U.S. patent number RE43,125 [Application Number 11/882,394] was granted by the patent office on 2012-01-24 for back light assembly for use with back-to-back flat-panel displays.
This patent grant is currently assigned to Seiko Epson Corporation. Invention is credited to Donald P. Seraphim, Dean W. Skinner.
United States Patent |
RE43,125 |
Seraphim , et al. |
January 24, 2012 |
Back light assembly for use with back-to-back flat-panel
displays
Abstract
The present invention features a system for uniformly
distributing luminance and a high degree of collimation from a back
light module for flat-panel, liquid crystal displays (LCDs)
simultaneously. A constant and uniform luminance output of the back
light module in two directions is obtained through appropriate
selection of lamps, geometry and optical components. An appropriate
balance of lamps, lamp spacing, diffusers and light collimating
optics are chosen to produce a high brightness back light module
with very high intensity output over two very large surfaces.
Variations in intensity over the illuminated area are minimized
using light recycling in conjunction with the reflective diffusers
and collimating optics. Precision collimators eliminate light
beyond a defined angle, as required in tiled or monolithic
flat-panel LCDs with predetermined display specifications.
Inventors: |
Seraphim; Donald P. (Vestal,
NY), Skinner; Dean W. (Beverly Hills, FL) |
Assignee: |
Seiko Epson Corporation (Tokyo,
JP)
|
Family
ID: |
25422818 |
Appl.
No.: |
11/882,394 |
Filed: |
August 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
11154995 |
Jun 17, 2005 |
Re. 40355 |
|
|
Reissue of: |
09906691 |
Jul 18, 2001 |
6578985 |
Jun 17, 2003 |
|
|
Current U.S.
Class: |
362/243;
362/97.1; 349/70; 359/599; 362/330; 362/342; 40/572; 362/290 |
Current CPC
Class: |
G02F
1/133611 (20130101); G02F 1/133604 (20130101); G02F
1/133342 (20210101); G02F 1/133607 (20210101); G02F
1/133613 (20210101); G02F 1/133507 (20210101) |
Current International
Class: |
F21V
13/00 (20060101) |
Field of
Search: |
;362/26,27,97.1-97.3,330,606,607,243,255,290,294,342
;349/64,70,61-63,66,68,71,73 ;40/572 ;359/599 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Husar; Stephen F
Attorney, Agent or Firm: Oliff & Berridge, PLC
Parent Case Text
.Iadd.This application is a divisional reissue application of
application Ser. No. 11/154,995 filed Jun. 17, 2005 (now U.S. Pat.
No. Re. 40,355), which is a reissue application of U.S. Pat. No. 6
578,985..Iaddend.
.Iadd.This divisional reissue application is one of three related
reissue applications for the reissue of U.S. Pat. No. 6,578,985.
The reissue applications are 11/154,995 (the issued parent
reissue), 11/882,394 (the present reissue), and 11/882,393 (another
divisional reissue of 11/154,995, that was filed on the same date
as the present reissue application)..Iaddend.
Claims
What is claimed is:
.[.1. A high-output back light module for use with two back-to-back
flat-panel displays, comprising: a) a housing having an open front
and an open back and defining a lamp cavity, said lamp cavity
having substantially solid, optically-reflective side walls; b) an
array of lamps disposed within said lamp cavity; and c) lamp
control means operatively connected to at least one lamp of said
array of lamps to provide power thereto and to optimize light
output therefrom; wherein said housing, said lamp cavity and said
lamp array are disposed intermediate two back-to-back flat-panel
displays at a predetermined distance from each of said two
back-to-back flat-panel displays..].
.[.2. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 1, wherein said lamp cavity
is substantially rectangular and oriented such that the longer side
of said rectangle is disposed horizontally..].
.[.3. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 2, wherein said array of
lamps is disposed horizontally within said lamp cavity..].
.[.4. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 2, wherein said lamp array
comprises fluorescent lamps..].
.[.5. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 4, wherein said fluorescent
lamps comprise hot cathode fluorescent lamps..].
.[.6. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 5, further comprising at
least one from group: collimating means, diffuser means and
brightness enhancing films (BEFs) disposed intermediate said
housing, and at least one of said back-to-back flat-panel
displays..].
