U.S. patent application number 13/308066 was filed with the patent office on 2012-06-07 for solid-state light emitting devices and signage with photoluminescence wavelength conversion and photoluminescent compositions therefor.
This patent application is currently assigned to INTEMATIX CORPORATION. Invention is credited to Bing Dai, Charles Edwards, Jonathan Melman, Xianglong Yuan.
Application Number | 20120138874 13/308066 |
Document ID | / |
Family ID | 46161350 |
Filed Date | 2012-06-07 |
United States Patent
Application |
20120138874 |
Kind Code |
A1 |
Yuan; Xianglong ; et
al. |
June 7, 2012 |
SOLID-STATE LIGHT EMITTING DEVICES AND SIGNAGE WITH
PHOTOLUMINESCENCE WAVELENGTH CONVERSION AND PHOTOLUMINESCENT
COMPOSITIONS THEREFOR
Abstract
A photoluminescent composition ("phosphor ink") comprises a
suspension of particles of at least one blue light (380 nm to 480
nm) excitable phosphor material in a light transmissive liquid
binder in which the weight loading of at least one phosphor
material to binder material is in a range 40% to 75%. The binder
can be U.V. curable, thermally curable, solvent based or a
combination thereof and comprise a polymer resin; a monomer resin,
an acrylic, a silicone or a fluorinated polymer. The composition
can further comprise particles of a light reflective material
suspended in the liquid binder. Photoluminescence wavelength
conversion components; solid-state light emitting devices; light
emitting signage surfaces and light emitting signage utilizing the
composition are disclosed.
Inventors: |
Yuan; Xianglong; (Fremont,
CA) ; Dai; Bing; (Fremont, CA) ; Melman;
Jonathan; (San Ramon, CA) ; Edwards; Charles;
(Pleasanton, CA) |
Assignee: |
INTEMATIX CORPORATION
Fremont
CA
|
Family ID: |
46161350 |
Appl. No.: |
13/308066 |
Filed: |
November 30, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61419099 |
Dec 2, 2010 |
|
|
|
Current U.S.
Class: |
252/582 ;
252/301.36; 359/326; 385/141; 427/157 |
Current CPC
Class: |
G02F 1/133615 20130101;
C09K 11/7734 20130101; C09K 11/7774 20130101; H01L 2933/0058
20130101; G02B 6/005 20130101; B41F 15/00 20130101; H01L 2933/0041
20130101; H01L 33/60 20130101; H01L 33/502 20130101; C09K 11/02
20130101; G02F 1/133614 20210101; H01L 33/501 20130101 |
Class at
Publication: |
252/582 ;
359/326; 385/141; 252/301.36; 427/157 |
International
Class: |
C09K 11/08 20060101
C09K011/08; G02B 6/00 20060101 G02B006/00; B05D 5/06 20060101
B05D005/06; C09K 11/59 20060101 C09K011/59; C09K 11/56 20060101
C09K011/56; C09K 11/80 20060101 C09K011/80; G02F 2/02 20060101
G02F002/02; C09K 11/02 20060101 C09K011/02 |
Claims
1. A photoluminescent composition comprising: a suspension of
particles of at least one phosphor material in a light transmissive
liquid binder wherein the at least one phosphor material is
excitable by blue light of wavelength 380 nm to 480 nm and wherein
the weight loading of at least one phosphor material to binder
material is in a range 40% to 75%.
2. The composition of claim 1, wherein the composition is
configured to be deposited using a method selected from the group
consisting of: screen printing; inkjet printing; letterpress
printing, gravure printing; flexograph printing and pad
printing.
3. The composition of claim 1, wherein the binder is selected from
the group consisting of being: U.V. curable; thermally curable;
solvent based and combinations thereof.
4. The composition of claim 1, wherein the binder has a viscosity
selected from the group consisting of: 0.5 Pas to 5 Pas and 1 Pas
to 2.5 Pas.
5. The composition of claim 1, wherein the binder is selected from
the group consisting of: a polymer resin; a monomer resin; an
acrylic, a silicone and a fluorinated polymer.
6. The composition of claim 1, wherein the binder has in a cured
state an elasticity in a range 300% to 500%.
7. The composition of claim 1, wherein the at least one phosphor
material has an average particle size selected from the group
consisting of: 2 .mu.m to 60 .mu.m, 10 .mu.m to 20 .mu.m and about
15 .mu.m.
8. The composition of claim 1, wherein the at least one phosphor
material is selected from the group consisting of: a silicate
phosphor; an orthosilicate phosphor; a nitride phosphor; an
oxy-nitride phosphor; a sulfate phosphor, an oxy-sulfate phosphor;
and a garnet (YAG) phosphor.
9. The composition of claim 1, and further comprising particles of
a light reflective material suspended in the liquid binder.
10. The composition of claim 9, wherein the light reflective
material is selected from the group consisting of: magnesium oxide,
titanium dioxide, barium sulfate and combinations thereof.
11. The composition of claim 10, wherein the light reflective
material has a particle size in a range selected from the group
consisting of: 0.1 .mu.m to 10 .mu.m and 0.1 .mu.m to 1 .mu.m.
12. The composition of claim 10, wherein a weight percent loading
of light reflective material to phosphor material is in a range
selected from the group consisting of: 0.01% to 10%; 0.01% to 1%;
0.1% to 1% and 0.5% to 1%.
13. A photoluminescence wavelength conversion component for a
solid-state light emitting device comprising a substrate having
deposited on a surface thereof at least one layer of the
composition of claim 1.
14. The component of claim 13, wherein the composition is provided
over an area of at least 0.8 cm.sup.2 to 180 cm.sup.2.
15. The component of claim 13, wherein the substrate is light
transmissive.
16. The component of claim 15, wherein the substrate and binder in
a cured state have refractive indices that are within 0.02 of each
other.
17. The component of claim 15, wherein the substrate is configured
as a light guide and the composition provided on at least a part of
a light emitting face of the light guide.
18. The component of claim 17, wherein the substrate is selected
from the group consisting of: an acrylic, a polycarbonate, a
silicone and a glass.
19. The component of claim 13, wherein substrate comprises a light
reflective surface.
20. The component of claim 19, wherein the light reflective surface
is selected from the group consisting of: silver, aluminum,
chromium, a light reflective polymer, a light reflective paper, a
light reflective paint and a light reflective metal.
21. A photoluminescence wavelength conversion component for a
solid-state light emitting device comprising: a substrate having on
a surface thereof a layer of a photoluminescent composition that is
excitable by blue light of wavelength 380 nm to 480 nm and wherein
the composition is deposited on the substrate using a method
selected from the group consisting of: screen printing; inkjet
printing; letterpress printing, gravure printing; flexograph
printing and pad printing.
22. The component of claim 21, wherein the composition comprises a
suspension of particles of at least one blue light excitable
phosphor material in a light transmissive liquid binder and wherein
the weight loading of at least one phosphor material to binder
material is in a range 40% to 75%.
23. The component of claim 21, wherein the composition is printed
as a first order stochastic pattern comprising a pseudo random
array of dots of substantially the same size.
24. The component of claim 21, wherein the composition is printed
as a second order stochastic pattern comprising a pseudo random
array of dots of varying size.
25. The component of claim 21, wherein the composition is printed
as a half tone pattern comprising a regular array of dots of
varying size.
26. The component of claim 21, wherein the substrate is light
transmissive.
27. The component of claim 26, wherein the substrate is configured
as a light guide and the composition provided on at least a part of
a light emitting face of the light guide.
28. The component of claim 26, wherein the substrate is selected
from the group consisting of: an acrylic, a polycarbonate, a
silicone and a glass.
