U.S. patent application number 14/487788 was filed with the patent office on 2016-03-17 for compositions and methods for modifying lumen maintenance characteristics of phosphor-containing coatings.
The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to William Winder BEERS, William Erwin COHEN, Fangming DU, Jon Bennett JANSMA.
Application Number | 20160079053 14/487788 |
Document ID | / |
Family ID | 55314737 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160079053 |
Kind Code |
A1 |
JANSMA; Jon Bennett ; et
al. |
March 17, 2016 |
COMPOSITIONS AND METHODS FOR MODIFYING LUMEN MAINTENANCE
CHARACTERISTICS OF PHOSPHOR-CONTAINING COATINGS
Abstract
Phosphor-containing coating compositions and methods capable of
changing the lumen maintenance characteristics of
phosphor-containing coatings and fluorescent lamps that utilize
such coatings. Lumen maintenance of a fluorescent lamp can be
modified by forming a phosphor-containing coating to contain at
least a first phosphor that depreciates during operation of the
fluorescent lamp, and forming the phosphor-containing coating to
further contain an additive composition in a sufficient amount and
sufficiently uniformly distributed in the phosphor-containing
coating to inhibit depreciation of the first phosphor during
operation of the fluorescent lamp.
Inventors: |
JANSMA; Jon Bennett; (Pepper
Pike, OH) ; BEERS; William Winder; (Chesterland,
OH) ; COHEN; William Erwin; (Solon, OH) ; DU;
Fangming; (Northfield, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
Schenectady |
NY |
US |
|
|
Family ID: |
55314737 |
Appl. No.: |
14/487788 |
Filed: |
September 16, 2014 |
Current U.S.
Class: |
313/486 ;
252/301.4R; 252/301.6F; 252/301.6P |
Current CPC
Class: |
H01J 61/44 20130101;
C09K 11/025 20130101; C09K 11/778 20130101; C09K 11/7734 20130101;
C09K 11/73 20130101; C09K 11/7478 20130101; C09D 5/22 20130101;
C09K 11/676 20130101; C09K 11/71 20130101 |
International
Class: |
H01J 61/44 20060101
H01J061/44; C09K 11/67 20060101 C09K011/67; C09K 11/73 20060101
C09K011/73; C09K 11/77 20060101 C09K011/77; C09K 11/71 20060101
C09K011/71; C09D 5/22 20060101 C09D005/22; C09K 11/02 20060101
C09K011/02; C09K 11/74 20060101 C09K011/74 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. A method for improving the lumen maintenance of a fluorescent
lamp, the method comprising: providing a phosphor blend containing
a first phosphor that exhibits a poorer lumen maintenance than at
least a second phosphor in the phosphor blend; and forming with the
phosphor blend a phosphor-containing coating of a fluorescent lamp,
wherein the phosphor-containing coating contains an additive
composition in a sufficient amount and sufficiently uniformly
distributed to improve the lumen maintenance of the first phosphor;
wherein the forming step comprises: combining the phosphor blend
with a liquid vehicle that contains at least one soluble salt of
the additive composition; and converting the soluble salt to the
additive composition.
14. The method of claim 13, wherein the additive composition is
yttria and/or lanthana.
15. The method of claim 13, wherein the forming step causes the
additive composition to define a uniform coating over and
surrounding individual particles of the first phosphor.
16. (canceled)
17. A method for improving the lumen maintenance of a fluorescent
lamp, the method comprising: forming a phosphor-containing coating
to contain at least a first phosphor that depreciates during
operation of the fluorescent lamp; and forming the
phosphor-containing coating to further contain an additive
composition in a sufficient amount and sufficiently uniformly
distributed in the phosphor-containing coating to inhibit
depreciation of the first phosphor during operation of the
fluorescent lamp; wherein the step of forming the
phosphor-containing coating to further contain an additive
composition comprises: combining a phosphor blend with a liquid
vehicle that contains at least one soluble salt of the additive
composition; and converting the soluble salt to the additive
composition.
18. The method of claim 17, wherein the additive composition is
yttria and/or lanthana.
19. (canceled)
20. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to lighting systems
and related technologies. More particularly, this invention relates
to fluorescent lamps and coating systems utilized by fluorescent
lamps to generate visible light.
