Note: Descriptions are shown in the official language in which they were submitted.
CA 02239477 1998-07-23
LIQUID CRYSTAL DISPLAY
The present invention relates to a liquid crystal
display and, more particularly, to a lighting device using a
prism lens for converging back light rays and for use in a
back lighting device for liquid crystal displays, and a
transmission type liquid crystal display for use in a
portable type personal computer, word processor and liquid
crystal television set and so on, and a back lighting device
suitable for use in a transmission type liquid crystal
display such as a portable type personal computer, a word
processor, a liquid crystal television set etc., and more
particularly, to a back lighting device using a light guiding
plate. This application is a divisional of Application
Serial No. 2,097,109, filed May 27, 1993.
Liquid crystal displays (LCD) do not radiate light and
are required to be illuminated when they are being used.
Accordingly, liquid crystal displays are each provided with
a back light for ensuring the necessary luminosity. A back
light is composed of a light source and a light diffusing
element. Lighting from a light source such as an
incandescent lamp (point-light source), a fluorescent lamp
(line-light source) and the like is converted from a light
diffusing element to surface light for illuminating a liquid
crystal display from the back. There are many kinds of
proposed liquid crystal displays provided with back lights.
For example, the publication of the unexamined patent
CA 02239477 1998-07-23
application JP,A,2-77726 discloses a liquid crystal display
which has an aspherical condenser lens placed between a
point light source and a liquid crystal panel and has an
aspherical Fresnel lens.
The publication of the unexamined patent application
JP,A,61-15104 discloses a liquid crystal display which has a
light diffuser composed of a plurality of photoconductors
disposed stepwise between a line light source and a
diffusing plate having triangle-pole prisms disposed
thereon.
The publication of the unexamined utility model
application JP,U,2-62417 discloses a liquid crystal display
which has a transparent plate between the light source and
the diffusing plate and the transparent plate has a
plurality of grooves placed thereon to function as a prism
opposite the light source.
As one of the means to increase the luminosity of the
display light a method is proposed for increasing the
luminosity of the display light in a specified direction by
converging diffused light of the back lighting device by the
use of a prism lens.
As mentioned above, the application of a conventional
prism lens may increase the luminosity of a li~uid crystal
~LC) display in a specified direction but it produces a view
angle with no quantity of light. This may impair the
quality of the image on the LC display. Accordingly, a
diffusing sheet is usually used between a prism lens and a
liquid crystal ~LC) panel. This means that light rays are
CA 02239477 1998-07-23
c~nverged and diffused aga1n. Therefore the intended effect
of the use of the prism lens cannot be attained. It is
possible to omit the diffusing sheet by increasing the
vertex angle of a prism lens to the level at which the
effect of a non-lighting quantity angle may be negligible.
However, this may have little effect on the prism lens since
the prism has an increased vertex angle that has decreased
the converging power.
The conventional liquid crystal display produces such a
proble~ that external light may ~e reflected by front and
rear surfaces of a transparent protection plate, a front
surface of and in a liquid crystal panel, thereby its image
visibility is impaired.
~ he conventional liquid crystal displays have a limit
in achieving reduction of their thickness and, at the same
time, the increase of the brightness of their display
screens. For instance, it is difficult to reduce the
thickness of the liquid crystal display using a direct
bottom-reflecting type back light. An attempt to increase
luminance of the lamp by increasing current is accompanied
by heating of the lamp, which leads to the damaging of the
IC chips for the horizontal and vertica~ drivers disposed at
the periphery of the display screen. It was difficult to
satisfy both above-mentioned requirements at the same time.
One of the disadvantages of the conventional
light-guiding type bac~ lighting device is that separate
components may expand to create uneven spacing therebetween
or be displaced during the assembly thereby causing uneven
CA 02239477 1998-07-23
luminosity ~y the back light rays during the operation of
the devices. In particular, in the case of assembling very
thin components, e.g., 0.2mm thick converging, diffusing and
reflecting plates, extreme care must be taken to prevent the
expansion and displacement and also to avoid the erroneous
positioning of the components and of damage from dirt on
their surfaces that may cause dark and luminescent spots on
an image on the display screen of the li~uid crystal panel.
In addition, any conventional back light device may leak
light rays through a gap between its lamp and light guiding
plate and the device and its holder that causes the weaker
luminosity of an image on the liquid crystal panel.
The other disadvantage of the prior art is as follows:
liquid crystal panels have a view angle characteristic that
is particularly narrow in its vertical direction.
Therefore, even if back light rays are applied uniformly at
all visual angles to a liquid crystal display panel, the
picture image quality of the display can be scarcely
improved because of the very narrow viewing angle of the
panel in a vertical direction.
The present invention aims to achieve an increase in
luminosity and to decrease the reflection of a li~uid
crystal display provided with an edge, light-type back
lighting unit.
Technical subjects are tl) increasing the luminosity of
a back lighting unit and (2) reducing the reflection of a
liquid crystal panel.
Regarding the subject (1), the present invention
CA 02239477 1998-07-23
proposes a back light that comprises a lamp (light source),
a light guiding p~ate, a diffusing plate and a converging
plate. On the other hand, Japanese publications of
unexamined application JP,A,61-15104 and JP,U,2-62417
disclose the relevant prior art. The former prior art is
similar to the present invention in the application of an
edge light-type back lighting unit, but said unit is
composed of a light source, a light-guiding plate and a
double-layered diffusing plate to eliminate the unevenness
of the light for illuminating a liquid crystal panel. The
latter prior art used a direct bottom-reflecting type back
light which was composed of a light source, a converging
plate and a diffusing plate through which light rays
illuminated the liquid crystal panel while the present back
lighting device, according to the present invention, is to
illuminate the liquid crystal panel with back light rays
through the use o~ a converging plate.
Essentially, the former prior art was intended to
prevent uneven luminosity within the display screen only and
the latter prior art was intended to increase (to the
m~Ximum) the luminosity of a specified position within the
display screen not taking into account the uneven luminosity
therein. The present invention proposes to increase the
front axial luminosity of the display screen and at the same
time to prevent the uneven luminosity thereon as much as
possi~le.