.[.7. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 6, wherein said lamp array
is defined by parameters comprising the number of lamps in said
lamp array, the type of lamps, the lamp diameter and the inter-lamp
spacing, and wherein at least one of said parameters is chosen to
optimize light. output from said lamp array disposed in said lamp
cavity..].
.[.8. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 7, wherein lamps of said
array of lamps are spaced apart from one another at a
predetermined, inter-lamp spacing; said array of lamps being
disposed a predetermined, optimized distance from each of said
back-to-back flat-panel displays, said distance being functionally
related to at least one of the parameters: lamp diameter, lamp
type, inter-lamp spacing, collimator means, BEFs and
diffusers..].
.[.9. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 8, wherein said inter-lamp
spacing between each lamp of said array of lamps is substantially
equal..].
.[.10. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 6, wherein said collimating
means comprises a collimator having an array of open cells having a
regular, repeating cell geometry, said geometry defining a cell
width, each of said cells having a thickness defining a cell
depth..].
.[.11. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 10, wherein said cell width
and said cell depth have an aspect ratio therebetween, defining a
cut-off angle..].
.[.12. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 11, wherein said cell width
and said cell depth define cell walls..].
.[.13. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 12, wherein said cell walls
are coated with a light-absorbing coating..].
.[.14. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 12, wherein said
light-absorbing coating comprises flat, black paint..].
.[.15. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 8, further comprising a
high efficiency exit diffuser placed proximate at least one of said
open front and open back of said housing..].
.[.16. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 15, wherein said high
efficiency exit diffuser produces a substantially Lambertian
distribution and efficiently reflects light for
recirculation..].
.[.17. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 16, wherein said high
efficiency exit diffuser is disposed at a predetermined distance
from said lamps whereby luminance gradients are reduced across said
illuminated areas below a predetermined value..].
.[.18. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 17, further comprising
brightness-enhancing means disposed proximate said high efficiency
exit diffuser..].
.[.19. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 18, wherein said
brightness-enhancing means comprises at least two
brightness-enhancing films (BEFs) for collimating light, disposed
intermediate said lamps and at least one of said back-to-back
displays, said BEFs comprising parallel V-grooves having a
predetermined wall angle relative to a first surface thereof, said
BEFs being arranged substantially orthogonally to one
another..].
.[.20. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 19, wherein said first
surfaces of said BEFs face away from said array of lamps, said BEFs
interacting with said high efficiency exit diffusers to enhance the
forward gain of light collimated by said collimating means..].
.[.21. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 1, wherein said lamp
control means comprises lamp temperature regulation means adapted
to maintain the surface temperature of each of said lamps within a
predetermined range of operating temperatures..].
.[.22. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 21, wherein said lamp
temperature regulation means comprises at least one from the group:
heat sinks, dimming controls and fan speed controls..].
.[.23. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 22, wherein at least one of
said dimming controls and fan speed controls comprises a
temperature sensor proximate at least one of said array of
lamps..].
.[.24. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 23, wherein said
temperature sensor generates a variable output voltage
representative of the temperature of said at least one of said
array of lamps..].
.[.25. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 24, wherein said
temperature sensor generates a variable output voltage
representative of the temperature of at least one of said lamps,
said variable output voltage controlling the speed of a cooling
fan..].
.[.26. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 24, wherein said
temperature sensor generates a variable output voltage
representative of the temperature of at least one of said lamps,
said variable output voltage controlling the output of a dimming
ballast..].
.[.27. The high-output back light module for use with back-to-back
flat-panel displays as recited in claim 24, wherein said
temperature sensor comprises a thermistor..].
.[.28. The high-output back light module for use with back-to-back
flat-panel display as recited in claim 1, wherein at least one of
said flat-panel displays comprises one from the group: monolithic
display, monolithic-like display, tiled display..].
.Iadd.29. A back light module comprising: a diffuser having an
incidence side and an output side; a brightness enhancing film
having an incidence side and an output side, the incidence side of
the brightness enhancing film being faced toward the output side of
the diffuser; and a second diffuser having an opposing side that is
disposed at a predetermined distance from the incidence side of the
diffuser; wherein the second diffuser produces a substantially
Lambertian distribution; and wherein the incidence side of the
diffuser at least partially transmits light toward the brightness
enhancing film and at least partially reflects light transmitted
from the output side of the diffuser back toward the brightness
enhancing film, the opposing side of the second diffuser at least
partially reflects light toward the diffuser..Iaddend.