[0002] Fluorescent lamps have been in use and commercialization
since the 1930s. More recently, both consumers and producers have
voiced increased concerns for energy efficiency and environmental
impact of products, spanning all industries, including the lighting
industry. As such, fluorescent lamps have seen an increase in usage
due to their increased energy efficiency as compared to
conventional incandescent lights. Fluorescent lamps see a great
deal of competition from light-emitting diode (LED) lights due to a
potential for greater efficiency and luminosity of LEDs.
Significant effort and research have been made in the interest of
improving fluorescent light lumen output without increasing power
requirements or significantly increasing material costs.
[0003] A nonlimiting example of a fluorescent lamp 10 is
schematically represented in FIG. 1. The lamp 10 is represented as
having a sealed glass tube comprising of a glass envelope or shell
12 enclosing an interior chamber 14. The chamber 14 is preferably
at very low pressure, for example, around 0.3% atmospheric
pressure, and contains a gas mixture having at least one
constituent that can be ionized to generate radiation that includes
ultraviolet (UV) wavelengths. According to the current state of the
art, such a gas mixture includes one or more inert gases (for
example, argon) or a mixture of one or more inert gases and other
gases at a low pressure, along with a small quantity of mercury
vapor. Electrodes 16 inside the chamber 14 are electrically
connected to electrical contact pins 18 that extend from
oppositely-disposed bases 20 of the lamp 10. When the contact pins
18 are connected to a power source, the applied voltage causes
current to flow through the electrodes 16 and electrons to migrate
from one electrode 16 to the other electrode 16 at the other end of
the chamber 14. In the process, this energy converts a small amount
of the liquid mercury from the liquid state to a charged (ionized)
gaseous (vapor) state. The electrons and charged gas molecules move
through the chamber 14, occasionally colliding with and exciting
the gaseous mercury molecules, raising the energy level of the
electrons in the mercury atoms. In order to return to their
original energy level, the electrons release photons.
[0004] Due to the arrangement of electrons in mercury atoms, most
of the photons released by these electrons are in the ultraviolet
(UV) wavelengths. This is not visible light, and as such for the
lamp 10 to emit visible light these photons must be converted to a
visible light wavelength. Such a conversion can be performed by a
coating 22 disposed at the interior surface of the glass shell 12.
The coating 22 comprises phosphor powders and, as represented in
FIG. 1, is separated from the glass shell 12 by a UV-reflecting
barrier layer 24 of, for example, alumina (Al.sub.2O.sub.3). As
known in the art, the coating 22 can be produced by applying to the
shell 12 a suspension containing particles of the desired
phosphor(s) combined with one or more surfactants, dispersants,
thickening agents, etc., and then performing a lehring operation
that involves heating the applied suspension to remove suspension
components, leaving the phosphor particles (and potentially other
particle materials) to form the coating 22 on the shell 12. The UV
wavelengths emitted by the ionized mercury vapor are absorbed by
the phosphor composition within the coating 22, resulting in
excitation of the phosphor composition to produce visible light
that is emitted through the glass shell 12. More particularly, when
electrons of the phosphor atoms are struck by photons, the
electrons become excited to a higher energy level and emit a photon
to return to their original energy level. The emitted photon has
less energy than the impinging photon and is in the visible light
spectrum to provide the lighting function of the lamp 10. The color
and luminosity of the lamp 10 are largely the result of the
phosphor or phosphors used in the coating 22.
[0005] The mercury in low pressure fluorescent lamps predominantly
emits UV radiation having a wavelength of 254 nm, and to a lesser
extent a wavelength of 185 nm. As used herein, "predominantly" and
"predominant" mean that something contains more of one constituent
(the "predominant constituent"), e.g., by weight, volume, molar, or
other quantitative percent, than any other individual constituent.
As these terms are used herein in relation to radiation,
"predominantly" and "predominant" signify a wavelength that is more
prevalent in a band of radiation than any other individual
wavelength. Some estimates are that roughly 90% of UV radiation
generated by low pressure fluorescent lamps is at the predominant
254 nm wavelength, with the balance (roughly 10%) being the 185 nm
wavelength. Both of these wavelengths fall within a wavelength
range known as ultraviolet subtype C. Phosphors used in low
pressure mercury lamps are typically excited by different ranges of
wavelengths that encompass the primary wavelength (254 nm) to
absorb as much UV radiation as possible. The efficiency and
effectiveness of fluorescent lamps and their coating systems can
differ based on what phosphors are used and what wavelengths of
light are absorbed.