29. The component of claim 21, wherein substrate comprises a light
reflective surface.
30. The component of claim 29, wherein the light reflective surface
is selected from the group consisting of: silver, aluminum,
chromium, a light reflective polymer, a light reflective paper, a
light reflective paint and a light reflective metal.
31. A method of manufacturing a photoluminescence wavelength
conversion component for a solid-state light emitting device
wherein the component comprises a substrate having a layer of a
phosphor material that is excitable by blue light of wavelength 380
nm to 480 nm, the method comprising: a) mixing a blue light
excitable phosphor material with a light transmissive binder
wherein the weight loading of at least one phosphor material to
binder material is in a range 40% to 75%; b) printing the
composition as a layer over at least a part of a substrate; and c)
at least partially curing the light transmissive binder.
32. The method of claim 31, wherein the composition is printed
using a method selected from the group consisting of: screen
printing; inkjet printing; letterpress printing, gravure printing;
flexograph printing and pad printing.
33. The method of claim 31, wherein the substrate comprises a
thermoplastics material and further comprising heating the
substrate and forming the component into a selected shape.
34. The method of claim 33, and further comprising during printing
the composition, selectively varying the thickness of the layer
such that after forming the component into a selected shape the
resulting composition layer is of a substantially uniform
thickness.
35. The method of claim 33, wherein the binder has in a cured state
an elasticity of 300% to 500%.
36. A component manufactured according to the method of claim
31.
37. A photoluminescent composition comprising: a suspension of
particles of at least one phosphor material in a light transmissive
liquid binder wherein the phosphor material is excitable by blue
light of wavelength 380 nm to 480 nm and wherein the binder has in
a cured state an elasticity of 300% to 500%.
38. A light emitting device comprising: at least one solid-state
light emitter operable to generate blue light of wavelength 380 nm
to 480 nm and a component according to claim 13.
39. A photoluminescence light emitting signage surface comprising:
a substrate having printed on a surface thereof at least one layer
of the composition according to claim 1.
40. The signage surface of claim 39, wherein the composition is
provided over an area of at least 100 cm.sup.2.
41. The signage surface of claim 39, wherein the substrate is light
transmissive and is selected from the group consisting of: an
acrylic, a polycarbonate, a silicone and a glass.
42. A light emitting sign comprising: at least one solid state
light emitter operable to generate blue light of wavelength 380 nm
to 480 nm and a signage surface according to claim 39.
Description
CLAIM OF PRIORITY
[0001] This application claims the benefit of priority to U.S.
Provisional Patent Application No. 61/419,099, filed Dec. 2, 2010,
entitled "Solid-State Light Emitting Devices and Signage with
Photoluminescence Wavelength Conversion and Photoluminescent
Compositions Therefor", by Yuan et al., the specification and
drawings of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to solid-state light emitting devices
and signage with photoluminescence wavelength conversion and more
particularly photoluminescent compositions therefor. In particular,
although not exclusively, the invention concerns devices and
signage based on LEDs (Light Emitting Diodes). The invention
further concerns a method of manufacturing photoluminescence
wavelength conversion components for solid-state light emitting
devices and photoluminescence signage surfaces for light emitting
signage.
[0004] 2. Description of the Related Art
[0005] White light emitting LEDs ("white LEDs") are known and are a
relatively recent innovation. It was not until LEDs emitting in the
blue/ultraviolet part of the electromagnetic spectrum were
developed that it became practical to develop white light sources
based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925,
white LEDs include one or more phosphor materials, that is
photoluminescent materials, which absorb a portion of the radiation
emitted by the LED and re-emit light of a different color
(wavelength). Typically, the LED chip or die generates blue light
and the phosphor(s) absorbs a percentage of the blue light and
re-emits yellow light or a combination of green and red light,
green and yellow light, green and orange or yellow and red light.
The portion of the blue light generated by the LED that is not
absorbed by the phosphor material combined with the light emitted
by the phosphor provides light which appears to the eye as being
nearly white in color.
[0006] Due to their long operating life expectancy (>50,000
hours) and high luminous efficacy (70 lumens per watt and higher)
high brightness white LEDs are increasingly being used to replace
conventional fluorescent, compact fluorescent and incandescent
light sources.
[0007] Typically the phosphor material is mixed with a silicone or
epoxy material and the mixture applied to the light emitting
surface of the LED die. As disclosed in United States patent
application US 2008/0218992 A1 to Li, it is also known to provide
the phosphor material as a layer on, or incorporate the phosphor
material within an, optical component that is located remotely to
the LED die.
[0008] United States patent application US 2007/0240346 A1, to Li
et al., teach a solid-state light emitting sign in which blue light
from an LED is used to excite phosphor materials on a light
emitting signage surface to generate a desired color of light.
[0009] It is an object of the present invention to provide a
photoluminescent composition for solid-state light emitting devices
and signage with photoluminescence wavelength conversion that in
part at least overcomes the limitations of the known devices.
SUMMARY OF THE INVENTION
[0010] Embodiments of the invention concern photoluminescent
compositions that contain one or more phosphor materials that are
excitable by blue light of wavelength 380 nm to 480 nm. In
accordance with the invention the photoluminescent composition can
be deposited onto a substrate as one or more layers of a uniform
thickness by printing, in particular screen printing, letterpress
printing, gravure printing, flexograph printing or pad
printing.
[0011] According to the invention a photoluminescent composition
comprises: a suspension of particles of at least one phosphor
material in a light transmissive liquid binder in which the at
least one phosphor material is excitable by blue light of
wavelength 380 nm to 480 nm and wherein the weight loading of at
least one phosphor material to binder material is in a range 40% to
75%.
[0012] The photoluminescent composition can be configured to be
screen printable and the binder has a viscosity in a range 0.5 Pas
to 5 Pas (Pascal-second=1 kgm.sup.-1s.sup.-1). Preferably the
binder has a viscosity of about 1 Pas (1000 cps (centipoise)) to
2.5 Pas.
[0013] Alternatively the photoluminescent composition can be
configured to be inkjet printable with a viscosity of 0.004 Pas to
0.020 Pas. Inkjet printing can enable fine patterns of the
composition to be printed.
[0014] In other arrangements the photoluminescent composition can
be configured to be gravure or flexograph printable with a
viscosity of 0.05 Pas to 5 Pas. A particular benefit of gravure and
flexograph printing is that since the quantity of composition
printed is determined by the size of the cavities in a plate or
drum this enables accurate control of the thickness and uniformity
of the printed layer.
[0015] In further arrangements the photoluminescent composition can
be letterpress printable with a viscosity 50 Pas to 150 Pas.
[0016] In another arrangement the photoluminescent composition can
be pad printable. Pad printing can be beneficial where the
substrate onto which the composition is to be printed is non-planar
for example where the substrate comprises a curved lens or a domed
shaped cover.
[0017] The binder can be U.V. curable, thermally curable, solvent
based or a combination thereof. The binder can comprise a polymer
resin; a monomer resin, an acrylic or a silicone. The binder can
comprise a fluorinated polymer to reduce degradation of the
phosphor material by the uptake of moisture. Preferably the binder
has a transmissivity of at least 0.9 for light of wavelength 400 nm
to 750 nm. Advantageously where it is intended to print the
composition on a light transmissive substrate the binder is
selected such that, in a cured state, it has a refractive index
that substantially matches the refractive index of the substrate.
Such refractive index matching reduces refraction of light at the
interface between the substrate and layer of photoluminescent
composition. Typically the refractive index of the binder is
greater than about 1.48 and is advantageously matched with the
substrate to within 0.02. Preferably the binder has, in a cured
state, an elasticity in a range 300% to 500%.