.Iadd.30. The back light module as recited in claim 29, wherein the
output side of the brightness enhancing film at least partially
reflects light transmitted by the diffuser back toward the
diffuser..Iaddend.
.Iadd.31. The back light module as recited in claim 30, further
comprising another brightness enhancing film, the brightness
enhancing films being arranged substantially orthogonally to one
another..Iaddend.
.Iadd.32. The back light module as recited in claim 29, wherein the
incidence side of the diffuser produces a substantially Lambertian
distribution..Iaddend.
.Iadd.33. The back light module as recited in claim 29, wherein the
opposing side of the second diffuser at least partially transmits
light toward a back side of the second diffuser..Iaddend.
.Iadd.34. A flat panel apparatus, comprising: a flat-panel display;
and a back light module; the back light module comprising: a) a
diffuser having an incidence side and an output side; b) a
brightness enhancing film having an incidence side and an output
side, the incidence side of the brightness enhancing film being
faced toward the output side of the diffuser; and c) a second
diffuser having an opposing side that is disposed at a
predetermined distance from the incidence side of the diffuser;
wherein the second diffuser produces a substantially Lambertian
distribution; and wherein the incidence side of the diffuser at
least partially transmits light toward the flat-panel display
through the brightness enhancing film and at least partially
reflects light transmitted from the output side of the diffuser
back toward the flat-panel display through the brightness enhancing
film, and the opposing side of the second diffuser at least
partially reflects light toward the diffuser..Iaddend.
.Iadd.35. The flat panel apparatus as recited in claim 34, wherein
the output side of the brightness enhancing film at least partially
reflects light transmitted by the diffuser back toward the
diffuser..Iaddend.
.Iadd.36. The flat panel apparatus as recited in claim 35, further
comprising another brightness enhancing film, the brightness
enhancing films being arranged substantially orthogonally to one
another..Iaddend.
.Iadd.37. The flat panel apparatus as recited in claim 34, wherein
the incidence side of the diffuser produces a substantially
Lambertian distribution..Iaddend.
.Iadd.38. The flat panel apparatus as recited in claim 34, wherein
the opposing side of the second diffuser at least partially
transmits light toward a back side of the second
diffuser..Iaddend.
.Iadd.39. A back light module comprising: a diffuser having an
incidence side and an output side; a brightness enhancing film
having an incidence side and an output side, the incidence side of
the brightness enhancing film being faced toward the output side of
the diffuser; and a second diffuser having an opposing side that is
disposed at a predetermined distance from the incidence side of the
diffuser; wherein the incidence side of the diffuser at least
partially transmits light toward the brightness enhancing film, the
opposing side of the second diffuser at least partially reflects
light toward the diffuser and the second diffuser produces a
substantially Lambertian distribution..Iaddend.
.Iadd.40. The back light module as recited in claim 39, wherein the
incidence side of the diffuser at least partially reflects light
transmitted from the output side of the diffuser back toward the
brightness enhancing film..Iaddend.
.Iadd.41. A flat panel apparatus, comprising: a flat-panel display;
and a back light module; the back light module comprising: a) a
diffuser having an incidence side and an output side; b) a
brightness enhancing film having an incidence side and an output
side, the incidence side of the brightness enhancing film being
faced toward the output side of the diffuser; and c) a second
diffuser having an opposing side that is disposed at a
predetermined distance from the incidence side of the diffuser;
wherein the incidence side of the diffuser at least partially
transmits light toward the flat-panel display through the
brightness enhancing film, the opposing side of the diffuser at
least partially reflects light toward the diffuser and the second
diffuser produces a substantially Lambertian
distribution..Iaddend.
.Iadd.42. The flat panel apparatus as recited in claim 41, wherein
the incidence side of the diffuser at least partially reflects
light transmitted from the output side of the diffuser back toward
the flat-panel display through the brightness enhancing
film..Iaddend.
Description
This application is related to U.S. patent application Ser. No.
09/368,921 filed Aug. 6, 1999 .Iadd.(now U.S. Pat. No.