Regarding the subject (2), the present invention
proposes that a liquid crystal panel and a transparent
CA 02239477 1998-07-23
protection plate having an AR coat (reflecting coat) applied
on its front surface are stuck together with adhesive.
The publication of unexamined patent application,
JP,A,3-18~420 discloses a device which applies a combination
of a circular light-deflecting disc and a 1/4-phasing
substrate to suppress light reflection in and at surface of
a panel and at the rear surface of a protection plate. This
combination, however, requires provision of an air gap
between the panel and the transparent protection plate. The
present invention aims to eliminate the light reflection by
surface of the liquid crystal panel and by the rear surface
of the transparent protection plate by sticking the
protection plate and the panel together with adhesive.
It is an object of the present invention to provide a
converging unit which is capable of effectively converging
light rays by using a prism lens without the use of a
diffusing sheet and which is suitable for use in a back
lighting device, making it possible to manufacture the back
lighting unit in a thinner size and at a lower cost.
It is another object of the present invention to
provide a liquid crystal display which is capable of having
an increased luminosity of its display screen with a reduced
thickness of its whole system.
It is another object of the present invention to
provide a liquid crystal display with a back light, which
CA 02239477 1998-07-23
has an improved visibility and an increased front axial
luminosity of the display screen by effectively using
diffused light rays in diagonal direction from the back
light through a converging system having a directional
characteristic with a peak luminosity in the front direction
and substantially corresponding to that of the liquid
crystal panel and by suppressing the reflection of external
light by the liquid crystal panel.
To achieve the above-mentioned objects, the present
invention proposes that a liquid crystal panel comprising at
least a sheet of color filter glass and a light deflecting
plate is covered with a transparent protection plate bonded
thereon to protect the front surface of the panel and form
the anti-reflection layer for suppressing the reflection of
external light by the surface thereof.
It is possible to provide a prism lens unit which is
capable of effectively converging back light for a liquid
crystal display by eliminating a non-lighting ~uantity angle
and by giving variety to vertex angles of the prism portions
with no diffusing sheet which was usually applied in the
prior art. Application of this prism lens therefore enables
a back lighting device to be thinner in size and adapted for
low cost manufacturing.
A li~uid crystal display, according to the present
invention, can be made thinner in size since a fluorescent
lamp is disposed at the side surface of the back lighting
unit. IC chips for drivers are disposed on a liquid crystal
panel so far from a lamp unit that they may not damage, with
CA 02239477 1998-07-23
increased luminosity, the display screen. A lamp unit can be
removably secured to a liquid crystal display holder and
therefore it can be easily replaced with a new one.
A back lighting device, according to the present
invention, wherein between a liquid crystal panel and a
diffusing plate is disposed a converging plate having a large
number of prism-like grooves arranged at intervals
sufficiently narrower than the spacing of picture elements in
the vertical direction of a liquid crystal panel, can
converge upward reflecting and downward reflecting light rays
in an axial direction to increase front luminosity on the
display screens thereby effectively using the back light
rays.
Accordingly, the present invention provides a liquid
crystal display comprising a display means for displaying
characters and graphics, said means including a display
portion comprising at least a liquid crystal panel; an
illuminating means for illuminating the display portion of
the liquid crystal panel, said illuminating means comprising
a lamp unit integrally formed of at least a lamp and a lamp
reflector for reflecting light rays from the lamp in one
direction, and a back light unit integrally formed of a
light-guiding plate for receiving therein light rays of the
lamp through a lateral surface thereof and for uniformly
propagating and diffusing the light rays therein, and a
reflecting plate disposed on the rear surface of the light-
guiding plate to reflect light rays from the rear surface of
the light-guiding plate; and a diffusing plate disposed on
CA 02239477 1998-07-23
the front surface of the light-guiding plate to diffuse light
rays from the front surface of the light-guiding plate; a
converging means for converging light rays from the
illuminating means at the display portion of the liquid
crystal panel; and a liquid crystal display holder for
holding the lamp unit, the back light unit and the liquid
crystal panel in definite positions, wherein, an integrated-
circuit chip for a horizontal driver and an integrated-
circuit chip for a vertical driver are mounted each on one of
two adjacent peripheral edges of the liquid crystal panel and
the lamp unit is mounted on one of two remaining peripheral
edges of the liquid crystal panel, whereon no IC chip is
mounted.
According to the present invention, it is possible to
provide a light-guiding type back lighting unit comprising a
light guiding plate, a lower reflecting plate, a diffusing
plate, a converging plate having a large number of prism-like
grooves arranged at intervals sufficiently close in relation
to the vertical picture elements of the liquid crystal
display screen, wherein all above-mentioned components are
each welded at a portion of the periphery to form an
integrated back lighting unit with elements of high
reflecting power covering the side surfaces of the light
guiding plate, internal surfaces of a back light's holder
and the protrusion of the diffusing plate. A thus
constructed back light unit is free from uneven luminosity
due to the displacement of the components during the unit
assembly and has no leakage of light rays thereby ensuring
- 8a -
CA 02239477 1998-07-23
the highly effective use of the back light's rays.
Embodiments of the invention will now be described with
reference to the accompanying drawings, in which:
Fig.1 is a view for explaining the converging action of
a prism lens of the prior art.
Fig.2 is a view for explaining the law of light
refraction of a prism lens from the prior art.
Fig.3 is a view for explaining the critical reflecting
angle of a light for a prism lens from the prior art.
Fig.4 is a view for explaining the refraction state of
a prism lens of the prior art.
Fig.5 is a view for explaining another refraction state
of a prism lens of the prior art.
Fig.6 is a typical conventional liquid crystal display
of a transmission back-lighted type.
Fig.7A is a front view of an example of a conventional
liquid crystal display.
Fig.7B is a sectional view along line VIIB-VIIB of
Fig.7A.
Fig.8A is a front view of another example of a
conventional liquid crystal display.
Flg.8B is a sectional view along line VI~IB-VIIIB of
Fig.8A.
Fig.g is a schematic view of a conventional back
lighting device of a light-guiding type.
Fig.10 is a schematic view of a conventional back
lighting device of a direct bottom reflecting type.