[0018] The at least one phosphor material can have an average
particle size in a range 1 .mu.m to 60 .mu.m with the preferred
particle size range being dependent on the intended printing
technique for depositing the composition. For example for screen
printable phosphor compositions the average particle is preferably
in a range 10 .mu.m to 20 .mu.m and more preferably about 15 .mu.m.
In contrast inkjet printable compositions generally require the
smallest particle size typically up to 5 .mu.m. The at least one
phosphor material preferably comprises a silicate phosphor; an
orthosilicate phosphor; a nitride phosphor; an oxy-nitride
phosphor; a sulfate phosphor, an oxy-sulfate phosphor, a garnet
(YAG) phosphor or a combination thereof.
[0019] The inventors have discovered that by further suspending
particles of a light reflective material in the liquid binder, this
can increase photoluminescence light generation by the
photoluminescent composition. It is believed that the increase in
photoluminescence generated light results from the particles of
light reflective material increasing the probability of collisions
of photons with particles of the phosphor material. Initial test
results indicate that the inclusion of the light reflective
material can potentially, for a given color and intensity of light
generated by the composition, reduce phosphor material usage by 33%
or more. Such a reduction of phosphor material usage is
particularly significant in applications where the composition is
required over a large area such as signage applications in which
the composition may be provided over several hundreds or even
thousands of square centimeters. The light reflective material has
a reflectivity that is as high as possible and is preferably at
least 0.9. The light reflective material preferably comprises
particles of magnesium oxide (MgO), titanium dioxide (TiO.sub.2),
barium sulfate (BaSO.sub.4) or mixtures thereof. Typically the
light reflective material has a particle size in a range 0.1 .mu.m
to 20 .mu.m and preferably in a range 0.5 .mu.m to 2 .mu.m. The
weight percent loading of light reflective material to phosphor
material can be in a range 0.01% to 10%, preferably in a range
0.01% to 1% and more preferably in ranges 0.1% to 1% or 0.5% to
1%.
[0020] According to another aspect of the invention a wavelength
conversion component for a solid-state light emitting device
comprises a substrate having printed on a surface thereof at least
one layer of the photoluminescent composition of the invention.
Typically for solid-state light emitting devices as used in general
lighting the photoluminescent composition is provided over an area
of at least 0.8 cm.sup.2 to 180 cm.sup.2.
[0021] The wavelength conversion component can be light
transmissive and configured to convert the wavelength of at least a
portion of the light transmitted through the component. In one
arrangement the wavelength conversion component comprises a light
transmissive substrate on which the photoluminescent composition is
provided as at least one layer. Alternatively the light
transmissive substrate can be configured as a light guide and the
photoluminescent composition provided on at least a part of a light
emitting face of the light guide. To reduce light being refracted
at the interface between the light transmissive substrate and the
layer of photoluminescent composition the substrate and binder in a
cured state have refractive indices that are within 0.02 of each
other. The light transmissive substrate can comprise an acrylic
poly(methyl methacrylate) (PMMA), a polycarbonate, a silicone or a
glass.
[0022] Alternatively the wavelength conversion component can be
light reflective and configured to convert the wavelength of at
least a portion of the light reflected by the component. One such
wavelength conversion component comprises a light reflective
surface on which the photoluminescent composition is provided as at
least one layer. The light reflective substrate can comprise any
light reflective surface and preferably has a reflectance of at
least 0.9. The light reflective surface can comprise a polished
metallic surface such as silver (Ag), aluminum (Al), chromium (Cr);
a light reflective polymer, a light reflective paper or a light
reflective paint.
[0023] The concept of manufacturing a solid-state light emitting
device or manufacturing a photoluminescence wavelength conversion
component for a solid-state light emitting device by printing a
photoluminescent composition containing one or blue light excitable
phosphor materials is considered inventive in its own right.
Accordingly in accordance with a further aspect of the invention a
photoluminescence wavelength conversion component for a solid-state
light emitting device comprises: a substrate having on a surface
thereof a layer of a photoluminescent composition that is excitable
by blue light of wavelength 380 nm to 480 nm and wherein the
composition is deposited on the substrate using screen printing;
inkjet printing; letterpress printing, gravure printing; flexograph
printing or pad printing. Preferably the composition comprises a
suspension of particles of at least one blue light excitable
phosphor material in a light transmissive liquid binder and wherein
the weight loading of at least one phosphor material to binder
material is in a range 40% to 75%.
[0024] The composition can be printed such that it covers the
entire light emitting surface of the substrate. Alternatively the
composition can be printed as a pattern covering at least a part of
the light emitting surface of the substrate. In one arrangement the
composition is printed as a first order stochastic pattern
comprising a pseudo random array of dots of substantially the same
size. A particular benefit of using a stochastic pattern is that
this can largely eliminate alignment issues when printing more than
one layer or where it is required to print multiple different
compositions. Moreover since the dots are the same size a first
order stochastic pattern is particularly suited to screen printing
where the dot size can correspond the screen mesh hole size.
Alternatively the composition can be printed as a second order
stochastic pattern comprising a pseudo random array of dots of
varying size. In yet further arrangements the composition can be
printed as a half tone pattern comprising a regular array of dots
of varying size.
[0025] The photoluminescent composition can be printed as a pattern
on substrates that are light transmissive or light reflective.
[0026] In accordance with another aspect of the invention a method
of fabricating a photoluminescence wavelength conversion component
for a solid-state light emitting device in which the component
comprises a substrate having a layer of a phosphor material that is
excitable by blue light of wavelength 380 nm to 480 nm, comprises:
a) mixing particles of a blue light excitable phosphor material
with a light transmissive binder wherein the weight loading of at
least one phosphor material to binder material is in a range 40% to
75%; b) printing the composition as a layer over at least a part of
a substrate; and c) at least partially curing the light
transmissive binder. Where the surface of the substrate onto which
the composition is to be printed is substantially planar the
composition can be printed by screen printing; inkjet printing;
letterpress printing, gravure printing or flexograph printing.
Where the substrate surface is curved the composition can be
printed by pad printing using a resiliently deformable printing
pad.
[0027] In some applications it may be desirable for the substrate
to be shaped rather than planar such as for example a dome-shaped
or hemispherical shell. To fabricate such components the substrate
preferably comprises a thermoplastics material having a
substantially planar surface on which the composition is printed.
The method further comprises heating the substrate and forming the
component into a selected shape. To ensure a uniform thickness
layer of the composition in the finished component the method
further comprises, during printing the composition, selectively
varying the thickness of the layer such that after forming the
component into a selected shape the resulting composition layer is
of a substantially uniform thickness. To prevent the layer(s) of
photoluminescent composition separating (delaminating) from the
substrate during forming of the substrate the binder advantageously
has in a cured state an elasticity of 300% to 500%. Such a method
of fabrication a wavelength conversion component is considered to
be inventive in its own right.
[0028] According to yet a further aspect of the invention a
photoluminescent composition comprises a suspension of particles of
at least one phosphor material in a light transmissive liquid
binder wherein the phosphor material is excitable by blue light of
wavelength 380 nm to 480 nm and wherein the binder has in a cured
state an elasticity of 300% to 500%.
[0029] According to yet another aspect of the invention a light
emitting device comprises at least one solid-state light emitter,
typically an LED, that is operable to generate blue light and a
wavelength conversion component in accordance with the various
aspects of the invention.