6,657,698).Iaddend.; U.S. patent application Ser. No. 09/406,977,
filed Sep. 28, 1999 .Iadd.(now U.S. Pat. No. 6,417,832).Iaddend.;
U.S. patent application Ser. No. 09/407,619, filed Sep. 28, 1999
.Iadd.(now U.S. Pat. No. 6,447,146).Iaddend.; U.S. patent
application Ser. No. 09/407,620, filed Sep. 28, 1999 .Iadd.(now
U.S. Pat. No. 6,341,879).Iaddend.; and U.S. patent application Ser.
No. 09/490,776, filed Jan. 24, 2000 .Iadd.(now U.S. Pat. No.
6,680,761).Iaddend., all of which are included herein by reference.
In addition, this application is related to U.S. Pat. Nos.
5,661,531, 5,867,236 and 5,903,328, all of which are also included
herein by reference. These copending applications and issued
patents are all commonly assigned to the assignee of the present
application.
FIELD OF THE INVENTION
This invention pertains to back light assemblies for flat-panel
displays and, more particularly, to a back light module with a
single array of lamps that produces high intensity, collimated
light in two directions suitable for use with large, back-to-back,
tiled flat-panel displays.
BACKGROUND OF THE INVENTION
Flat-panel displays (FPDs) made in accordance with known active
matrix (e.g., TFT, etc.) liquid crystal display technologies (e.g.,
AMLCD) are typically mounted in front of a back light module which
contains an array of fluorescent lamps. AMLCD flat-panel displays
of this type have been increasing in size by about 1 to 2 inches
diagonal, yearly. The median size in 1999 for use in desktop PCs
was about 15 inches diagonal viewing area. A few very large
displays are made in the range of 20 to 28 inches diagonal. Tiled
AMLCD FPDs may be made in the range of 40 inches diagonal, as
described in copending U.S. patent applications Ser. Nos.
09/368,921 1999 .Iadd.(now U.S. Pat. No. 6,657,698) .Iaddend.and
09/490,776 .Iadd.(now U.S. Pat. No. 6,680,761).Iaddend.. Tiled
FPDs, as described in U.S. Pat. No. 5,661,531, require extremely
intense back light sources with highly collimated light, masked
optical stacks, and pixel apertures that may have low emitted light
efficiency. Thus, lighting with unusually high intensity ranges of
50,000 to 150,000 nits is desirable. Also, intensity uniformity
over the very large areas of tiled FPDs is very important. Unique
back light designs, including temperature control features, are
necessary to achieve such high intensities at reasonable power
consumption.
Maintaining bright (i.e., high intensity) and uniform illumination
of the display over its entire active area is difficult to do. The
intensity required for some applications and, in particular, that
required for large, tiled, seamless flat-panel LCD displays, causes
the lamps to produce a significant amount of heat. In addition,
since fluorescent lamps are designed to run most efficiently at an
elevated temperature, it is desirable to operate them at or near
their ideal design temperature, which is usually about 50 to 60
degrees Centigrade.
Small, edge-lit back light modules, such as those used in notebook
or laptop PCs, do not produce sufficient brightness for use in a
large area display, nor are they capable of illuminating that large
an area uniformly. Thus, it is .[.necessary.]. .Iadd.preferable
.Iaddend.to illuminate these larger areas with an array of large
fluorescent lamps. The number of lamps required depends on the size
of the area to be illuminated and the display brightness
requirements. A large area display generally requires multiple
lamps to illuminate it properly. A large area display that can be
viewed from two sides (i.e., a back-to-back display) requires
proportionally more lamps, as well as unique design features to
achieve the desired intensities and maintain optimized lamp
efficiency through temperature control of the lamps.
Since most displays are designed to be wider than they are tall, it
is advantageous, from a reliability and power perspective, to place
the lamps in a horizontal orientation. This typically results in
the use of fewer lamps and, consequently, lower power consumption,
since fewer lamp cathodes are present. The resulting preferred
designs orient lamp tubes horizontally, one above the other with
predetermined, preferred spacing relationships to each other and to
each of the back-to-back displays, one disposed on each side of the
lamp array.
It is, therefore, a principal object of the invention to provide a
back light module designed to illuminate back-to-back displays.
It is an additional object of the invention to provide a back light
module for use with large flat panel displays, either monolithic or
tiled.
It is another object of the invention to provide a back light
module designed to provide a high intensity light output.