~ g _
CA 02239477 1998-07-23
Fig.llA shows a prism lens embodied in the present
invention.
Fig.llB shows the visual characteristic of the prism
lens shown in Fig.llA.
Fig.12 shows another embodiment of the prism lens,
according to the present invention, which is capable of
adiusting the visual characteristic by means of a surface
area of a flat portion and a quantity of prism portions.
Fig.13A shows another em~odiment of a prism lens
according to the present invention.
Fig.13B shows a visual characteristic by means of a
vertex angle, the surface area of a flat portion and a
quantity of prism portions.
Fig.13C shows another embodiment of the prism lens.
Fig.13D shows another embodiment of the prism lens.
Fig.14A shows still another embodiment of a prism lens
according to the present invention.
Fig.14B shows a prism lens, which is capable of marking
by means of a non-lighting quantity angle of a prism.
Fig.15A shows another example of a prism lens of
Fig.14A.
Fig.15B shows a display screen at a viewing angle of
36.5~ shown in Fig.15A.
Fig.15C shows a display screen at a viewing angle of
38.6~ shown in Fig.15A.
Fig.16 shows a liquid crystal display embodying the
present invention.
Fig.17 is an enlarged view of a portion A of Fig.16.
-- 10 --
CA 02239477 1998-07-23
Fig.18A is a front view of a liquid crystal display
embodied in the present invention.
Fig.18B is a sectional view along line XVIIIB-XVIIIB of
Fig.18A.
Fig.19A is a front view of a lamp unit used in the
embodiment of liquid crystal display.
Fig.19B is a sectional view along line XIXB-XIXB of
Fig.19A.
Fig.20 is a view for explaining the construction of a
back lighting device embodied in the present invention.
Fig.2~ is an enlarged section of a portion A of Fig.20.
Fig.22 is a view of another embodiment of a back
lighting device according to the present invention.
Fig.23A is a view of still another embodiment of a bac~
lighting device according to the present invention.
Fig.23B is a sectional view along line XXIIIB-XXIIIB of
Fig.23A.
Fig.24 is an enlarged section of a portion B of
Fig.23B.
Fig.25A is a view of another embodiment of a back
lighting device according to the present invention.
Fig.25B is a view of another embodiment of a back
lighting device shown in Fig.25A.
Fig.26 shows a view angle versus the luminosity of a
back lighting device according to the present invention.
CA 02239477 1998-07-23
As one of the means to increase the luminosity of the
display light, a method is proposed for increasing the
luminosity of the display light in a specified direction by
converging diffused light of the back lighting device by the
use of a prism lens. The mechanism of a prism lens
according to the prior art will be described in detail with
reference to the accompanying drawings.
Figs.lA,lB are views for explaining the converging
function of a prism lens: Fig.lA is a general construction
view of a liquid crystal display and Fig.lB shows a view
angle characteristic of a prism lens, wherein A and B
represent portions of the light's quantity distribution. A
sheet 1 comprises small assembled pieces of prism lenses.
Number 2 designates a liquid crystal panel and number 3
designates a back light composed of a light guiding plate, a
reflecting plate, a diffusing plate and a light source, e.g.
a fluorescent tube, electroluminescence or daylight and
number 4 designates a diffusing sheet.
Fig.2 shows the law of light refraction. When a light
beam enters at an incident angle e1 into a medium having a
refractive index n, the light beam refracts at 00 as
expressed by the following formula:
eO = sin 1 ((sin 01)/n) ... (1)
When the refracted beam passes from the medium to the
air, it refracts again at an angle e expressed by the
following formula:
= sin 1 (n x sin ~0)
= sin~1 (n x sin (sin~1 (sin e1)/n))
- 12 -
CA 02239477 1998-07-23
= el ... (2)
The formula ( 2) shows that an incident angle of light
is e~ual to the departure angle if the incident plane and
departing plane are parallel to each other.
Fig.3 shows a critical reflecting angle of light. A
light ray enters a medium at an incident angle of 9O~ and
refracts at an angle elimit expressed by the following
formula. Therefore, if an incident angle of a light beam
exceeds this angle, no refraction can occur and all the
light energy is reflected by the interface. This angle is
called a critical reflection angle.
limit = Sin ((sin 90~)/n)
= sin~1 (1/n) ... (3)
Fig.4 shows the refractive state of a prism lens. A
light beam enters a medium at an incident angle of ai and
refracts at a refraction angle of e1 expressed by the
formula (1). If a vertex angle of a prism lens is denoted
by 0p the refracted light beam encounters the boundary of
the prism lens at the angle e2 expressed by the following
formula.
02 = 90~ - e1 - ~p/2 ... (4)
The light beam departs from the prism lens at an angle
e3 (the angle made by the ray to an imaginary line
perpendicular to the prism's surface) expressed by the
following formula:
~3 = sin 1 (n x sin ~2) ... (5)
Finally, the light beam appears as refracted at an
angle ~O expressed by the formula:
- 13 -
CA 02239477 1998-07-23
~o = 90~ - ~p/2 - e3
= a - sin 1 (n ~ sin (a-sin 1(sin ~1)/n))
(a = 90~ - ~p/2) ... (6)
This departing light beam corresponds to portion A of
the quantity of light distributed as shown in Fig.1.
When an incident angle e1 of the light beam
substantially equals 90~, its departure angle may have the
following expression:
eO = a - sin 1 (n-sin(a - sin 1 (1/n))) ... (7)
If the departure angle of a light beam exceeds the
above-mentioned value, no light appears and brightness is
sharply reduced. This critical angle will be called
no-light angle.
Fig.5 shows another refraction state of a prism lens.