[0030] Whilst the various aspects of the invention arose in
relation to solid-state light emitting devices the photoluminescent
compositions of the invention further find application to
solid-state light emitting signage in which the photoluminescent
wavelength conversion component comprises a photoluminescence light
emitting signage surface. According to another aspect of the
invention a signage surface comprises a substrate having printed on
a surface thereof at least one layer of the photoluminescent
composition of the invention. The composition can be configured as
a pattern to define an image, picture, letter, numeral, device,
pattern or other signage information. Alternatively, as for example
is required for channel lettering, the shape of the signage
surface, substrate, can be configured to define signage
information. Typically the substrate is light transmissive and can
be backlit or edge lit. The substrate can comprise an acrylic, a
polycarbonate, an epoxy, a silicone or a glass. Where the signage
surface is backlit, that is one or more solid-state light emitters
are located behind the signage surface which is configured as a
light transmissive window such that a proportion of blue light
passing through the component will be converted to light of a
different color by the phosphor material, the signage surface is
preferably located at a distance of at least 5 mm from the light
emitters. Alternatively where the sign is edge lit the substrate is
configured as a light guide and the photoluminescent composition
printed on at least a part of a light emitting face of the light
guide. Typically the substrate will be planar and light can coupled
into the light guide from one or more edges of the substrate.
[0031] In signage applications the light emitting surface will
often be much greater than that in lighting applications and the
composition will typically be provided over an area of at least 100
cm.sup.2.
[0032] In accordance with yet another aspect of the invention a
light emitting sign comprises at least one solid state light
emitter operable to generate blue light and a signage surface in
accordance with the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In order that the present invention is better understood
photoluminescent compositions, solid-state light emitting devices,
photoluminescence wavelength conversion components and light
emitting signage in accordance with embodiments of the invention
will now be described, by way of example only, with reference to
the accompanying drawings in which:
[0034] FIG. 1 is schematic representation of an LED-based light
emitting device with a light transmissive photoluminescence
wavelength conversion component in accordance with an embodiment of
the invention;
[0035] FIG. 2 is a plot of emission intensity versus chromaticity
CIE x for an LED-based light emitting device in accordance with the
invention for different weight percent loadings of light reflective
material;
[0036] FIG. 3 is a schematic representation of an LED-based light
emitting device with a light transmissive photoluminescence
wavelength conversion component configured as a light guide in
accordance with an embodiment of the invention;
[0037] FIG. 4 is a schematic representation of an LED-based light
emitting device with a light transmissive photoluminescence
wavelength conversion component configured as a light guide in
accordance with another embodiment of the invention;
[0038] FIG. 5 is a schematic representation of an LED-based light
emitting device with a light reflective photoluminescence
wavelength conversion component in accordance with an embodiment of
the invention;
[0039] FIG. 6a printed phosphor ink pattern based on AM (amplitude
modulated) half tone screening;
[0040] FIG. 6b printed phosphor ink pattern based on a first order
stochastic or FM (frequency modulated) screening; and
[0041] FIGS. 7a to 7e are schematic representations illustrating a
method of manufacture of a dome-shaped photoluminescence wavelength
conversion component.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Embodiments of the invention concern photoluminescent
compositions for use with photoluminescence wavelength conversion
components for solid-state light emitting devices and light
emitting signage. In particular, although not exclusively, the
invention relates to photoluminescent compositions that can be
deposited by printing. The concept of printing the photoluminescent
composition to manufacture a photoluminescence wavelength
conversion component for solid-state light emitting devices is
believed to be inventive in its own right. Since the
photoluminescent compositions are preferably printable they will in
this specification and for the sake of brevity, be referred to as
"phosphor inks". It will be appreciated however that the use of
this term does not restrict the invention to printable
photoluminescent compositions and that the phosphor ink can be
deposited by other methods such as for example spraying, spin
coating, slit coating, dipping or sweeping the phosphor ink over
the surface of a substrate using a blade such as a squeegee (e.g.
doctor blading).
[0043] Throughout this patent specification like reference numerals
are used to denote like parts.
[0044] FIG. 1 is a schematic representation of an LED-based white
light emitting device 10 in accordance with an embodiment of the
invention. The device 10 comprises a blue light emitting LED (blue
LED) 12 and a light transmissive photoluminescence wavelength
conversion component 14 located remotely to the LED 12. The
wavelength conversion component 14 can comprise a light
transmissive substrate (window) 16 having, on at least one face,
one or more layers of a phosphor ink 18 that constitutes a
photoluminescence wavelength conversion layer. The phosphor ink 18
comprises a light transmissive binder material 20 with particles of
a blue light excitable phosphor material 22 homogeneously
distributed throughout its volume. As indicated in FIG. 1 the
phosphor ink 18 can optionally further comprise particles of a
light reflective material 24 homogeneously distributed throughout
its volume. To provide physical protection of the wavelength
conversion layer 18 it is preferred, as indicated in FIG. 1, that
the wavelength conversion component 14 is configured such that the
wavelength conversion layer 18 faces the LED.
[0045] The substrate 16 can comprise any light transmissive
material such as a polymer material for example an acrylic
poly(methyl methacrylate) (PMMA) or polycarbonate or a glass such
as fused silica or a borosilicate glass such as Pyrex (Pyrex is a
brand name of Corning Inc). The light transmissive material can
also comprise an optical quality silicone or epoxy. The substrate
16 can be planar such as a circular disc though it can be square,
rectangular or other shapes depending on the intended application.
Where the substrate is disc-shaped the diameter can be between
about 1 cm and 153 cm (6 inches) that is an aperture of area of
between 0.8 cm.sup.2 and 184 cm.sup.2. In alternative embodiments
the substrate 16 can be non-planar and comprise other geometries
such as a spherical or dome-shaped shell, a part cylindrical
surface or an optical component that directs light in a selected
direction such as a convex or concave lens. To reduce the transfer
of heat from the LED 12 to the wavelength conversion component 14,
in particular heat transfer to the phosphor material, the
wavelength conversion component is located remote to the LED, that
is physically separated from the LED, by a distance L of at least 5
mm. Locating the wavelength conversion component 14 remote to the
LED provides a number of benefits namely reduced thermal
degradation of the phosphor material. Additionally compared with
devices in which the phosphor material is provided in direct
contact with the light emitting surface of the LED die, providing
the phosphor material remote to the LED reduces absorption of
backscattered light by the LED die. Furthermore locating the
phosphor material remotely enables generation of light of a more
consistent color and/or CCT since the phosphor material is provided
over a much greater area as compared to providing the phosphor
directly to the light emitting surface of the LED die.
[0046] The blue LED 12 can comprise a GaN-based (gallium
nitride-based) LED that is operable to generate blue light 26
having a peak wavelength .lamda..sub.1 in a wavelength range 380 nm
to 480 nm (typically 440 nm to 450 nm). The blue LED 12 is
configured to irradiate the wavelength conversion component 14 with
blue light 26 whereat a proportion is absorbed by the phosphor
material 22 which in response emits light 28 of a different
wavelength .lamda..sub.2, typically yellow-green in color for a
cold white light emitting device. The emission product 30 of the
device 10 which is configured to appear white in color comprises
the combined light 26 emitted by the LED and the light 28 generated
by the phosphor material 20.
[0047] The phosphor material 22 and light reflective material 24,
which are in powder form, are thoroughly mixed in known proportions
with the liquid binder material 20 to form a suspension and the
resulting phosphor ink deposited onto the substrate 16 to form a
layer of substantially uniform thickness. In preferred embodiments
the phosphor ink is deposited onto the substrate 16 by screen
printing and the thickness t of the layer controlled by the number
of printing passes. As will further be described the phosphor ink
can be applied using other printing methods including inkjet,
letterpress, gravure, flexograph or pad printing.
