US 3707642 A
Fluorescent lamp incorporates a finely divided phosphor coating of different particle sizes. The phosphor particles of the coating which are positioned next to the envelope inner surface are preferably relatively fine and are smaller than the relatively coarse particles which are positioned farther from the envelope inner surface. Such a special phosphor coating enables the total amount of coated phosphor to be considerably reduced.
Claims available in
Description (OCR text may contain errors)
United States Patent Thornton, Jr.
451 Dec. 26, 1972  VAPOR LAMP WHICH INCORPORATES A SPECIAL PHOSPHOR COATING  Inventor:
 Assignee: Westinghouse Electric Corporation,
 Filed: Aug. 31, 1970  Appl.No.: 68,221
 U.S. Cl ..3l3/109  Int. Cl. ..-..'H01j 61/44, l-lOlj 61/48  Field of Search ..3l3/109, 221
[5 6] References Cited UNITED STATES PATENTS 7/1942 Holman et al. ..3l3/l09 William A. Thornton, Jr., Cranford,
2,854,600 9/1958 Van De Weijer et al ..3 1 3/109 2,774,903 12/1956 Burns ..3 1 3/109 3,581,139 5/1971 l-laft et al ..313 /109 Primary Examiner-Palmer C. Demeo Attorney-A. T. Stratton and W. D. Palmer  1 ABSTRACT Fluorescent lamp incorporates a finely divided phosphor coating' of different particle sizes. The phosphor particles of the coating which are positioned next to the envelope inner surface are preferably relatively fine and are smaller than the relatively coarse particles which are positioned farther from the envelope inner surface. Such a special phosphor coating enables the total amount of coated phosphor to be considerably reduced.
7 Claims, 7 Drawing Figures PATENTEDMBZ I912 3, 701.642
SHEET 1 OF 3 WITNESSES TOR 09 M) 4% William A. Thornion AT TORNEY BACKGROUND OF THE INVENTION This invention generally relates to discharge devices and, more particularly, to a fluorescent lamp which incorporates a particular phosphor coating which permits less phosphor to be used.
The phosphor material which is most used in fluorescent lamps is a so-called halophosphate phosphor as generally described in U.S. Pat. No. 2,488,733, dated Nov. 22, 1949. It is known that a small increase in the lumen output of a fluorescent lamp can be obtained by increasing the average particle diameter of the fluorescent lamp phosphor. It is also known that the smaller the average particle diameter of the phosphor, the greater the reflectance thereof for the ultraviolet radiations which are used to excite the phosphor.
SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a fluorescent lamp which comprises an elongated, light-transmitting envelope having electrodes operatively disposed proximate the ends thereof and enclosing a discharge-sustaining filling. A phosphor coating of predetermined thickness and comprising finely divided particles of different predetermined average particle size is carried on the inner surface of the envelope. The phosphor particles comprising the coating are positioned relative to the envelope inner surface according to their average particle size, with the phosphor particles which are positioned closer to the envelope inner surface having a smaller average particle size than those proximate phosphor particles which are positioned farther from the envelope inner surface.
BRIEF DESCRIPTION OF THE DRAWINGS For better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings in which:
FIG. 1 is an elevational view, partly broken away, illustrating a fluorescent lamp which incorporates a phosphor coating of the present invention;
FIG. 2 is a fragmentary enlarged view showing the phosphor coating in greater detail;
FIG. 3 is a symbolic representation which illustrates the partition of incoming diffuse ultraviolet radiation in atop phosphor layer and a bottom phosphor layer, which layers are carried on an ultraviolet-absorbing glass substrate; I
FIG. 4 is a graph of absorption loss and reflection loss, plotted on the ordinates, versus phosphor coating weight plotted on the abscissa, illustrating optical performance characteristics for a phosphor layer in a conventional fluorescent lamp;
FIG. 5 is a family of curves representing ultraviolet reflection loss and ultraviolet absorption loss for a double layer phosphor coating of the present invention, wherein the coating weight of the top layer is plotted on the ordinate and the coating weight of the bottom layer is plotted on the abscissa,
FIG. 6 is a graph similar to FIG. 5, but taken for a phosphor bottom layer which has optical characteristics differing from those of the bottom layer used in taking the curves shown in FIG. 5;
FIG. 7 is a graph similar to FIG. 5, but wherein the optical characteristics of the phosphor comprising the top layer differ from those optical characteristics of the top layer used in taking the curves shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT The numeral 10 in FIG. 1 indicates a fluorescent lamp comprising a tubular, vitreous envelope l2 coated internally with a double phosphor layer 14, 16, which varies in particle size in accordance with the present invention. Sealed in each end of the envelope 12 are mounts, each comprising an electrode 18, supported by lead-in conductors 20. Base caps 22.and base pins 24 are provided at the envelope ends. Except for the special phosphor layer 14, 16, the construction of the lamp 10 is conventional, and the envelope encloses a small charge of mercury and inert, ionizable starting gas to sustain a low-pressure, ultraviolet-generating discharge between the lamp electrodes during operation. It should be understood that the present invention is applicable to any type of fluorescent discharge device wherein a source of ultraviolet radiations is surrounded or enclosed by an envelope which carries on its inner surface a coating of phosphor. An example of such a fluorescent discharge device is a so-called color-corrected high-pressure mercury-vapor discharge device, as disclosed in U.S. Pat. No. 2,748,303, dated May 29, 1956.