It is a further object of the invention to provide a back light
module capable of delivering highly collimated light.
It is an additional object of the invention to provide a back light
module having a very high operating efficiency.
It is a still further object of the invention to provide a back
light module having a cooling structure to maintain a substantially
uniform operating temperature.
It is yet another object of the invention to provide a back light
module utilizing an array of horizontally-mounted fluorescent
tubes.
It is an additional object of the invention to provide a back light
module incorporating a cavity to maximize and control light
recirculation.
It is another object of the invention to provide a back light
assembly incorporating diffusers, collimators and
brightness-enhancing films (BEFs).
It is a further object of the invention to provide a back light
assembly suitable for illuminating large, back-to-back, tiled
flat-panel displays having visually imperceptible seams.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a back
light module which uniformly distributes luminance to back-to-back
flat-panel, liquid crystal displays (LCDs) simultaneously.
Fluorescent lamps are used due to their high efficiency. However,
luminance, efficiency, and lamp life of fluorescent lamps are all
functions of lamp tube temperature. The present invention provides
an apparatus and method for achieving luminance uniformity and a
high degree of light collimation in back-to-back displays with one
single back light module source.
In particular, a constant and uniform luminance output of the back
light module is obtained through appropriate selection of lamps,
optimization of back light module geometry and use of additional
optical components. A preferred balance of lamps, lamp spacing,
diffuser and collimating optics is chosen to produce a high
brightness back light module with very high, uniform intensity
output over very large surface areas. Light is recycled from one
display module to the other as the light is reflected from each of
the display's optical stacks. The optical stacks of the two display
modules typically include polarizers, masks, diffusers etc. In
addition, light is reflected from the light collimating optics and
the light enhancing and diffusing films also typically present in
the optical stacks.
This invention provides a method for achieving this goal through
selection of combinations of components and appropriate design
geometries. A particular application of the inventive back light
module is for use in integrating two large, tiled, flat-panel
displays having visually imperceptible seams as described in the
aforementioned U.S. patent application Ser. Nos. 08/652,032
.Iadd.(now U.S. Pat. No. 5,867,236).Iaddend., and .[.09/368,291.].
.Iadd.09/368,921 (now U.S. Pat. No. 6,657,698), .Iaddend.and U.S.
Pat. No. 5,903,328. The back light module system, with thermal
enhancements such as those disclosed in U.S. patent application
Ser. No. 09/406,977 .Iadd.(now U.S. Pat. No. 6,417,832)
.Iaddend.and applicable controls, such as those disclosed in U.S.
patent application Ser. No. 09/407,619 .Iadd.(now U.S. Pat. No.
6,447,146).Iaddend., provides for an efficient, reliable, large
area, high intensity light source usable with back-to-back
flat-panel displays.
Additionally, optimum geometries are determined for the purpose of
maximizing light output at high efficiencies, while minimizing
luminance gradients across the two displays. These optimum
geometries are also determined for maximizing light output using
brightness enhancing films (BEFs) and light recycling.
Finally, precise collimators such as that disclosed in U.S. patent
application Ser. Nos. 09/024,481 .Iadd.(now U.S. Pat. No.
6,152,580) .Iaddend.and 60/177,447 .Iadd.(now U.S. Pat. No.
6,654,449).Iaddend., eliminate light beyond a defined cut-off angle
for each flat panel display, as required in a tiled flat-panel
LCD.
It will be obvious that while the back light assembly of the
invention is optimized for use with tiled, AMLCD flat-panel
displays, it may also be used with monolithic and monolithic-like
displays.
BRIEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the present invention may be obtained
by reference to the accompanying drawings, when considered in
conjunction with the subsequent, detailed description, in
which:
FIG. 1 is a graph of luminance vs. temperature in a typical
fluorescent lamp;
FIG. 2a is a schematic, cross-sectional view of a multiple lamp
back light simultaneously illuminating back-to-back displays;
FIG. 2b is a plan view of the multiple lamp back light shown in
FIG. 2a;
FIG. 3 is a schematic diagram illustrating lamp and diffuser
spacing relationships;
FIG. 4 is a graph showing light output as a function of the number
of lamps installed;
FIG. 5 is a schematic, sectional view of a back light assembly in
use with back-to-back flat panel displays in accordance with the
present invention;
FIG. 6 is a graph showing luminance as a function of deviation from
a normal caused by the collimation attributes of the optics;
and
FIG. 7 is a ray diagram showing typical reflections of light rays
between diffusers and light collimating (i.e. brightness enhancing)
films.