A part of a light beam appears from the prism lens after
reflecting therein. A light beam enters into the prism at
an incident angle ei and becomes a refracted beam bent at
the angle e1 to be determined by the formula (1). When the
vertex angle of the prism is denoted by ~p, the refracted
light beam encountering the boundary of the prism lens at
the angle e2 is expressed by the following formula:
= ~pl2 - ~
( 90~ ~ e1 = ~2 + 90~ - ep/2) ... (8)
If e2 is smaller than the critical reflecting angle
determined by the formula (2), the light beam totally
reflects at the boundary surface of the prism lens. This
refracted beam encounters the boundary of the prism lens at
the angle e3 expressed by the following formula:
- 14 -
CA 02239477 1998-07-23
3 ~2 ~ ~p ~ 90~
( e3 = 180~ - 90~ - (180~-ep-~2)) ... ~9)
This light beam passes through the prism lens at an
angle ~4 (the angle made by the ray to an imaginary line
perpendicular to the prism's surface) expressed by the
formula:
9~ = sin 1 (n x sin e3) ... (10)
Finally, the light beam appears refracted at an angle
expressed by the following formula:
eO = 90~ ~ ep/2 - ~4
= a + sin 1 (n sin(ep-a-sin (sin el)/n))
(a = 90~ - ~3p/2) . . . ( 11)
This departing light beam corresponds to portion B of
the quantity of light distributed as shown in Fig.lB.
Fig.6 is a typical conventional liquid crystal display
of transmission back-lighted type that comprises a liquid
crystal panel 5 including a color filter glass 5a, a
deflection plate 5b, a black matrix 5c and so on, for
obtaining an image tr~n.cmission light from a white light
emitted from a back light (not shown), and a front
transparent plate 7 made of acrylic resin and the like and
disposed with an air-gap layer 6 on the front side of the
liquid crystal pane~ (opposite to the side facing to the
back light). The front transparent plate 7 serves to
protect the liquid crystal panel 5 against direct
application of mechanical force and contamination by dusts
and dirt.
In thus constructed liquid crystal display, white light
CA 02239477 1998-07-23
from a back light tnot shown) enters the liquid crystal
panel 5 and passes therethrough to form an image on a
display screen by light rays transmitted through the air-gap
layer 6 and the front transparent plate 7.
On the other hand, one of the factors affecting the
visibility of the liquid crystal display is reflection of
the external light at the front surface of the liquid
crystal display. Generally, the reflectance of light at an
interface between two media having different refractive
indices has the following expression:
Light Reflectance R=~n1-n2) /(n1+n2) ] x 100 (%) where
n1 and n2 are refractive indices.
Reflected light rays are produced thereat at a the
above-mentioned reflectance.
The reflection of external light is descri~ed further
in detail as follows:
When the external light encounters a front (incident)
surface 7a of the front transparent plate 7 that is assumed
to have a refractive index 1.5% and a transmittance 92%,
about 4.3% of the incident light is reflected by the surface
7a and about 4.1% of the incident light is reflected by the
rear surface 7b of the front transparent plate 7. When the
incident light passing through the front transparent plate 7
strikes a front surface 5b1 of the deflecting plate 5b that
is assumed to have a refractive index 1.49% and a
transmittance 41%, about 3.3% of the incident light is
reflected by the surface 5b1. When the incident light
passing through the deflecting plate 5~ encounters a surface
- 16 -
CA 02239477 1998-07-23
5c1 of the black matrix 5c on the color filter glass 5a on
the assumption that a reflectance of the black matrix is
30%, an aperture ratio of picture elements is 4~% and a
transmittance of the color filter glass 5a is 95~, about
11.6% of the incident light is reflected by the surface 5c1
of the black matrix 5c. The reflection of the external
light at a front surface 5a1 (the boundary on the deflecting
plate 5b) of the color filter glass 5a is negligible since
the refractive index of color filter glass 5a is nearly
equal to that of the deflecting plate 5b.
As mentioned above, the conventional liquid crystal
display reflects totally about 23.3% of external incident
light from its front surface, that may impair the visibility
of an image indicated on the display screen. Particularly,
under a plenty of external light, the reflected light
becomes brighter than the picture image of the liquid
crystal display, that may remar~ably affect the display
image contrast, i.e. impairing its visibility.
Accordingly, to improve the visibility of the li~uid
crystal display by reducing reflected light rays at its
front surface it has been proposed and adopted to apply
antireflection coating to the front transparent plate of the
liquid crystal display.
The prevention of light reflection by this thin film
having an anti-reflection coating is based on that the
anti-reflection coating may cause an incident light beam
having a certain wavelength entered thereinto to be reverse
in phase to a light beam reflected by rear surface thereof
- 17 -
CA 02239477 1998-07-23
to cancel each other. This eliminates the reflection of the
light toward the front surface thereof.
Increasing the quantity of anti-reflection coatings may
suppress reflection of light beams of all wavelengths.
Conventional transmission-type liquid crystal displays
for use in liquid crystal television sets have used mainly a
direct reflecting type back lighting units shown in Figs.7A,
7B or Figs.8A,8B.
Figs.7A and 7B show a back lighting unit using a
U-shaped fluorescent lamp, wherein IC chips 14a and 14b for
a horizontal driver are mounted by TAB (tape automated
bonding) on the upper edge and the lower edge, respectively,
of a glass plate 12, and IC chips 15a and l5b for a vertical
driver are mounted by TAB on the left edge of the glass
plate 12. The IC chips 14a, 14b and 15a, 15~ are connected
to a drivers' substrate 16 by means of a member enclosing a
liquid crystal display holder 17. A lamp unit 13 composed
of a U-shaped fluorescent lamp 18, a reflecting plate 19, a
diffusing plate 20 and an inverter unit 21 is inserted into
the liquid crystal holder 17 from the right side thereof.
The inverter unit 21 includes a lamp driving circuit and
electrically connected to the U-shaped fluorescent lamp 18.
~he fluorescent lamp 18 can be exchanged with a new one
after taking out the lamp unit 13 from the holder 17 in the
direction shown by the arrows A.
In a thus constructed liquid crystal display, the
driver's TAB package is bendable to reduce the size of the
screen's frame, but it is difficult to make the U-shaped
- 18 -
CA 02239477 1998-07-23
lamp 18 smaller in diameter, thereby the lamp unit 13 must
be thick in size, increasing the total thickness of the
liquid crystal display.