[0006] The apparent, or perceived, color of a light source can be
described in terms of color temperature, which is the temperature
of a black body that emits radiation of about the same chromaticity
as the radiation considered. A light source having a color
temperature of 3000K has a larger red component than a light source
having a color temperature of 4100K. As additional examples, a
fluorescent lamp having a perceived "warm white" color may have a
correlated color temperature (CCT) of approximately 3000K, whereas
a fluorescent lamp having a perceived "cool white" color may have a
CCT of approximately 4000K. Another measure of fluorescent lamp
performance is the color rendering index (CRI). The CRI of a light
source does not indicate the apparent color of the light source,
but instead is a quantitative measure of the ability of a light
source to reproduce the colors of objects faithfully in comparison
with an ideal or natural light source. CRIs can only be accurately
compared among two light sources having the same CCT. The highest
possible numeric CRI value is 100. Incandescent lamps, which are
essentially blackbodies, have CRIs of 100. Typical LEDs have CRIs
of 80 or more, with CRIs of up to 98 being claimed, whereas
fluorescent lamps typically have CRIs in a range of about 50 to
about 90. In this regard, a high CRI for fluorescent lamps can be
considered to be about 80 and higher, particularly at least 87.
[0007] Another metric by which fluorescent lamp performance can be
gauged is light output or lumen maintenance, which characterizes
the ability of a lamp to provide roughly the same amount of
luminosity over its life span. All lamps exhibit some reduction in
luminosity over time, though some more so than others, depending on
the phosphors they utilize. Zinc silicate phosphors such as
manganese-doped zinc silicate green phosphor (ZSM) are particular
but nonlimiting examples of phosphors that can exhibit poor lumen
maintenance characteristics, with other notable examples including
strontium-based phosphors such as tin-doped strontium phosphate red
(SR) and tin-doped strontium phosphate blue phosphor (SB), and
typically to a lesser extent halophosphors. ZSM phosphor has been
used separately in lamps that emit green light and in combinations
with other phosphors to emit white light. As nonlimiting examples
of the latter, phosphor blends containing ZSM, SR and SB have been
used or considered for use in high CRI lamps formulated for color
temperatures of about 4100K, and phosphor blends containing ZSM,
cerium magnesium borate (CBM), europium-doped strontium aluminate
(SAE), and halophosphors have been used or considered for use in
high CRI lamps formulated for color temperatures ranging from 2700K
to 3500K. Though these phosphor blends have certain desirable
qualities, for example, excellent color rendering, initial color
properties, and/or initial light levels, they suffer from poor
lumen maintenance characteristics, attributable at least in part to
their ZSM content.
[0008] Poor lumen maintenance is characterized by a rapid
depreciation of a phosphor during normal operation of a lamp, and
can be particularly evident in highly loaded lamps such as T5,
T5HO, CFL, and BIAX types. The poor lumen maintenance
characteristics of ZSM, SR and SB may be attributed in part to
their propensity for mercury consumption (binding), and the poor
lumen maintenance characteristics of ZSM can be further attributed
at least in part to sensitivity to 185 nm radiation emitted by low
pressure mercury lamps. Various attempts have been made to improve
lumen maintenance in lamps that contain ZSM and/or other phosphors
that exhibit poor lumen maintenance characteristics. As examples,
chemical vapor deposition (CVD) coatings and surface washes have
been attempted, but with limited success. In addition, barrier
coatings and additions of alumina or silica have been investigated,
but have not been entirely successful.
[0009] In view of the above, it would be desirable to improve the
lumen maintenance characteristics of fluorescent lamps that contain
certain phosphors prone to poor lumen maintenance, including but
not limited to ZSM, SR, and SB.
BRIEF DESCRIPTION OF THE INVENTION
[0010] The present invention provides methods and
phosphor-containing coating compositions capable of modifying the
lumen maintenance characteristics of phosphor-containing coatings
and fluorescent lamps that utilize such coatings.
[0011] According to one aspect of the invention, a
phosphor-containing coating includes at least a first phosphor that
depreciates from exposure to ultraviolet radiation of at least a
first wavelength, and further includes an additive composition in a
sufficient amount and sufficiently uniformly distributed to
attenuate absorption by the first phosphor of the ultraviolet
radiation of the first wavelength.