The phosphor layer is shown in greater detail in the enlarged fragmentary view of FIG. 2, wherein the layer 14, which preferably has a relatively fine particle size, is positioned adjacent the inner surface of the envelope l2 and thus farthest from the ultraviolet-generating discharge. The layer 14 will be referred to as the bottom layer. The layer .16, which is positioned farthest from the inner surface of the envelope 12, and thus closest to the ultraviolet-generating discharge, has a relatively large particle size and will be referred to as the top layer. 7
In FIG. 3 is shown a symbolic representation for the partition of incoming diffuse ultraviolet radiation I in a top layer (subscript T) and a bottom layer (subscript B) on an absorbing glass substrate. R is the diffuse reflectance of one layer and T, is its diffuse transmittance. The rays are only symbolic and represent total radiation moving upward or downward at a given horizontalplane. The relative values of reflectance and transmittance will be referred to hereinafter in presenting the optical characteristics for phosphor layers.
In FIG. 4 are illustrated the optical characteristics of a single homogeneous layer of halophosphate phosphor having an average particle diameter of about 10 to 14 microns as used in a standard conventional fluorescent lamp. Such a fluorescent material has optical characteristics, as will be explained hereinafter, of a 0.70 and r 0.15. In explanation of these terms, the absorption coefficient for ultraviolet radiation, a,.of the phosphor material is fairly constant at about 0.70 per mg/cm of coated phosphor, irrespective of the phosphor particle size. The reflection coefficient for ultraviolet radiations striking a phosphor layer, r," varies with the phosphor particle size, and for a phosphor as generally used commercially in a fluorescent lamp, wherein the average particle size falls within the range of from about to 14 microns, r" about 0.15 per mg/cm of coated phosphor. With respect to the curves shown in FIG. 4, for such a phosphor, the absorption loss of 254nm ultraviolet radiation in the glass substrate 'due to transmission through the phosphor layer is plotted on one ordinate and the reflection loss of ultraviolet-radiation which strikes the phosphor layer and is reflected therefrom back toward the discharge is plotted on the other ordinate. As a matter of practicality, the phosphor layer is normally deposited to a coating weight of approximately 5 mg/cm of coated surface, and as shown in FIG. 4, both curves of absorption loss and reflection loss vs. coating weight are becoming be understood, of course, that all of this ultraviolet radiation which is reflected back is not lost through reabsorption in the discharge, but commensurate with a reasonable coating weight and other optical performancecharacteristics, 'it is desirable to minimize both the absorption loss and reflection loss as much as possible.
With respect to a given phosphor material, the opti calcharacteristics will vary primarily with the particle size of the phosphor and by way of specific example, a halophosphate phosphor is considered herein since this is the phosphor most used. While the term halophosphate phosphor encompasses within its genus many different species of halophosphate phosphor, the most common species is a calcium halophosphate phosphor. As a specific example for preparing a socalled cool white calcium halophosphate phosphor, the following ingredients are mixed in the following molar proportions: CaO, 4.7; P 0 1.5; Mn, 0.08; Sb, 0.1; F,
l 1.0; CI, 0.5; and Cd, 0.05. Of course these ingredients are normally included as the dicalcium phosphate and respective carbonates, etc., as is conventional. The raw-mix ingredients are fired in nitrogen at a temperature of about l,200 C for about 3 hours. The fired phosphor is then lightly milled, washed in dilute nitric acid, rinsed, and thereafter spray dried.