Figs.8A and 8B show a liquid crystal display which uses
a straight tube type fluorescent lamp 18 and is similar in
construction to the liquid crystal display of Figs.7A and
7B. In this case, IC chips 14a, 14b for a horizontal
driver, IC chips l5a, 15b for a vertical driver and a
drivers' substrate 16 are mounted on the same plane with a
li~uid crystal panel 11 to reduce the thickness of the
liquid crystal display. The straight tube-type fluorescent
lamp 18 may have a reduced diameter. This enables the lamp
unit 13 to be thinner thereby assuring the possibility of
reducing the thickness of the liquid crystal display.
Fig's.9 and 10 are construction views of conventional
transmission type liquid crystal displays which are provided
with, respectively, a back lighting device of a
light-guiding type ~Fig.9) and a back lighting device
causing bottom reflection (Fig.10). In Figs.9 and 10, there
are shown a liquid crystal panel 31, a reflector 32, a
diffusing plate 33, a light guiding plate 34 having a
lateral inlet surface 34a, a front surface 34b and a bottom
surface 34c, a reflecting plate 35, a fluorescent lamp 36
and a reflecting plate 37.
In Fig.9, there is illustrated the back lighting device
of a light-guiding type wherein light rays from the
fluorescent lamp 36 enter directly or, after being reflected
at the reflector 3~, into the light guiding plate 34,
-- 19 --
CA 02239477 1998-07-23
through the lateral inlet surface 34a, then repeatedly
reflected at the bottom surface 34c and the front surface
34b thereof and then are emitted as homogeneous back
lighting rays directed upward from the front surface
thereof. At the back of the light guiding plate 34 is
placed the reflecting plate 35 by which light rays, leaking
from the bottom surface of the light-guiding plate 34, are
reflected and returned into the light-guiding plate 34,
thereby the light rays can be effectively used. Light rays
emitted from the light-guiding plate 34 are dif used in the
diffusing plate 33 into homogeneous diffused light having
limited directionality and which illuminates the liquid
crystal panel 31.
Referring to Fig.26, in the case of such a construction
having a lamp arranged at the upper portion of the liquid
crystal display, light rays from the light-guiding plate 34
have directionality A with peak lighting obt~;n~hle when
observed from a lower position, i.e., the directionality
does not correspond to the view angle D of the liquid
crystal panel. This means that the maximum lighting on the
display cannot be obtained when observing the display screen
from the front.
Therefore, the light-guiding plate 34 is generally
placed at the front surface with the diffusing plate 33 to
obtain homogeneous light rays of a relatively small
directionality B' as shown in Fig.2~. Another similar
example of the prior art is a back lighting device which has
a converging plate placed at the front surface of the light-
- 20 -
CA 02239477 1998-07-23
guiding plate 34 and has a diffusing plate 33 placed on the
converging plate. However, such modifications cannot
produce the intended effect because the back light rays are
converged but then diffused again.
All the components and a holder for the back lighting
device are provided with ribs and assem~led on the holder
through the use of ribs. However, since the components are
thin and not attached to each other, they may become
displaced in the course of assembling thereby causing uneven
luminosity by the back light rays.
Another typical example of the prior art is a direct
bottom-reflecting type back lighting device which may employ
a variety of straight tubular, U-shaped and W-shaped lamps.
The device shown in Fig.10 includes a straight tubular type
fluorescent lamp. Back light rays from the lamp 36 are
injected directly or, after reflection by a reflecting plate
37, into a diffusing plate 33 through which homogeneously
diffused light rays are irradiated to a liquid crystal
display panel 31.
Referring now to the accompanying drawings, preferred
embodiments of the present invention will be described in
detail as follows:
Figs.llA and llB are views, respectively, showing a
refraction characteristic Fig.llA and a view angle
characteristic Fig.llB of a prism lens embodied in the
present invention. In Fig.llA, numerals 41 and 42 designate
a prism portion and a flat portion, respectively, of a prism
lens and numeral 43 designates the prism lens itself. In
CA 02239477 1998-07-23
Fig.llB, represents a portion of a view angle characteristic
of a prism portion and B represents a portion of a view
angle characteristic of a flat portion and C represent a
portion of a view angle characteristic of the whole system.
In the shown case, a flat portion 42 is provided on the top
of a prism portion 41, however, the same effect can be
achieved by providing a flat portion 42 between the prism
portions 41 or on the top of the prism portions and between
the prism portions. When the vertex angle of a prism is
expressed as ~p in Fig.llA, a non-lighting quantity angle of
the prism lens may have the following expression:
= a - sin 1 (n-sin 1 (a-sin l(l/n))
(where a = 90~ - ~pl2)
Since no light appears at angles greater than this
angle, a single prism lens sharply loses lllm;n~nce.
However, according to the present invention, as described in
relation to the expression (2), the flat portion at which an
incident angle of a light beam equal to its departure angle
and ~0 is smaller than 90~ so therefore the light beam
appears at the angle ~0. The intensity of the light beam,
directed in the direction of angle ~0, can be adjusted by
means of a duty ratio of the flat portioned area to the
prism lens area.
Referring to Fig.12, a plurality of prism portions 41
are arranged with flat portions 42 each between two prism
portions, wherein the view angle characteristic of the prism
lens unit can be adjusted by means of the flat area of the
portions and the number of prism portions.
CA 02239477 1998-07-23
For example, in case of the prism lens having a vertex
angle ~p=90~ of a prism and refraction coefficient 1.585 of
material (polycarbonate resin) the non-lighting quantity
angle is 35.6~ degrees according to the following formula:
= a - sin 1 (n-sin~a-sin 1(1/n)))
= ~5~ - sin 1 (1.585-sin(45~-sin ~ 1.585)))
. 35.6~
Since eO>gO~' the departure of the light beam occurs in
the angular direction eO thanks to the provision of the flat
portions.