For purposes of both clarity and brevity, like elements and
components will bear the same designations and numbering throughout
the figures.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Generally speaking, the invention features an apparatus and a
method for controlling the luminance level, luminance uniformity
and collimation of light exiting a large area back light suitable
for use with back-to-back flat-panel displays. The back light
assembly is suitable for use with large, tiled, flat-panel displays
which require high luminance levels and a precise, predetermined
degree of collimation. In addition, the present invention provides
an optimum design taking into account efficiency, cooling,
luminance and image quality for use in integrating back-to-back
flat-panel displays with a single light source. The design is
useful with tiled flat-panel displays and large monolithic or
monolithic-like LCD displays.
Referring first to FIG. 1, there is shown a graph 100 of the light
output (i.e., luminance) and efficiency (i.e., efficacy) of a
typical fluorescent lamp as a function of temperature. Fluorescent
lamps generally operate most efficiently at a predetermined,
optimum lamp tube wall temperature. Maximum brightness usually
occurs near the point 102 of maximum efficacy.
The ideal temperature T.sub.o 104 may then be determined from the
temperature axis of graph 100. The ideal temperature 104 is
determined by the lamp construction, particularly dependent on such
parameters as the phosphor, cathode construction and the mercury
vapor pressure. The most efficient lamps 128 are generally the
class of fluorescent lamps of the hot cathode type. Hot cathode
lamps have a preheat cycle during which the cathodes are heated,
thereby causing easier ignition (i.e., striking) of the gas within
the lamp.
Now referring to FIG. 2a, there is shown a side view 120 of
back-to-back flat-panel displays 122 and its back light assembly
124. The back light assembly 124 consists of a light box cavity
126, an array of fluorescent lamps 128, and light diffusers 130.
Lamps 128 are cooled by fans (not shown).
Some display applications require additional optical components 132
to enhance certain characteristics of the exiting light. For
example, tiled, flat-panel LCD displays require highly collimated
light. The additional optical components 132 required to collimate
the light may be somewhat inefficient. This necessitates that a
high luminance be produced by the back light assembly 124.
Referring now also to FIG. 2b, there is shown a front view of the
back light assembly 124 of FIG. 2a. The lamps 128 are held in the
light box cavity 126 by lamp holders 134. The lamps 128 are wired
to the ballast 136 by a wiring harness 138. The ballast 136
supplies high frequency (usually 20-30 KHz) AC power to the lamps
128. Efficient, high-frequency electronic ballasts are well known
to those skilled in the art and any suitable unit may be chosen for
use with the instant invention, the ballast forming no part
thereof.
It will be obvious that temperature sensing devices, fan speed
control circuitry, lamp dimming controls, heat sinks and other such
temperature control devices and methods which are known to those
skilled in the art could be used in conjunction with the back light
of the present invention to help control the surface temperatures
of the lamps 128. As an example, the lamp holder 134 can be a heat
sink with an attached thermistor (not shown) to measure lamp
temperature and its output used to regulate the voltage to one or
more fans thereby regulating fan speed, or the voltage may be used
to regulate the output of dimming ballast 136.
Referring now to FIG. 3, there is shown a schematic diagram 140 of
a portion of a back light assembly where certain critical
dimensions and/or distances are identified. Two lamps 128, each
having a diameter D 142, are arranged adjacent one another, spaced
apart a distance S 144. Lamps 128 are positioned a distance H 146
away from the diffusers 130. These dimensions may be used in design
calculations in manners well known to those skilled in the art.
If lamps 128 are assumed to be line sources, luminance may be
calculated according to the equation:
.times. ##EQU00001##
Assuming that the required luminance A is known, the number of
lamps may readily be calculated.
Referring now to FIG. 4, there is shown a graph 160 illustrating
the effect of varying the S 144 and H 146 dimensions on the light
output from a back light assembly. Having this information, the
required number of lamps 128 of a predetermined size (diameter) D
142 required to produce the necessary luminance may be
calculated.