[0048] The phosphor material can comprise an inorganic or organic
phosphor such as for example silicate-based phosphor of a general
composition A.sub.3Si(O,D).sub.5 or A.sub.2Si(O,D).sub.4 in which
Si is silicon, O is oxygen, A comprises strontium (Sr), barium
(Ba), magnesium (Mg) or calcium (Ca) and D comprises chlorine (Cl),
fluorine (F), nitrogen (N) or sulfur (S). Examples of
silicate-based phosphors are disclosed in U.S. Pat. No. 7,575,697
B2 "Silicate-based green phosphors" (assigned to Intematix Corp.),
U.S. Pat. No. 7,601,276 B2 "Two phase silicate-based yellow
phosphors" (assigned to Intematix Corp.), U.S. Pat. No. 7,655,156
B2 "Silicate-based orange phosphors" (assigned to Intematix Corp.)
and U.S. Pat. No. 7,311,858 B2 "Silicate-based yellow-green
phosphors" (assigned to Intematix Corp.). The phosphor can also
comprise an aluminate-based material such as is taught in our
co-pending patent application US 2006/0158090 A1 "Novel
aluminate-based green phosphors" and U.S. Pat. No. 7,390,437 B2
"Aluminate-based blue phosphors" (assigned to Intematix Corp.), an
aluminum-silicate phosphor as taught in co-pending application US
2008/0111472 A 1"Aluminum-silicate orange-red phosphor" or a
nitride-based red phosphor material such as is taught in our
co-pending United States patent application US 2009/0283721 A1
"Nitride-based red phosphors" and International patent application
WO2010/074963 A1 "Nitride-based red-emitting in RGB
(red-green-blue) lighting systems". It will be appreciated that the
phosphor material is not limited to the examples described and can
comprise any phosphor material including nitride and/or sulfate
phosphor materials, oxy-nitrides and oxy-sulfate phosphors or
garnet materials (YAG).
[0049] The phosphor material comprises particles with an average
particle size of 10 .mu.m to 20 .mu.m and typically of order 15
.mu.m. The phosphor material can comprise particles of a size 2
.mu.m to 60 .mu.m depending in part on the intended technique to be
used to deposit the phosphor ink.
[0050] The light reflective material 22 comprises a powdered
material with as high a reflectivity as possible typically a
reflectance of 0.9 or higher. The particle size of the light
reflective material is typically in a range 0.1 .mu.m to 10 .mu.m
and in a preferred embodiment is within a range 0.1 .mu.m to 10
.mu.m. The weight percent loading of light reflective material to
phosphor material is in a range 0.1% to 10% and in a preferred
embodiment in a range 1% to 2%. Examples of light reflective
materials include magnesium oxide (MgO), titanium dioxide
(TiO.sub.2) and barium sulfate (BaSO.sub.4). The light reflective
material can also comprise a white ink such as for example Norcote
International Inc's super white ink GN-027SA which already includes
particles of a highly light reflective material, typically
TiO.sub.2 (up to about 42% by weight).
[0051] In operation blue light 26 from the LED passes through the
binder material 20 until it strikes a particle of phosphor material
22. It is believed that on average as little as 1 in 1000
interactions of a photon with a phosphor material particle results
in absorption and generation of photoluminescence light 28. The
majority, about 99.9%, of interactions of photons with a phosphor
particle result in scattering of the photon. Due to the isotropic
nature of the scattering process on average half of the photons
will scattered in a direction back towards the LED. Tests indicate
that typically about 10% of the total incident blue light 26 is
scattered and emitted from the wavelength conversion component 14
in a direction back towards the LED. For a cool white light
emitting device the amount of phosphor material is selected to
allow approximately 10% of the total incident blue light to be
emitted from the window and contribute to the emission product 30.
The majority, approximately 80%, of the incident light is absorbed
by the phosphor material and re-emitted as photoluminescence light
28. Due to the isotropic nature of photoluminescence light
generation, approximately half of the light 28 generated by the
phosphor material will be emitted in a direction towards the LED.
As a result only up to about 40% of the total incident light will
be emitted as light 28 of wavelength .lamda..sub.2 and contributes
to the emission product 30 with the remaining (up to about 40%) of
the total incident light being emitted as light 28 of wavelength
.lamda..sub.2 in a direction back towards the LED. Typically light
emitted towards the LED from the wavelength conversion component is
re-directed by a light reflective chamber (not shown) to contribute
to the emission product and to increase the overall efficiency of
the device.
[0052] The inventors have discovered that the inclusion of
particles of a light reflective material 24 increases the number of
times a photon is scattered and thereby increases the probability
that a photon will result in the generation of photoluminescence
light. Preliminary tests indicate that by including particles of a
light reflective material with the phosphor material in the
phosphor ink this can reduce, by up to 33%, the amount of phosphor
material required to generate a given color emission product. It is
believed that the particles of light reflective material increase
the probability of photons striking a particle of phosphor material
and thus for an emission product of a given color less phosphor
material is required.
[0053] FIG. 2 is a plot of emission intensity versus chromaticity
CIE x for a light emitting device in accordance with the invention
for weight percent loadings of light reflective material of
.diamond-solid.--0%, .box-solid.--0.4%, .tangle-solidup.--1.1% and
--2%. The data are for screen printed wavelength conversion layers
18 in which the binder material 20 comprises Nazdar's.RTM. UV
curable litho clear overprint PSLC-294 and the phosphor material
comprises Intematix Corp.'s yellow (565 nm) silicate phosphor
EY4453 with an average particle size of 15 .mu.m. The ratio of
phosphor material 22 to binder material 20 is in a proportion of
2:1 by weight. The light reflective material 24 comprises Norcote
International Inc's super white ink GN-027SA. The numbers for
loading of light reflective material refer to weight percent of
super white ink 24 to total phosphor ink composition (binder
material+phosphor material). The smaller reference numerals
associated with each data point indicate the number `n` of print
passes used to form the wavelength conversion layer 18. It will be
appreciated that the thickness t of the wavelength conversion layer
18 is proportional to the number of print passes. The ovals 32, 34,
36, 38 are used to group data points for emission products that
have substantially the same intensity and CIE x values. For example
oval 32 indicates that an emission product of similar intensity and
color can be produced for wavelength conversion layers 18
comprising a) 3 print passes without light reflective material and
b) 2 print passes with a 2% loading of light reflective material.
These data indicate that by the inclusion of a 2% weight loading of
light reflective material 24 it is possible to generate the same
color and intensity of light with a phosphor ink comprising about
33% less phosphor material. Oval 34 indicates that the same
intensity and color of emission product is produced for wavelength
conversion layers 18 comprising a) 4 print passes without light
reflective material and b) 3 print passes with a 0.4% loading of
light reflective material. These data indicate that by the
inclusion of a 0.4% weight loading of light reflective material the
same color and intensity of light can be produced with a phosphor
ink comprising about 25% less phosphor material. Oval 36 indicates
that the same intensity and color of emission product is produced
for wavelength conversion layers 18 comprising a) 4 print passes
without light reflective material and b) 3 print passes with a 1.1%
loading of light reflective material. These data indicate that by
the inclusion of a 1.1% weight loading of light reflective material
the same color and intensity of light can be produced with a
phosphor ink comprising about 25% less phosphor. Oval 38 indicates
that the same intensity and color of emission product is produced
for wavelength conversion layers 18 comprising a) 4 print passes
with a 0.4% weight loading of light reflective material and b) 3
print passes with a 2% weight loading of light reflective material.