Figs.13A to 13D are construction views of another
embodiment of the prism lens according to the present
invention. In Figs.13A to 13D, numerals 41 and 43 designate
a prism portion and a prism lens. In Fig.13B, ~ represents
a portion of a view angle characteristic of a prism P1 and B
represents a portion of a view angle characteristic of a
prism P2 and C represents a portion of a view angle
characgeristic of the whole system. The design of a prism
lens unit may be determined by the view angle
characteristics and the necessary visual field angles of a
liquid crystal display and a back lighting system. When the
prism portions of the prism lens unit of Fig.13A have vertex
angles ep1 and ep2 respectively, the non-lighting quantity
angle is defined according to the expression 16):
eO1(2) = a ~ sin 1 (n sin(a-sin 1(1/n)))
(where a = ~0~ - ~pl(2)l2)
Since no light appears at angles greater than the
above-mentioned angle, a single prism lens sharply loses its
CA 02239477 1998-07-23
luminosity. However, according to the present invention,
the provision of prism lenses having different vertex angles
enables the light beam to pass at the direction of angle
eOl(2). The intensity of the light beam directed in the
angular direction ~01(2) can be adjusted by means of a duty
ratio of the vertex angles and the area of the prism
portions. Fig.13C shows a lens unit having different vertex
angles at each prism portion. Referring to Fig.13D, a
plurality of prism portions are arranged with the prism
portions each having a different vertex angle between two
prism portions wherein the view angle, characteristic of the
prism lens unit, can be adiusted by means of the prism
portions' area and the number of prism portions.
For example, in the case of the prism lens having the
vertex angle of 70~ ~pl) and lOQ~ (ep2) of a prism and a
refraction coefficient of 1.585, the non-lighting ~uantity
angles are calculated as follows:
eOl = 33~ - sin 1 (1.585-sin(33~-sin 1(1/1.585)))
_.29.30
~02 = 40~ - sin 1 (1.585 sin(40~-sin 1(1/1.585)))
38.6~
Since the non-lighting quantity angles are shifted from
each other by 10~, the combination of the two prisms, having
different vertex angles, may generally remove the
non-lighting portion.
As described above in the embodiments shown in
Figs.llA,llB and Fig.13A to 13D, it is possible to eliminate
the non-lighting quantity angles by adjusting the vertex
- 24 -
CA 02239477 1998-07-23
angles of the prism portions. This means the further
possibility to see completely even an image on an LC display
when viewing it from the front of the display but you may
have any character or figure relieved against the background
on the screen of the display when changing the viewing angle
to a specified direction. This is achieved by selectively
e~iminating the non-lighting quantity angle from only a
specified portion of the prism lens unit.
Fig.s 14A and 14B show an example for relieving a
character or symbol against a background on the LC display
screen only at a specified viewing angle by means of
providing a flat portion on a prism sheet. For example, in
the case of the prism lens having a vertex angle of 90~ (~p)
and a material refraction coefficient of 1.585, the
non-lighting quantity angles are calculated as follows:
= 45~-sin 1(1.585 sin 1(45~-sin 1(1/1.585)))
~. 35.60
When viewing the display screen at a viewing angle of
35.6, you may see thereon the symbols ( O ~ X O in Fig.14B)
that can never be seen when viewing them from the front.
This enables you to put any symbol, e.g. mar~ing for quality
control on the display screen with no fear of affecting the
image's quality on the LC display. Furthermore, it is also
possible to obtain such an effect that a specified portion
of the screen's image may be emphasized with increased
luminosity.
Fig.15A to l~C show examples for giving a plurality of
non-lighting quantity angles to a prism lens unit by
CA 02239477 1998-07-23
providing a prism having a different vertex angle on the
part of the prism sheet. For instance, in the case of
Fig.15B which includes a prism having a vertex angle of 70~,
normally invisible symbols ~ ~ ~ ~ become visible only at a
viewing angle of 29.3~ and in the case of Fig.15C having a
vertex angle of 100~ normally invisible symbols O O O O
become visible only at a viewing angle of 3~.6~. In both
cases such visibility is obtained by making the
correspondin~ portions flat.
Fig.16 shows a back-lighted transmission type liquid
crystal display, by means of example, according to the
present invention.
A liquid crystal disp~ay according to the present
invention comprises a liquid crystal panel 44 which includes
a color filter glass 44a, a deflecting plate 44b and a black
matrix 44c and so on for obtaining an image transmitting
light from a white light emitted by a back light (not
shown), and a front transparent plate 45 made of acrylic
resin and the like and closely attached to the front surface
of the liquid crystal panel ~opposite to the side facing to
the back light). The front transparent plate 45 serves to
protect the liquid crystal panel 44 against direct
application of mechanical force and con~min~tion with dusts
and dirt.
In thus constructed liquid crystal display, the front
transparent plate 45 and the deflecting plate 44b of liquid
crystal panel 44 are bonded to each other with adhesive 4~
o~ which the refractive index is nearly equal to the
- 26 -
CA 02239477 1998-07-23
~le-clecting plate 44b and the front transparent plate 45.
Fig.17 is an enlarged view of the portion A of Fig.16,
showing a front transparent panel 43 which has a non-glaring
uneven surface portion 45a for converting reflected light to
scattered and is coated at the uneven surface with a
anti-reflection layer 46 of several angstroms in thickness.
In thus constructed liquid crystal display, white light
emitted by a back light (not shown) enters the liquid
crystal panel 44 and passes therethrough to form an image on
a screen thereof by transmitted light, which is visible
through the front transparent plate 45.
The reflection of external light at the front surface
of the liquid crystal display is as follows:
The external light reflection at a front (incident)
surface 45a of the front transparent plate 45 is reduced to
an amount of about ~.1% of incident light by means of the
antireflection film 46. No reflection occurs at a surface
45a and a rear surface 45b of the front transparent plate 45
because the front transparent plate 45 bonded to the
deflecting plate with the adhesive having nearly e~ual
refractive index that the two plates have. When the
incident light passing through the deflecting plate
encounters a surface 44cl of the black matrix 44c on the
color filter glass 44a on the assumption that reflectance of
the black matrix is 30%, an aperture ratio of picture
elements is 40% and the a transmittance of the color filter
glass is 95%, about 11.6% of the incident light is reflected
by the surface 44c1 of the black matrix 44c. Total
CA 02239477 1998-07-23
reflection of external incident light at the front surface
of the liquid crystal display corresponds to about 11.8% of
the external incident light and this figure means that the
reflection of external light is reduced to 1/2 in comparison
with the previously described conventional liquid crystal
display. The reflection of light at a front surface 44a1
(the boundary on the deflecting plate 44b~ of the color
filter glass 44a is negligible since the refractive index of
color filter 44a is nearly equal to that of the deflecting
plate 44b.