The curve of total light output from the back light cavity 126 is a
function of the number of lamps 168 installed. The desired light
level 162 is also shown. It will be noted that, as the number of
lamps increases, the light output increases until a maximum
illumination 164 occurs prior to reaching the point of maximum lamp
capacity 166. Also, as more lamps 168 are used, or the lamps are
spaced closer together, they block light from each other. The
number of lamps 168 corresponding to the desired light output 162
is also shown.
It is also necessary that the diffusers 130 be highly efficient,
but not of high transmissivity. One diffuser 130 behaves as a
diffuser for the display on its side of the lamps 128. However, the
same diffuser 130 behaves as a reflector for the opposite display.
Since the collimating films 182 & 184 (BEFs) require
recirculating light in order to be efficient, the diffusers 130
must both transmit and reflect light. A transmission of 50-75% has
been found to be effective in this application.
Now referring to FIG. 7, several light rays are traced to explain
the interaction between the diffusers 130 and the collimating films
182 and 184. The efficiency of these collimating films 182, 184 are
conditional on a good optical coupling with their reflective
surfaces. Consider a light ray that emanates from the lamp 128 and
strikes the upper diffuser 130 at a point A. If the diffuser 130
is, for example, 60% transmissive, then 60% of the light will be
transmitted through the diffuser 130 and result in a "Lambertian"
distribution (i.e., be uniformly distributed at all angles relative
to the surface of diffuser 130) of light aimed at the collimating
film 182. However, 40% of the light is reflected (also in a
Lambertian distribution) toward the lower diffuser 130. One light
ray from the transmitted light at point A heads toward point B on
the collimating film. The angle of incidence (for example, less
than 60.degree. from normal) of this light ray is such that it is
reflected back toward the diffuser at point C. At point C, 40% of
the light ray is reflected back toward the collimating film 182.
This type of reflection is highly efficient compared to light that
re-enters the lamp cavity. Light which enters the light cavity must
cross two diffuser/air interfaces (thus losing light) and some may
be absorbed or scattered by the lamps.
Consider now another light ray reflected from point C, and is
directed toward point D. This light ray has a favorable angle of
incidence (for example, 60-85.degree.) and is sent forward to the
next collimating film 184 (FIG. 5) and eventually the LCD tile 194
(FIG. 5). Some of the reflected light from point C is sent to the
lower diffuser 130 at point E. Some of this light will end up in
the lower display and some will be reflected from the lower
diffuser 130 and be sent toward point F and some of this light will
make it to point G on the collimating film 182 and be sent forward
to the upper display. As can be seen, light rays will continue to
be reflected between the elements 130 and 182.
The nature of the efficient coupling of reflected light between the
collimating film 182 and the adjacent diffuser 130 improves the
forward gain of the collimated light output. The key to the
collimation efficiency is the highly efficient, but relatively low
transmission diffuser.
A good approximation of the total light output of the back light
assembly, without considering collimation and related light
re-circulation, can be obtained by considering the geometry. A lamp
tube 128 produces light rays substantially uniformly over 360
degrees. The light exits forward toward a first display, is
absorbed by neighboring lamps or exits rearward and hits the
alternative display. The light reflecting off one display either
exits through the lamp array and into the second display or is
absorbed into the array of fluorescent lamps.
The light absorbed by a neighboring lamp can be expressed by the
angle of light rays leaving the lamp:
.PHI..function. ##EQU00002##
The space S is given by the number of lamps N housed in the width W
of the back light cavity, and is:
##EQU00003##
The light exiting forward is given by its angle:
.phi..sub.forward=180-2.phi..sub.1
The light exiting rearward is the same as that exiting forward;
thus, the total light exiting from the back light assembly is:
.times..times..PHI..times..PHI. ##EQU00004## where [1] l is the
total light output of one lamp. The results are plotted in FIG.
4.
Since the power consumed by each lamp 128 is constant, efficiency
is related to light output and the number of lamps. The curve 170
is nearly linear until the number of lamps approaches one-half of
the maximum that can be installed in the allotted space. It is
desirable then to choose a light output design point near this
inflection point. Thus, an optimum number of lamps 168 are shown in
FIG. 4.
Referring now to FIG. 5, there is shown a schematic,
cross-sectional view 180 of the inventive back light assembly with
back-to-back displays. Many optical components typically used in
both single and back-to-back configurations are shown.