In determining the particle the size for halophosphate phosphor, it is convenient to utilize a sured and the particle size calculated using Stokes law.
In order to separate a prepared phosphor into individual phosphor batches of varying particle size, it is convenient to use a water settling technique as is disclosed in US. Pat. No. 3,255,373, dated June 7, .1966. Alternatively, the particle size of the raw materials can control the particle size of the final phosphor to some degree.
' As stated hereinbefore, the optical characteristics for the usual halophosphate phosphor for 254nm ultraviolet radiations are a 0.70 per mg/cm and r 0. l 5 per mg/cm of coated phosphor. The reflectance of a very thick layer of the phosphoris simply related to these coefficients a and r in accordance with the following formula:
Applying the foregoing formula to halophosphate phosphor of varying particle sizes, the varying interrelationships of reflectance, characteristic reflectance coefficient and particle size are shown in the following Table I:
TABLE I acmlmg R m rcm lmg 1 Average Particle Size l 0.70 0.033 0.05 coarse (20 microns) (2)0.70 0.09 v 0.15 normal 12 microns) (3) 0:70 0.27 0.70 (4 microns) (4) 0.70 0.51 l 3.0 fine 1 micron) If a fluorescent lamp is first coated with a bottom layer'of fine halophosphate, such as designated (4) in Table l, and this first-applied fine layer has coated thereover a layer of coarse halophosphate, such as is designated (1) in Table l, the optical characteristics of the resulting fluorescent lamp will be as set forth in the family ofcurves illustrated in FIG. 5. In this figure, the heavy, generallyhorizontal lines represent reflection loss of ultraviolet radiation which is directed back from the top phosphor'layer toward the discharge, and absorption loss is represented by the generally slanted, parallel lines which represent the ultraviolet loss in the glass substrate. [n.order to duplicate the reflection loss and absorption loss characteristics for the standard fluorescent lamp, the optical performance of which is shown in FIG. 4, the reflection loss and absorption loss characteristics should fall within the dashed circle shown in FIG. 5. Significantly, when the bottom layer is coated to a weight of approximately 1.1 mg/cm of coated area and the top layer is coated to a weight of approximately 1.6 mg/cm of coated area, the reflection loss and absorption loss characteristics for a standard fluorescent lamp will be substantially duplicated. While in a standard fluorescent lamp, this required approximately 5 mg/cm of coated phosphor, with the special double layer of coating as described herein, only 1.1 1.6 2.7 mg/cm of coated phosphor is required. Accordingly, with the particular phosphor constituents which comprise the coating, only about.
one half the phosphor need be used to obtain the same reflection and the transmission characteristics which are used to optimize output in the standard fluorescent lamp.
A different family of curves will exist for each individual variation in particle size which is used forv each of the two layers. In FIG. 6 is shown a different family mmm of curves wherein the bottom layer is formed of a less finely divided phosphor, such as represented by (3) in Table I, with the top layer being the coarse material represented by (1 in Table I. The dashed circle in this figure represents the 2 percent absorption loss and 9 percent reflection loss which is achieved with standard fluorescent lamp. As shown in FIG. 6, approximately 2.6 mg/cm of coating weight for the bottom layer and 0.8 mg/cm of coating weight of the top layer are required to duplicate the absorption loss and reflection loss characteristics of the standard lamp. Thus'with this modification, the total phosphor weight required is still substantially reduced.
In FIG. 7 is shown still another family of curves wherein the top layer comprised normal halophosphate as represented by (2) in Table I and the bottom layer was formed of fine halophosphate as represented by (4) of FIG. 1. In this case the total phosphor weight required to produce the same absorption and reflection loss characteristics is approximately 3 mg/cm as represented by dashed circle on this figure.
While the foregoing example has considered in detail a halophosphate phosphor, any phosphor can be substituted therefore, such as manganese activated zinc silicate.
Weight reductions of phosphor required normally can be achieved only by utilizing a phosphor particle size for the bottom layer which is smaller than the particle size for a single homogeneous layer which is normally used. Nevertheless, if the bottom layer is finer than the top layer, some improvement in lamp characteristics will be obtained, whatever the relative particle sizes of the two layers. As a general rule, in the case of two discrete layers for example, the particles comprising the layer which is positioned closest to the envelope inner surface should have an ultraviolet reflectivity of at least about 0.2, and the particles comprising the top layer which is positioned farthest from the envelope should have an ultraviolet reflectivity of less than about 0.1.