The front transparent plate 45 prevents the reflection
of external incident light by its antireflection film 46 and
assures the improved visibility of display screen even at a
bright place by scattering the reflected external light by
the irregular surface portion on the antireflection
coating.
Fig.18A and 18B are construction views of a liquid crystal
display embodying the present invention, which is comprised
of a thin-film transistor (TFT), a glass plate 52, a li~uid
crystal holder 53, IC chips for a horizontal driver 54a,
54b, IC chips for a vertical driver 55a a driver's substrate
56, a reflecting plate 57, a straight tube fluorescent lamp
58, a lamp reflector 59, a lamp holder 60, a di~fusing plate
61, a converging plate 62, a light-guiding plate 63. In
Figs.18A and 18B, numeral 51 designates a flat liquid
crystal panel which comprises a glass plate 52 filled with
liquid crystal and having a thin-film transistor (TFT)
composing picture elements, a color filter RGB, black
CA 02239477 1998-07-23
matrices between picture elements and ~us lines to each
picture element, and a driver substrate 56 on which IC chips
54b, 54b for a horizontal driver and IC chips 55a, 55b for a
vertical driver are mounted. The IC chips 54a and 54b are
disposed at the lower edge of the glass plate 52 and the IC
chips 55a and 55b are disposed at the left side edge of the
glass plate 52. There IC chips are electrically connected
with the TFT picture elements of the glass plate 52. The
lamp unit contains a straight tu~e-type fluorescent lamp 58
and is disposed on a side surface (the upper surface in
Fig.18B) of a back lighting unit. A lamp holder 60 holds
the straight tube-type fluorescent lamp 58, a lamp reflector
59 enclosing the lamp, as shown in detail in Figs.19A and
l9B. It also includes safety thermal fuses 61a, 61b. Lead
wires of the lamp 58 through the thermal fuses 61a, 61b are
electrically connected to electrode terminals 62a and 62b
provided at the external surface of the lamp holder 60.
The back light unit is composed of a light-guiding
plate 63 for receiving therein light rays from the
fluorescent lamp 58 through the lateral surface thereof and
uniformly propagating and diffusing the light over a whole
display screen, a reflecting plate 57 disposed on the rear
surface of the light-guiding plate 63 so as to reflect the
light therefrom, a diffusing plate 61 for diffusing light
from the front surface of the light-guiding plate 63 and a
converging plate 62 for converging light from the diffusing
plate according to a directional characteristic
substantially corresponding to a view angle characteristic
- 29 -
CA 02239477 1998-07-23
of a liquid crystal panel 51. These components are disposed
all in flat layers on the rear surface of the glass plate
52.
The above-mentioned liquid crystal panel 51, the lamp
unit and the back light unit are held by a liquid crystal
display holder 53.
Light rays emitted by the fluorescent lamp 58 and light
rays reflected by the reflector 59 enter the light-guiding
plate 63 through its lateral surface, wherein they uniformly
propagate and are diffused. The reflecting plate 57
reflects light rays leaking from the bottom surface of the
light-guiding plate 63 and injects them again into the
light-guiding plate 63. Light rays pass through the front
surface of the light-guiding plate 63 and enter into the
converging plate 62 whereby they are further converged in
the direction toward the front surface of the liquid crystal
panel 51. Consequently, the light rays from the converging
plate 62 can have a directional characteristic substantially
corresponding to the view angle characteristic of the liquid
crystal panel, remarkably increasing the front axial
luminosity of the display screen.
The lamp unit integrally holding the fluorescent lamp
58 together with the lamp reflector 59 can be removably
secured to the liquid crystal display holder 60. Therefore,
when the lamp 58 reaches the end of its useful life or is
damaged, anyone can easily replace the entire lamp unit with
a new one.
Fig.20 is a construction view of a light-guiding type
- 30 -
CA 02239477 1998-07-23
back lighting device embodying the present invention, which
is comprised of a liquid crystal panel 71, a converging
plate 72, a diffusing plate 73, a light-guiding plate 74
having a lateral inlet portion 74a, a front surface 74b and
a bottom surface 74c, a reflecting plate 75, a lamp 76 and a
reflector 77.
Light rays from the fluorescent lamp 76 enter, directly
or after reflection by the reflector 77, into the light-
guiding plate 74 through the lateral inlet surface 74a,
repeatedly reflecting at the bottom surface 74c and on the
front surface 74b thereof and then being emitted as
homogeneous back light rays directed upward from the front
surface thereof. At the back of the light-guiding plate 74
is placed the reflecting plate 75 by which light rays,
leaking from the bottom surface of the light-guiding plate
74 are reflected and returned into the light-guiding plate
74, thereby light rays can be effectively used. Light rays
emitted from the light-guiding plate 74 are diffused in the
diffusing plate 73 to produce homogeneous by diffused light
having small directionality which illuminates the liquid
crystal panel 71.
This embodiment differs from the conventional back
lighting device shown in Fig.9 in that a prism-like
converging plate 72 is provided between the liquid crystal
panel 71 and the diffusing plate 73. In the embodiment,
back lighting rays directed upward and downward at an angle
are converged in an axial direction by the prism-like
converging plate 72. Consequently, the embodiment may have
CA 02239477 1998-07-23
the view angle characteristic C shown in Fig.26, which is
similar in directionality with characteristic D of the
liquid crystal panel and may have a remarkably increased
axial luminosity in comparison with the view angle
characteristics B and B', shown in Fig.26, of the
conventional diffusing plates.
Application of the converging plate 72 having
prism-like grooves, shown in the enlarged section in Fig.21,
in a horizontal direction is effective for obtaining
converging characteristics in a vertical view angle
direction. If the spacing between prism-like grooves of the
converging plate 72 is substantially equal to that of the
vertical picture elements of the liquid crystal panel 71,
there may occur periodical uneven luminosity which appears
as more fringes on an image of the liquid crystal display.