Light collimating optics 132 consist of crossed BEFs 182 and 184
and a collimator 186. The diffusers and collimating optics 132 are
sandwiched between glass plates 188 and 190. These plates 188 and
190 may be optically clear, with enough stiffness to support the
film optics over the expanse needed. Flat-panel displays 122 are
placed in front of the optics assemblies 192 and separated by a
distance F, leaving air spaces 194. These air spaces 194 are vented
to ambient air to allow for further cooling of the displays
122.
As was previously stated, the collimating optics use BEFs which
accept light at high angles of incidence and send light at near
normal angles of incidence back towards the back light assembly for
recycling. It is desirable to have as much reflective area
available as possible for the BEFs. However, more lamps produce
more light output. The first pass design choice for lamp spacing S
is increased slightly. It has been found that increasing lamp
spacing such that the number of lamps is reduced by approximately
10% provides satisfactory results. The coupling of light into the
BEFs 182 and 184 is also affected by the distance B that they are
placed from the lamps 128.
The luminance output of the BEFs increases with proximity to the
lamps, but luminance uniformity decreases with proximity to the
lamps. For practical purposes, a reasonable space H 146 is required
between the lamps 128 and the glass optics holder for air flow to
cool the cavity 126 (FIG. 2a).
The preferred diffuser 130 is a high efficiency, low transmission
diffuser which is chosen to have a near Lambertian distribution in
order to couple a maximum amount of light into the BEFs 182 and 184
and to permit a maximum amount of recycling in the back light
cavity 126. The diffuser 130 must efficiently reflect light, it
must have high transmission efficiency, and it must produce a
Lambertian distribution of light. Additionally, the lamps are not
100% absorbing. Consequently, fine tuning is necessary in the
design parameters of lamp spacing, back plane space, and BEF
spacing to the lamps.
The collimators 186, also described in detail in the aforementioned
U.S. Pat. No. 5,903,328, consist of open hexagonal cells in a honey
comb configuration, coated with a highly light-absorbing paint. The
aspect ratio of cell width to cell depth determines the cut-off
angle or collimation angle.
The use of a sharp cut-off collimator is preferred in a seamless,
tiled, flat-panel display. Non-tiled, large monolithic or
monolithic-like displays do not require cut-off angles as sharp as
those for tiled displays. A more efficient collimator design which
may be applied is disclosed in United States Provisional Patent
Application Serial No. 60/177,447. Unfortunately, collimators,
having a physical structure, create a shadow image which can be
seen on the display. To prevent imaging of the collimator, the
display is placed a predetermined distance F away so that cell
images overlap, or are defocused, and therefore are not visible to
the viewer.
FIG. 6 depicts the degree of collimation or angular distribution of
light emitted from each of the optical components. The diffuser 130
emits a Lambertian distribution 200, as stated hereinabove. The
BEFs 182, 184 focus light forward in a distribution 202 that has a
theoretical forward gain of 2.2 for the type used herein. Actual
achieved forward gain is about 1.9. The BEF distribution 202 has a
significant amount of light energy remaining beyond the cut-off
angle (.about.301 in the preferred embodiment) that is undesirable
for use with seamless, tiled, flat-panel displays.
The collimator 186 eliminates such unwanted light by cutting off
light beyond the collimation angle, as shown by its emission
distribution 204. The surface absorption of the collimator cell
must be sufficient to prevent luminance of more than 1% of normal
luminance beyond the collimation angle.
Brightness levels far exceeding existing industry capability have
been achieved with the inventive design. Luminance values exceeding
100,000 nits (candelas/square meter) have been reached. Reasonable
designs with exceptional efficiency have been prototyped with
luminance output exceeding 50,000 nits, a uniformity of luminance
of 10% at an efficiency better than any currently available
commercial back light unit, even those achieving lower brightness
levels.
Since other modifications such as in optical configurations can be
made to fit particular operating specifications and requirements,
it will be apparent to those skilled in the art that the invention
is not considered limited to the examples chosen for purposes of
disclosure, and covers all changes and modifications which do not
constitute departures from the true spirit and scope of this
invention.
Having thus described the invention, what is desired to be
protected by Letters Patent is presented in the subsequently
appended claims.
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