The lumen values reported in said Table for the remaining sample lamps demon .strates comparable or greater improvement if compared with conventional phosphor admixtures at the same color points.

As above indicated, the color coordinates for the present lamp constructions are shown in Fig.

3. A chromaticity diagram is therein illustrated whereby the x and y color coordinates obtained according to the well recognized C.I.E. method of measurement are applied for a representation of the white color points achieved with the present phosphor combinations. Also presented on said chromaticity diagram and the ANSI standard daylight and cool white color ovals along with other recognized color ovals in the daylight color region. The color points achieved with the blue-green phosphor component in the present phosphor combinations are illustrated in said diagram by a straight line 10 for a manganese activator level range from 03 mol fraction to 25 mole fraction.The color point for 40WT12 lamp coated with a representative SECA phosphor component only in the present admixture also appears in said diagram from which a further straight line 12 is shown extending from said color point to intersect the straight line 10 and from which the lamp color point using a particular binary phosphor mixture can be established.* The significance of said straight line relationships 10 and 12 can be appreciated by noting that, the color point for lamp sample B lies on straight line 12 within the daylight color oval and the intersection point between said straight lines is fixed by the manganese activator level in the blue-green phosphor component of this phosphor mixture.By varying the relative weight proportions of the same phosphor constituents in said phosphor mixture, different lamp emission color points are obtained in the daylight color region which lie on the straight line 12. It is understandably further possible to vary intersection points between straight lines 10 and 12 through adjustment of the manganese activator level in the blue-green phosphor component of said binary phosphor admixture and thereby obtain still other white color points in the daylight color region which can either reside within the other color ovals depicted or be located adjacent thereto.The remaining lamp color points reported in the above Table and shown in said chromaticity diagram adhere to the same general relationship so that it becomes possible to predetermine both the composition of the individual phosphor constituents as well as the relative weight proportions therebetween to be used in order to achieve a desired lamp emission color point. As can also be noted from said chromaticity diagram, all lamp color points A-C lie adjacent the black body locus line which allows the respective color temperatures to be established at about 4500"K or greater.

From the above described preferred embodiments, it will be evident that a particular two component phosphor combination has been provided which achieves significantly more light output than conventional phosphor combinations when employed in daylight type fluorescent lamps. It will be apparent, however, that further modification in the illustrated embodiments is contemplated such as by addition of a third phosphor component, for example, trivalent europium activated yttrium oxide to adjust the lamp color point or some other desirable lamp operating characteristics. Consequently, it is intended to limit the present invention only by the scope of the appended claims.

Claims (8)

1. A fluorescent lamp having a light transmissive sealed envelope coated with phosphor, a gaseous medium within said envelope which can be ionized to generate a low pressure mercury discharge for conversion by said phosphor of at least a portion of the radiation emitted from said discharge to visible light having a white color, the phosphor comprising a blended mixture of:: a first phosphor which has a narrow emission spectrum in a blue portion of the visible spectrum, and a second phosphor which has a broad bimodal emission spectrum in the blue-green portion of the visible spectrum and is represented by the formula Ca,O w x-yCdwMnXsby(po4)6F2-y-2clZoy where w is 0-0-0-2, x is 0-03-0-25, z is 0-0-0-09 and y is 0-02-0-2; said first and second phosphors being uniformly blended together in proportional relationship preselected to provide increased luminous efficiency for said mixture.

2. A lamp as claimed in claim 1 wherein said phosphor is Sr,O zEuz(PO4)6Cl2 wherein z is 0-02-0-2.

3. A lamp as claimed in claim 1 or claim 2 wherein the proportional relations between said first and second phosphor achieves a daylight white color.

4. A lamp as claimed in claim 3 wherein the blended phosphor mixture is in parts by weight 3-12 parts of said first phosphor and 97-88 parts of said second phosphor.

5. A lamp as claimed in claim 3 wherrein the mol fraction of manganese in said first phosphor is varied to achieve said daylight white color.

6. A lamp as claimed in claim 1 wherein said first phosphor is Ba2 zEuzMg2Al22037 wherein z is 0-1-0-4, said first phosphor having a peak emission of approximately 450 nanometers wavelength.

7. A lamp as claimed in claim 1 wherein said second phosphor has one principal peak in the 550-600 nanometer region attributable to manganese emission and a second broader principal: peak in the 450-510 nanometer region attributable to antimony emission.

8. A lamp as claimed in claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.