[0012] According to another aspect of the invention, a method for
improving the lumen maintenance of a fluorescent lamp includes
forming a phosphor-containing coating to contain at least a first
phosphor that depreciates during operation of the fluorescent lamp,
and forming the phosphor-containing coating to further contain an
additive composition in a sufficient amount and sufficiently
uniformly distributed in the phosphor-containing coating to inhibit
depreciation of the first phosphor during operation of the
fluorescent lamp.
[0013] According to yet another aspect of the invention, a method
for improving the lumen maintenance of a fluorescent lamp includes
providing a phosphor blend containing a first phosphor that
exhibits a poorer lumen maintenance than at least a second phosphor
in the phosphor blend, and forming therefrom a phosphor-containing
coating of a fluorescent lamp, wherein the phosphor-containing
coating contains an additive composition in a sufficient amount and
sufficiently uniformly distributed to improve the lumen maintenance
of the first phosphor.
[0014] A technical effect of the invention is the ability to
improve the lumen maintenance exhibited by a phosphor-containing
coating of a fluorescent lamp by addressing the rapid depreciation
of one or more phosphors in the phosphor-containing coating, and
particularly those phosphors that exhibit rapid mercury consumption
(binding) or sensitivity to certain UV wavelengths, for example,
the 185 nm wavelength emitted by low pressure mercury lamps.
[0015] Other aspects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 represents a fluorescent lamp, a fragmentary
cross-sectional view of a tube of the lamp, and an inner surface of
the tube provided with a coating system that includes a
phosphor-containing coating.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The invention will be described hereinafter in reference to
the lamp 10 shown in FIG. 1, though it should be appreciated that
the teachings of the invention are not limited to the lamp 10 and
instead are more generally applicable to various applications in
which visible light is generated with the use of one or more
phosphor compounds. It should also be noted that the drawings are
drawn for purposes of clarity when viewed in combination with the
following description, and therefore are not necessarily to
scale.
[0018] The invention relates to coating systems that include a
phosphor-containing coating, such as the phosphor-containing
coating 22 of FIG. 1, typically applied to a transparent or
translucent substrate, such as the glass shell 12 of the
fluorescent lamp 10. Though the coating 22 is represented in FIG. 1
as a single layer, and in the case of FIG. 1 may be the only
phosphor-containing coating of the lamp 10 such that all phosphors
within the coating system of the lamp 10 are within the coating 22,
the coating system could comprise any number of phosphor-containing
layers and coatings. In addition, any such phosphor-containing
layers and/or coatings could contain constituents in addition to
phosphors, for example, a scattering agent selected on the basis of
its ability to scatter incoming UV radiation prior to being
absorbed by the phosphors. Such a scattering agent can be provided
within a phosphor-containing layer in lieu of or in addition to the
UV-reflecting barrier layer 24 represented in FIG. 1.
[0019] In the nonlimiting example of FIG. 1, UV radiation emitted
by an ionized constituent (for example, mercury) is absorbed by the
phosphor composition within the coating 22, resulting in excitation
of phosphor compounds within the phosphor composition to produce
visible light that is emitted through the shell 12. In preferred
examples in which the ionized constituent is mercury, the emitted
UV radiation is predominantly at a wavelength of about 254 nm, with
a secondary wavelength of about 185 nm. Consequently, one or more
phosphors within the coating 22 are chosen on the basis of their
ability to predominantly absorb and be excited by the predominant
254 nm wavelength of UV radiation, and then emit wavelengths of
visible light that will provide a desired lighting effect. Notable
but nonlimiting examples of such phosphors include ZSM
(Zn.sub.2SiO.sub.4:Mn.sup.2+), SR
(Sr.sub.3(PO.sub.4).sub.2:Sn.sup.2), SB
(Sr.sub.5(PO.sub.4).sub.3(F,Cl):Sb.sup.3+,Mn.sup.2+), CBM
(GdMgB.sub.5O.sub.10:Ce.sup.3+,Mn.sup.2+), SAE
(Sr.sub.4Al.sub.14O.sub.25:Eu.sup.2+), and halophosphors (e.g.,
Ca.sub.5F(PO.sub.4).sub.3:Sb and/or
Ca.sub.5(PO.sub.4).sub.3(F,Cl):Sb.sup.3+,Mn.sup.2+). Though these
and other phosphors absorb and are predominantly excited by the 254
nm wavelength of UV radiation to produce visible light, these
phosphors may also absorb the 185 nm wavelength of UV radiation
without generating a significant level of visible light therefrom.