These data indicate by the inclusion of a 0.4% weight loading of
light reflective material that the same color and intensity of
light can be produced with a phosphor ink comprising about 25% less
phosphor. Points 40 (n=4, 1.1% loading) and 42 (n=4, 2% loading)
suggest that a saturation point exists above which an increase in
light reflective material loading results in a decrease in emission
intensity with little effect on the color.
[0054] FIG. 3 is a schematic representation of an LED-based white
light emitting device 10 in accordance with another embodiment of
the invention. In this embodiment the light transmissive substrate
16 is configured as a light guide (waveguide) and the wavelength
conversion layer 18 is provided (printed) over the light emitting
face of the substrate. Typically the substrate 16 is substantially
planar and can be disc-shaped, square, rectangular or other shapes
depending on the application. Where the substrate is disc-shaped
the diameter can typically be between about 5 cm and 30 cm
corresponding to a light emitting face of area of between about 20
cm.sup.2 and about 700 cm.sup.2. Where the substrate is square or
rectangular in form the sides can typically be between about 5 cm
and 40 cm corresponding to a light emitting face of between about
80 cm.sup.2 and about 5000 cm.sup.2. On the non-light emitting face
(the lower face as illustrated) of the substrate 16 a layer of
light reflective material 44 can be provided to prevent the
emission of light from the rear of the device. The reflective
material 44 can comprise a metallic coating such as chromium or a
glossy white material such as a plastics material, paint or paper.
To minimize light being emitted from the edges of the substrate,
the edges of the substrate preferably include a light reflective
surface (not shown). One or more blue LEDs 12 are configured to
couple blue light 26 into one or more edges of the substrate 16. In
operation light 26 coupled into the substrate 16 is guided
throughout the entire volume of the substrate 16 by total internal
reflection. Light 26 striking the light emitting face of the
substrate at angles above a critical angle will be emitted through
the face and into the phosphor wavelength conversion layer 18.
Operation of the device is the same as that described with
reference to FIG. 1. As indicated in FIG. 3 phosphor generated
light 46 emitted in directions away from the light emitting face
can re-enter the substrate 16 and will eventually be emitted
through the light emitting face by being reflected by the light
reflective layer 44. The emission product 30 emitted by the device
is the combination of the blue light 26 generated by the LED and
wavelength converted light 28 generated by the phosphor wavelength
conversion layer 18.
[0055] FIG. 4 is a schematic representation of an alternative
LED-based white light emitting device 10 in which the light
transmissive substrate 16 is configured as a light guide. In this
embodiment the phosphor conversion layer 18 is provided on the face
of the substrate that is opposite to the light emitting face and
the light reflective layer 44 is provided over the phosphor
conversion layer 18.
[0056] FIG. 5 shows a schematic representation of an LED-based
white light emitting device 10 in accordance with a further
embodiment of the invention. In this embodiment the wavelength
conversion component 14 is light reflective and is configured to
convert the color of light reflected by the component. The
wavelength conversion component 14 comprises a light reflective
substrate 48 with the wavelength conversion layer 18 provided on a
light reflective surface of the component. As shown the light
reflective substrate 48 can comprise a parabloidal surface though
it can comprise any surface including planar, hemispherical, part
cylindrical, convex and concave shaped surfaces. To maximize light
emission from the device, the light reflective surface is as
reflective as possible and preferably has a reflectance of at least
0.9. The light reflective surface can comprise a polished metallic
surface such as silver, aluminum, chromium; a light reflective
polymer, a light reflective paper or a surface including a light
reflective coating such as a light reflective paint. To assist in
the dissipation of heat the light reflective surface can also be
thermally conductive.
[0057] Operation of the light emitting device of FIG. 5 is not
described in detail as it is similar to that of FIG. 1. However it
is to be appreciated that since on average up to half of the LED
generated light 26 will travel through the wavelength conversion
layer 18 twice, the thickness of the wavelength conversion layer 18
can be of up to half (.apprxeq.t/2) compared to arrangements with a
light transmissive wavelength conversion component (FIG. 1). As a
result of providing the phosphor material on a light reflective
surface the same color of emission product can be achieved with a
further potential reduction of up to 50% in phosphor material
usage.
[0058] The concept of manufacturing a photoluminescence wavelength
conversion component for a solid-state light emitting device by
printing a layer of phosphor ink on a substrate (light transmissive
or light reflective) is believed to be inventive in its own right.
To ensure that the wavelength conversion component produces a
uniform color of emitted light over its entire surface, the
wavelength conversion layer 18 can be printed as one or more
uniform thickness layers that cover the entire light emitting
surface of the substrate. In other embodiments the phosphor ink can
be printed as a pattern covering a part or the whole of the surface
of the substrate. For example the phosphor ink can be printed as a
series of parallel lines, a regular pattern such as a checkerboard
pattern or an irregular pattern.
[0059] It is further envisioned in other applications to print the
phosphor ink as a graded or graduated pattern. Such a graduated
patterning can be used to compensate for variations in the color of
emitted light across the substrate. For example it has been
discovered that for a white light emitting device with a circular
wavelength conversion component with a uniform phosphor conversion
layer, light emitted from the center of the component can have a
relatively higher proportion of blue light compared with light
emitted around the periphery of the component. The result is that
the emission product of such a device can be blue-white at the
center surrounded by a yellow-white halo. It is believed that by
printing relatively more phosphor ink at the center of the
substrate compared with the edges this can reduce at least in part
such haloing. More phosphor ink can be provided on a selected
region of the substrate by a) selectively printing more phosphor
ink layers at such regions or b) preferably by printing the
phosphor ink as a graduated pattern in which the phosphor ink
covers a greater proportion per unit area of the substrate in such
regions. FIGS. 6a and 6b respectively show graduated printed
phosphor ink pattern based on AM (amplitude modulated) half tone
screening and first order stochastic or FM (frequency modulated)
screening. In FIG. 6a the phosphor ink is printed as array of
regularly spaced dots of varying size. Such a patterning is
referred to as AM half tone screening as the amplitude (size) of
the dots is modulated (varied) whilst the frequency (spacing) of
the dots remains fixed. In FIG. 6b the phosphor ink is printed as a
first order stochastic pattern comprising a pseudo-random array of
phosphor dots of the same size in which the frequency (density) of
dots is varied. Compared with a half tone patterning a first order
stochastic pattern can be easier to print since the dot size is
fixed and is preferred for screen printing since the dot size can
correspond to the screen mesh size. Moreover a stochastic pattern
can be preferred where it is required to make multiple print passes
or to print patterns comprising two or more phosphor inks since
such a random patterning is less sensitive to alignment issues. It
is further envisioned to print the phosphor ink using a second
order stochastic screening in which both the frequency and
amplitude of the dots are modulated.
[0060] FIGS. 7a to 7e illustrate a method of manufacturing a shaped
(i.e. non-planar) wavelength conversion component 14 such as a
hemispherical, parabloidal or cylindrical-shaped components. The
wavelength conversion component 14 can be light transmissive (FIG.
1) or light reflective (FIG. 5). For either component the substrate
16, 48 onto which the phosphor ink is to be deposited comprises a
thermally formable material typically a thermoplastic polymer
material such as an acrylic poly(methyl methacrylate) (PMMA) or PET
(Polyethylene terephthalate). Polycarbonate can also be used but is
not preferred for phosphor inks in which the binder materials is
based on a conventional ink as these can exhibit de-lamination
problems. Acrylic and PET are preferred substrate materials due to
their strong adhesion and easy thermal forming capabilities. To
enable the phosphor ink to be readily deposited it is preferred
that at least the surface of the substrate onto which the phosphor
ink is to be deposited is substantially planar (FIG. 7a). The
phosphor ink is printed, typically screen printed, onto the
substrate to form a substantially uniform thickness layer (FIG.