In the case of a halophosphate phosphor, the portion of the phosphor coating which is positioned nearest to the inner surface of the envelope, and thus farthest from the operating discharge, preferably has an average particle size not exceeding about 3 microns, and the portion of the top phosphor coating preferably has an average particle size of at least about 6 microns. While two discrete layers are preferred, more than two layers may be utilized if desired, gradually varying the particle size from the bottom layer to the top layer.
In order to coat the two discrete layers onto the envelope 12, the bottom layer is first applied in conventional fashion by suspending the phosphor in a liquid vehicle with an organic binder to form a slurry, with the resulting phosphor slurry flushed over the envelope surface, and the vehicle then volatilized. The top layer is then applied in an identical fashion, using a different vehicle and binder, after which the coated envelope is lehred. More than two layers can be similarly applied. The phosphor can also be applied as a single layer, with the particle sizes so selected that the smallest particles are nearest to the envelope surface and the largest particles are farthest from the envelope, with a gradual, nondiscrete, monotonic variation in particle size existing between the innermost and outermost portion of the layer.
In the-case of the preferred halophosphate phosphor, the bottom layer which is adjacent the inner surface of the envelope desirably has a particle size of from about 0.1 to 3 microns, and the layer which is farthest from the inner surface of the envelope desirably has a particle size of from about 6 to 35 microns. The coating weights also can vary and desirably the coating weight of the bottom layer will be from 0.2 to 2 mg/cm of coated bulb surface and the coating weight-of the top layer will be from 0.5 to 2 mg/cm of coated bulb surface. For best performance this coating weight will vary from about 0.8 to 1.4 mg of phosphor per cm for the top layer, with a large divergence in particle size between the bottom layer and top layer.
1. In combination with a fluorescent discharge device comprising a light-transmitting envelope which encloses a source of ultraviolet radiations, the improved phosphor coating which is present in predetermined amount and comprises finely divided particles of different predetermined average particle size carried on the inner surface of said envelope, the phosphor particles comprising said coating being positioned relative to said envelope inner surface according to their average particle size, with phosphor particles which are positioned closer to said envelope inner surface having a smaller average particle size than those proximate phosphor particles which are positioned farther from said envelope inner surface.
2. The device as specified in claim 1, wherein said phosphor coating is carried on the inner surface of said envelope in at least two discrete layers, the particles comprising the layer which is positioned closer to said envelope inner surface havingan ultraviolet radiation reflectivity of at least about 0.2, and the particles comprising the layer which is positioned farther from said envelope having an ultraviolet radiation reflectivity of less than about 0. l.
3. A fluorescent lamp comprising:
a. an elongated light-transmitting envelope having electrodes operatively disposed proximate the ends thereof and enclosing a small charge of mercury and inert ionizable starting gas, and a lowpressure ultraviolet-generating discharge adapted to be sustained between said electrodes during operation of said lamp; and b. a halophosphate phosphor coating carried on the inner surface of said envelope, said phosphor coating comprising individual particles of different average particle size, the portion of said phosphor coating which is positioned nearest to the inner surface of said envelope, and thus farthest from said operating discharge, having an average particle size not exceeding about 3 microns, and the portion of said phosphor coating which is positioned farthest from the inner surface of said envelope, and thus closest to said operating discharge, having an average particle size of at least about 6 microns.
4. The lamp as specified in claim 3, wherein said phosphor coating is carried on the inner surface of said envelope in at least two discrete layers, and any said layer, which is positioned closer to the inner surface of said envelope than an adjacent layer, having a smaller particle size than the particle size of said adjacent layer.
5. The lamp as specified in claim 3, wherein said phosphor is present in two discrete layers, said layer weight of from 0.5 to 2 mg/cm of coated bulb surface.
7. Thelamp as specified in claim 6, wherein said layer which is closest to the inner surface of said envelope has a coating weight of from 0.8 to 1.4 mg/cm of coated bulb surface, and said layer-which is farthest from the inner surface of said envelope has a coating weight of from "1.3 to 1.8 mg/cm of coated bulb surface.
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