Accordingly, when the picture elements of the liquid
crystal panel 71 are arranged, for example, at intervals of
about 200~m in a vertical direction, the prism-like grooves
of the converging plate 72 shall be arranged at intervals of
not more than 50~m, thereby the possibility of the
occurrence of moire fringes can be eliminated.
Fig.22 is a construction view of a back lighting device
of a direct bottom-reflecting type wherein a reflecting
plate is designated by numeral 78 and other components
similar to those of Fig.20 shown with like numerals. The
embodiment differs from the prior art shown in Fig.10 in
that a converging plate ha~ing prism-~ike grooves 72
arranged at narrow intervals is placed on a diffusing plate
- 32 -
CA 02239477 1998-07-23
73.
In this case, back light rays directed upwards and
downwards at an angle come together in the axial direction
by the prism-like converging plate 72. Thereby, the
embodiment may have the view angle characteristic C shown in
Fig.26, which is similar in direction to characteristic D of
the liquid crystal pan-el and may attain a remarkable
improvement of axial luminosity similar to the
above-mentioned case.
Figs.23A and 23B are construction views of a back
lighting device using a welded light-guiding unit, and
Fig.24 shows the portion B of Fig.23B in an enlarged scale.
In Figs.23A,23B and Fig.24, there are shown a converging
plate 81, a diffusing plate 82 having a protrusion 82a, a
light guide plate 83 having an inlet portion 83a, a front
surface 83b and a bottom surface 83c, a reflecting plate 84,
and image area of a liquid crystal panel 85, a reflector 86,
a lamp 87, a liquid crystal panel 88, a back lighting unit
89 with a welded portion 89a and protrusions 89b, 89c, and a
reflecting element 90.
Light rays from the fluorescent lamp 87 are injected,
directly or after reflection by the reflector 86, into the
diffusing plate 83 through the inlet portion 83a thereof,
wherein they repeatedly reflect at the bottom surface 83c
and the front surface 83b and then exit from the front
surface 83b. The reflecting plate 84 reflects light rays
leaking from the bottom surface of the light guiding plate
83 and again injects them into the light guiding plate 83.
- 33 -
CA 02239477 1998-07-23
The light rays from the light guiding plate 83 enter into
the converging plate 82 wherein they are diffused to
homogeneous light rays having no direction. In the
embodiment, between the diffusing plate and the liquid
crystal panel, the converging plate 81 is provided which
enables light rays of conventional view angle characteristlc
B', substantlally even as shown in Fig.26, to have a view
angle characteristic C similar to that of the liquid crystal
panel, causing upward, downward and diagonal light rays to
converge in the front axial direction. Such light rays
assure the increased front luminosity of an image on the
liquid crystal panel. The converging plate, having
prism-like grooves in the horizontal direction, is effective
for obtaining the required converging characteristic in a
vertical visual, angled direction. However, if spacing
between prism-like grooves of the converging plate is
substantially equal to that of the vertical picture elements
of the liquid crystal panel, there may occur periodical
uneven luminosity which appears as moire fringes on an image
indicated on the liquid crystal display. ~ccordingly, when
the picture elements of the liquid crystal panel are
arranged, for example, at intervals of about 200~m in a
vertical direction, the prism~ e grooves of the converging
plate shall be arranged at intervals of not more than 50~m,
thereby eli m; n~ting moire fringes.
The present invention also proposes a method for
welding the above-mentioned components t~ form an integrated
unit which will be desc~ibed in detail as follows:
- 34 -
CA 02239477 1998-07-23
In the case when a light guiding plate 83 and a
diffusing plate 82, each made of acrylic resin having a
thermal expansion coefficient of 6 to 7 x 10 5cm~cmJ~C, a
converging plate 81 made of polycarbonate resin, having a
thermal expansion coefficient of about 2 x 10 5cm/cml~C and
a reflecting plate having a thermal expansion coefficient of
about 1.8 x 10 5cm/cm/~C are welded with each other at the
periphery to form one unit having an area size equal to a
4-inch display screen. The unit in operation at
temperatures of 0 to 50~C may be subjected to thermal
deformation by about 0.2 to 0.3mm there~y causing on uneven
luminosity of back light. Accordingly, the present
invention provides such a method as shown in Fig.23A the
back light unit 89 is made in a size larger than the liquid
crystal display screen 88 by about 3mm at each peripheral
edge and its components each are welded at one of two
locating protrusions 89b, 89c provided for mounting to a
back-lighting holder 91 as shown in Fig.25B. The proposed
method may elimlnate the possibility of causing an uneven
luminosity in the weld portion of the unit on the display
screen as well as el; mi n~ting the possibility of the
occurrence of uneven luminosity due to the thermal
deformation of the components since each component can
freely expand and shrink.
Protection from leakage of the light rays from the
periphery of the light guiding plate of the device is
improved as follows:
One of the proposed methods is to adhesively attach
- 35 -
CA 02239477 1998-07-23
rerlecting plates 83d,83e respectively to both side surfaces
of a lighting guide 83 as shown in Fig.25A.
The other method is such that the lighting guide plate
83 is enclosed at all surfaces ~y opposing surfaces of other
components and by surfaces, except for upper surface 91d for
mounting a liguid crystal panel, of a back lighting holder
91 shown in Fig.25B. All opposed surfaces of the components
are made of or coated with a material of a high reflective
power.
Fig.24 shows a portion B of Fig.23B to an enlarged
scale. The portion between a lamp 87 and a light inlet
surface 83a of a light guiding plate 83 is enclosed by a
reflector 86 and a reflecting plate 84, but, as shown in
Fig.23B, a diffusing plate 82 has an end 82a protruding in
the direction of the reflector 86. Therefore light rays
emitted by the lamp 87 may leak through the protruding end
82a of the diffusing plate 82 and travel toward the liquid
crystal panel 85 causing uneven lu~inosity on the display
screen and resulting in a loss of illumination.
Accordingly, a light reflecting element 90 is adhesively
attached to the lower surface of the protrusion 82a of the
diffusing plate 82 to prevent the leakage of light rays
therethough.
- 36 -