7b). As indicated the phosphor ink can be printed as a pattern or
as a layer covering the entire surface of the substrate. The
substrate is them formed into a selected shape by for example
vacuum and thermal forming using a mold or former 50 (FIGS. 7c and
7d). After cooling the finished wavelength conversion component 14
is removed from the mold (FIG. 7e).
[0061] When printing the phosphor ink on such components it is
contemplated to account for local changes in the thickness of the
wavelength conversion layer that can arise during the forming
process. Such expected changes can be compensated by printing
proportionally thicker layers of phosphor ink at regions of the
substrate that will be deformed most in the final wavelength
conversion component and proportionally thinner layers printed
where the substrate is deformed less or results in a compression of
the wavelength conversion layer. In this way it is possible to
fabricate complex shaped wavelength conversion components having a
consistent and predictable thickness wavelength conversion
layer.
Phosphor Inks
[0062] As described phosphor inks in accordance with the invention
comprise a light transmissive binder material 20 that is loaded
with at least one phosphor material 22 and can optionally further
include particles of a light reflective material 24.
[0063] The color of the emission product produced by the wavelength
conversion component will depend on the quantity of phosphor
material per unit area in the wavelength conversion layer. It will
be appreciated that the quantity of phosphor material per unit area
is dependent on the thickness of the wavelength conversion layer
and the weight loading of phosphor material to binder. In
applications in which the emission product is white or in
applications in which the emission product has a high saturation
color (i.e. the emission product comprises substantially all
photoluminescence generated light) the quantity of phosphor
material per unit area in the wavelength conversion layer will
typically be between 10 and 40 mgcm.sup.-2. To enable printing of
such a wavelength conversion layer in a minimal number of steps the
phosphor ink preferably has as high a solids loading of phosphor
material to binder material as is suitable for the selected
printing (deposition) method. Alternatively phosphor inks with a
lower solids loading of phosphor material can be used though
multiple layers will need to be deposited to achieve a selected
conversion emission product color.
[0064] For many applications, in particular those where the
emission product comprises white light, it is necessary for the
phosphor ink to allow a proportion of the blue light from the LED
to pass through. In general white light and cooler colors will use
the native blue light direct from the LED as the blue light
component in the emission product. Accordingly the wavelength
conversion layer must be configured to not only generate the
required proportion of photoluminescence light but also allow an
appropriate proportion of blue light to pass through. For cool
white light the proportion of blue light pass through is
approximately 10%-30% depending on the color temperature of the
emission product (a lower proportion is used for warm light).
Control of the amount of blue light pass through will also depend
on the solids loading of phosphor material in the phosphor ink and
the thickness t of the wavelength conversion layer. In pass through
mode the phosphor inks act as a diffuser to the blue light and aids
in color blending.
[0065] Red light emitting LEDs (red LEDs) can be a very efficient
and low cost way of generating the red component of warm colors and
warm white light. For some applications including general lighting,
LED backlights, transportation and especially those applications
requiring white light with a high CRI (Color Rendering Index) it
can be desirable to use red LEDs combined with blue LEDs in the
light source used to excite the wavelength conversion component. In
such devices the phosphor ink must allow for red light pass through
as well. Since none of the red light will be converted by the
phosphor material the phosphor ink should have a maximum
transmittance of red light (typically at least 60%). In operation
the blue pass through light would be used for the blue component,
green and yellow phosphors in the inks for the mid range colors and
red LED pass through light would used as the red component.
Phosphor Ink Binder Material
[0066] Typically the binder material 20 comprises a curable liquid
polymer such as a polymer resin, a monomer resin, an
acrylic-poly(methyl methacrylate) (PMMA), an epoxy (polyepoxide), a
silicone or a fluorinated polymer. It is important that the binder
material 20 is, in its cured state, transmissive to all wavelengths
of light generated by the phosphor material 24 and LED 12 and
preferably has a transmittance of at least 0.9 over the visible
spectrum (380 to 800 nm). The binder material 20 is preferably U.V.
curable though it can be thermally curable, solvent based or a
combination thereof. U.V. or thermally curable binders can be
preferable because, unlike solvent-based materials, they do not
"outgas" during polymerization. When a solvent evaporates the
volume and viscosity of the composition will change resulting in a
higher concentration of phosphor material which will affect the
emission product color of the device. With U.V. curable polymers,
the viscosity and solids ratios are more stable during the
deposition process with U.V. curing used as to polymerize and
solidify the layer after deposition is completed. Moreover since in
the case of screen printing of the phosphor ink multiple-pass
printing is often required to achieve a required layer thickness,
the use of a U.V. curable binder is preferred since each layer can
be cured virtually immediately after printing prior to printing of
the next layer.
[0067] As well as providing a light transmissive suspension medium
for the phosphor material the binder also acts as a protective
encapsulation for the phosphor material protecting it from moisture
and air which can damage the phosphor material's performance. To
enhance hermetic sealing of the phosphor material, the binder
material can comprise a polyimide, a fluorinated plastics material
or a silicone. In addition to the hermetic properties of the
binder, the permeability of the substrate to moisture and air
should also be taken into account. For optimal performance the
light transmissive substrate comprises a glass, a multilayered
structure including one or more light transmissive inorganic
hermetic layers or a plastics material with a low permeability to
water and air. The substrate and binder material thus combine to
create a protected phosphor layer during the printing process.
Additionally a protective layer can be printed, laminated or
otherwise deposited onto the substrate or onto the wavelength
conversion layer to maximize protection of the phosphor
material.
[0068] For applications in which the substrate 16, 48 is
substantially planar or in which the substrate is only mildly
re-formed after printing the wavelength conversion layer, the
binder material 20 can have a low elasticity (i.e. an elastic limit
100% or lower). Where it is intended to re-form the substrate after
printing the phosphor ink to create wavelength conversion
components of more complex shapes (e.g. domes, dishes, hemispheres,
spherical shells etc--FIGS. 7a to 7e) the binder material 20
preferably has an elasticity with an elastic limit of
300%-500%.
[0069] Where the photoluminescence wavelength component is light
transmissive it is desirable to match as closely as practicable the
refractive index of the binder material (in a cured state) with the
refractive index of the light transmissive substrate to minimize
the refraction of light at the interface between the substrate and
wavelength conversion layer. TABLE 1 gives refractive index values
for various substrate materials. A preferred binder material is a
U.V. curable acrylic adhesive that has a refractive index n=1.48.
When used in conjunction with acrylic, silica, Pyrex and silicone
substrates the refractive indices are matched to within about
.+-.0.02. Preferably the binder material has a refractive index in
a range 1.46 to 1.59.
TABLE-US-00001 TABLE 1 Substrate material Refractive index n
Polycarbonate 1.584 to 1.586 Acrylic (PMMA) 1.491 Fused silica
1.459 Pyrex glass 1.474 Silicone .apprxeq.1.46
Printable Phosphor Inks
[0070] The intended printing method used to deposit the phosphor
ink will affect the required properties of the binder material,
typically viscosity, weight loading of phosphor material to binder
and when present phosphor/light reflective material
loading/particle size. For example if the viscosity of the phosphor
ink is too high it will not be possible to print the ink.
Conversely if the viscosity is too low the phosphor material can
tend to agglomerate during printing resulting in clusters of
phosphor material in the printed layer.