In addition, certain phosphors may even be damaged by the
absorption of the 185 nm wavelength, notable but nonlimiting
examples of which include zinc silicate phosphors such as ZSM and
strontium-based phosphors such as SR and SB, and typically to a
lesser extent halophosphors. The performances of the SR and SB
phosphors and halophosphors are also known to be reduced by rapid
mercury consumption (binding). Such damage can lead to depreciation
of such phosphors with aging, characterized by lumen loss, color
instability and/or color shift of the phosphor, and leads to poor
lumen maintenance of the lamp 10, evident from decreasing light
brightness of the lamp 10.
[0020] In view of the above, a particular aspect of the invention
is to formulate the phosphor-containing coating 22 to contain at
least one additive composition that is capable of reducing the rate
and/or extent at which a phosphor depreciates within the coating
22, especially but not solely due to mercury binding and/or damage
from one or more UV wavelengths, without attenuating a wavelength
required to excite the phosphor for the purpose of emitting visible
light. Notable examples are the 185 nm and 254 nm wavelengths of
low pressure mercury lamps, the former wavelength which may damage
SR, SB, ZSM, and halophosphors, and the latter wavelength being
required by these same phosphors to produce visible light.
Particularly preferred additive compositions are believed to be
capable of selectively attenuating the 185 nm wavelength emitted by
low pressure mercury lamps to reduce the damaging effect of this
wavelength on various phosphors, including SR, SB, and ZSM, while
not attenuating the 254 nm wavelength. While not wishing to be held
to any particular theories, by attenuating 185 nm radiation the
additive compositions are believed to be able to reduce undesirable
mercury binding reactions induced by 185 nm radiation, for example,
mercury binding reactions enabled as a result of H.sub.2O and OH
breakdown caused by 185 nm radiation. Other potential benefits
include improved lamp performance, improved lumen maintenance, and
reduced color shift, enabling the use of such phosphors in a wider
range of applications. As nonlimiting examples, ZSM, often
recognized as a high color rendition phosphor, may be used in
phosphor blends comprising CBM, SAE, and/or halophosphors in high
CRI (approaching 100) lamps formulated for color temperatures
ranging from 2700K to 3500K, and phosphor blends containing ZSM, SR
and SB may be used in high CRI lamps formulated for color
temperatures of about 4100K and higher. Particular types of lamps
that may be capable of using either or both of the above-noted
formulations of the coating 22 include CFL, T5, T5HO, T8, T12,
BIAX, Chroma 50, Chroma 75, and specialty lamps, for example, lamps
used for stage and studio lighting applications where both high CRI
and maximum light levels are desired.
[0021] Yttria (yttrium oxide; Y.sub.2O.sub.3) is a particularly
notable candidate for the one or more additive compositions that
can be used in the coating 22 to attenuate the 185 nm wavelength
without absorbing the 254 nm wavelength. Another notable candidate
having these capabilities is lanthana (lanthanum oxide;
La.sub.2O.sub.3). Other additive compositions potentially exist and
can be used alone or in combination with yttria and/or lanthana if
able to attenuate the 185 nm wavelength without absorbing the 254
nm wavelength (or another wavelength) required by phosphors in the
coating 22 to produce visible light. More broadly, additive
compositions should be capable of selectively attenuating a
wavelength that would damage one or more phosphors in the coating
22 without attenuating one or more wavelengths required to excite
the phosphors. To be effective, it is further believed that the
additive composition(s) should be of high purity (e.g., purity
levels of 99.999% weight percent or more) and uniformly distributed
throughout the coating 22 to provide a uniform coating over and
surrounding those phosphor particles within the coating 22 that are
particularly susceptible to depreciation, e.g., particles of ZSM,
SR and/or SB within the coating 22. For use in coatings 22
containing ZSM in combination with CBM, SAE, and/or halophosphors,
a suitable content for yttria as the additive composition is
believed to be in a range of about 2 to about 4 weight percent of
the phosphor blend with the coating 22 to promote the likelihood
that the ZSM particles will be surrounded by yttria. For use in
coatings 22 containing ZSM in combination with SB and/or SR, a
suitable content for yttria as the additive composition is believed
to be in a range of about 1 to about 3 weight percent of the
phosphor blend with the coating 22 to promote the likelihood that
the ZSM, SB and/or SR particles will be surrounded by yttria. In
both examples, yttria amounts of as low as about 0.1 weight percent
of the phosphor blend and as high as about 10 weight percent of the
phosphor blend could possibly be used. Similar or identical amounts
are believed to be appropriate if lanthana is used alone or in
combination with yttria and/or other candidates for the additive
composition. Such resulting compositions for the coating 22 may
have various desirable attributes, for example, excellent color
rendering, improved light output maintenance, color stability
and/or initial light levels, and reduced mercury loss.
[0022] To provide a high purity and uniformly distributed additive
composition throughout the coating 22, the additive composition can
be incorporated as a precursor (e.g., salt) into the phosphor blend
of the coating 22. The precursor can be present in a liquid vehicle
that, when combined with the phosphor blend and then dried, will
yield a coating layer ready for lehring or some other process
capable of producing the coating 22. The liquid vehicle may
comprise one or more non-ionic thickening agents and/or surfactants
to promote the formation of a uniform coating of the additive
composition over the phosphor particles being combined therewith to
form the coating 22. Particularly suitable thickening agents and
surfactants for use in this process will depend on the nature of
the additive composition(s) and phosphor(s) being combined to form
the coating 22, as well as the type of lamp. As such, examples of
potentially suitable thickening agents include but are not limited
to polyethers such as polyethylene oxide and cellulose types such
as hydroxyethylcellulose, and examples of potentially suitable
surfactants include but are not limited to nonionic types such as
NPE (nonylphenolethoxylate) and block copolymers of ethylene oxide
and propylene oxide. Other potentially suitable thickening agents
and surfactants, as well as other potential constituents for the
vehicle containing the additive composition, may be known to those
skilled in the art, and such constituents can be employed if they
do not precipitate or interact with the additive composition in any
manner that would adversely inhibit or prevent the uniform
dispersion of the additive composition in the coating 22 or
adversely inhibit or prevent the ability of the additive
composition to uniformly coat the phosphor particles.
[0023] In combination with the surfactant(s) and thickening
agent(s), the additive composition can be present in the form of
dissolved ions in the liquid vehicle during incorporation of the
additive composition into a mixture of phosphor particles of the
one or more phosphors desired for the coating 22. A particular but
nonlimiting example involves carefully selecting one or more
soluble salts that are capable of being converted to the desired
additive composition, for example during a lehring operation
performed on the lamp 10, and then combining the one or more
soluble salts with a suitable solvent and one or more suitable
thickening agents and surfactants to form a liquid vehicle capable
of being mixed with a suspension containing particles of the
desired phosphor(s) for the coating 22. Nonlimiting examples of
suitable salts capable of being converted into yttria are yttrium
acetate, yttrium chloride, and yttrium nitrate, and nonlimiting
examples of suitable salts capable of being converted into lanthana
are lanthanum acetate, lanthanum chloride, and lanthanum nitrate.
These salts are capable of being dissolved in water and converted
into yttria or lanthana at temperatures that are compatible with
lehring and other processing of shells (glass envelopes) performed
during the manufacture of fluorescent lamps.
[0024] Various formulations can be utilized as the suspension that
contains the phosphor particles and is combined with the liquid
vehicle containing the additive composition. As an example, such
suspensions will often comprise one or more surfactants, thickening
agents, dispersants, etc., creating a liquid vehicle in which the
phosphor particles are suspended. Suitable particle sizes for the
phosphor particles are generally on the order of about 2 to about
20 micrometers, and in any event are preferably of sufficiently
small size to enable the salts of the additive composition to
become uniformly distributed throughout the suspension, and
thereafter, after conversion of the salts to form the additive
composition, enable the additive composition to provide a uniform
coating over and surrounding the individual phosphor particles
within the coating 22.
[0025] While the invention has been described in terms of specific
embodiments it is apparent that other forms could be adopted by one
skilled in the art. Therefore, the scope of the invention is to be
limited only by the following claims.
* * * * *