[0071] The viscosity of the phosphor ink is primarily determined by
the viscosity of the binder material and weight loading of
phosphor/light reflective material Thinning additives can be used
during initial formulation of the phosphor ink to achieve a
required viscosity and to "thin" the phosphor ink during printing.
However care must exercised when thinning to maintain the solids
loading since it is the phosphor material content (loading) and
layer thickness, not viscosity, that determines the color of light
generated by the phosphor ink.
[0072] As well as viscosity the surface tension of the binder
material can affect the phosphor inks performance. For example if
the surface tension of the phosphor ink is too high, bubbles can
form during printing resulting in poor layer formation. Bubbles can
also form in phosphor inks with a low surface tension and it is
preferred to additionally add a de-foaming agent to the phosphor
ink.
TABLE-US-00002 TABLE 2 Viscosity (Pa s) Phosphor weight Phosphor
ink Range Preferred loading Inkjet 0.004 to 0.020 .apprxeq.0.014
.apprxeq.25% Gravure/flexograph 0.05 to 0.5 Screen printing 0.5 to
5.sup. .apprxeq.1 40% to 75% Letterpress 50 to 150
Screen Printing Phosphor Inks
[0073] For screen printing the binder material preferably has a
viscosity in a range 0.1 to 5 Pas (100 to 5000 cps) and preferably
about 1 Pas to 2.5 PaS (1000 to 2500 cps). To reduce the number of
print passes necessary to achieve a required quantity of phosphor
material per unit area in the deposited wavelength conversion
layer, the weight loading of phosphor material to binder is as high
as possible and is preferably in a range 40% to 75% depending on
the phosphor material density. It has been found that above about a
75% weight loading it can be difficult to ensure strong cohesion,
adhesion and maintain printability of the phosphor ink. For weight
loadings below about 40% it is found that five or more print passes
are necessary to achieve a required phosphor material per unit
area. In the phosphor inks of the invention the weight loading of
phosphor material to binder material is much higher that weight
loading of pigment in a conventional screen print ink. It has been
discovered that efficient light conversion can be achieved when the
mean particle size of the phosphor material is about 15 .mu.m.
Gravure and Flexograph Phosphor Inks
[0074] In both gravure and flexograph printing a plate or drum with
a pattern of holes or cavities are filed with ink during an
application step. The plate or drum is then rolled over the
substrate and the ink passes from the plate to the substrate with
the quantity of ink being controlled by the size of the cavities.
Flexograph printing generally uses a polymer plate and gravure
generally uses a metal plate suitable for longer production runs. A
particular advantage of gravure and flexograph printing for
fabricating wavelength conversion layers is that each is capable of
depositing relatively high volumes of phosphor ink in a single
pass. Moreover, since the volume of ink is determined by the volume
of the cavities the quantity of phosphor ink deposited is also
accurately controlled. For gravure and flexograph printing the
phosphor ink preferably has a viscosity in a range 0.05 to 0.5 Pas
(50 to 500 cps).
Inkjet Phosphor Inks
[0075] For inkjet printing it is preferred that the phosphor ink
has a very low viscosity typically in a range 0.004 to 0.020 Pas (4
to 20 cps), preferably about 0.014 Pas (14 cps), with about a 25%
weight loading of phosphor material to binder (TABLE 2). For
operation with piezoelectric inkjet printers with nozzles up to 100
.mu.m the phosphor/light reflective material have an average
particle size of less than about 10 .mu.m Inkjet printable phosphor
inks can be preferable for custom applications and low volume
production. It is envisioned to use the elevated temperatures
during inkjet printing (up to 100.degree. C.) to reduce the
viscosity of the phosphor ink.
[0076] Compared with conventional inkjet ink pigments, the larger
phosphor material particle size can result in a settling of the
phosphor material within the phosphor ink. To reduce settling of
the phosphor material it is envisaged to use an inkjet printer with
a re-circulating system, such as for example those with a dual port
print head from Spectra Inc, to continuously circulate the phosphor
ink and thereby maintain a consistent solids loading during
printing. This can be especially important where it is required to
print very small areas of phosphor ink.
[0077] Additionally phosphor inks for inkjet printing can further
include a surfactant to lower surface tension between the phosphor
and binder materials assist in uniformly dispersing the phosphor
material in the binder material for both steric and/or
electrostatic dispersion.
Pad Printing Phosphor Inks
[0078] Pad printing is a well established technique for printing
onto shaped (i.e. non-planar) substrates such as for example
keyboard keys, golf balls and pen barrels. As is known a
resiliently deformable pad, often made of silicone rubber, is used
to transfer the ink from the printing plate to the substrate. The
pad can be round in form "round pad", bar shaped "bar pad" or other
shapes such as square or rectangular termed "loaf pads". Pad
printing of phosphor inks is particularly preferred where the
substrate has a simple shaped surface such as for example is part
cylindrical, dished or dome shaped.
Blended Phosphor Inks
[0079] So far the phosphor inks have been described as including a
single phosphor material. Such single phosphor inks are
particularly useful where it is required to generate saturated
colors. A selected emission product color can be attained by
printing multiple layers or patterns, for example dot patterns, of
different single phosphor inks.
[0080] In further embodiments the phosphor ink can comprise a blend
of two or more phosphor materials having different emission
characteristics to generate a selected emission product color. By
blending different ratios of the phosphor materials different
target emission colors can be achieved. A blended phosphor ink can
provide a consistent ratio of color and simplifies printing by
eliminating the need for multi-pass printing. Accordingly blended
phosphor inks are particularly suitable for coating processes such
as spraying, spin coating, slit coating or dipping for single
wavelength conversion layers.
Light Emitting Signage with Photoluminescence Wavelength
Conversion
[0081] Whilst the phosphor inks of the invention have been
described in relation to wavelength conversion components for
solid-state light emitting devices the invention can also be
applied to other applications. In particular phosphor inks in
accordance with the invention are particularly suitable for light
emitting signs that use photoluminescence wavelength conversion to
generate a selected color of light such as for example is taught in
co-pending United States patent application US 2007/0240346 A1, to
Li et al., the specification of which is incorporated herein by way
of reference thereto.
[0082] It will be appreciated that in such light emitting signs the
wavelength conversion component 14 can be used as the
photoluminescence signage surface to generate signage information
of a desired light color. The phosphor ink can be configured as a
pattern to define an image, picture, letter, numeral, device,
pattern or other signage information on the light transmissive
substrate. Alternatively, as for example is required for channel
lettering, the shape of the signage surface, that is the light
transmissive substrate, can be configured to define signage
information. Phosphor ink can be particularly advantageous in
signage applications where the area of the light emitting signage
surface is many hundreds of square centimeters requiring the
phosphor material to be distributed over a minimum area of 100
cm.sup.2 (10 cm by 10 cm) and more typically over many hundreds or
even thousands of square centimeters. For such applications the
inclusion of a light reflective material in the phosphor ink can
provide a significant saving in phosphor material usage and a
substantial reduction in manufacturing costs.
[0083] The signs can be backlit, that is, the LEDs are located
behind the signage surface within for example a light box, and the
signage surface provided overlaying the light box opening.
Typically the signage surface is located at a distance of at least
5 mm from the LEDs. Alternatively the sign can be edge lit and the
light transmissive signage surface configured as a light guide and
the mixture of phosphor material and light reflective material
provided on at least a part of a light emitting face of the light
guide.
[0084] It will be appreciated that the invention is not limited to
the exemplary embodiments described and that variations can be made
within the scope of the invention. For example whilst the invention
has been described in relation to LED-based light emitting devices
and signage the invention also applies to devices based on other
solid-state light emitters including solid-state lasers and laser
diodes.
* * * * *