U.S. patent application number 08/622204 was filed with the patent office on 2002-01-03 for surface light source device and liquid crystal display.
Invention is credited to SASAKO, HIROMI, WATAI, KAYOKO.
Application Number | 20020001186 08/622204 |
Document ID | / |
Family ID | 14192734 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001186 |
Kind Code |
A1 |
SASAKO, HIROMI ; et
al. |
January 3, 2002 |
SURFACE LIGHT SOURCE DEVICE AND LIQUID CRYSTAL DISPLAY
Abstract
A surface light source device has a narrowed visual field and
increased brightness. The surface light source device can be
applied to a liquid crystal display. The back side of a fluorescent
lamp is covered with silver foil. Light emitted from the lamp
enters a wedge-shaped light guide plate through its incident
surface. The light guide plate is designed so that directional
light exits from the guide plate. When the light is guided toward a
thin-walled end surface through the light guide plate, the light is
scattered, reflected, and undergoes other action. Collimated light
flux gradually exits from the exiting surface. The light flux
passes through two prism sheets successively. As a result, the
direction of propagation of the light is restricted in two
dimensions. The light flux of increased brightness is directed to
the liquid crystal panel. V-shaped channels formed in the prism
faces of the two prism sheets PS1, PS2 are arrayed in two mutually
perpendicular directions while facing outward. The prismatic
vertical angles of the two sheets PS1, PS2 have various desirable
combinations of values. For example, where the first sheet PS1 is
disposed vertical to the lamp, the vertical angles of the two
sheets PS1, PS2 are preferably 90.degree. and 70.degree.,
respectively.
Inventors: |
SASAKO, HIROMI; (TOKYO,
JP) ; WATAI, KAYOKO; (SAITAMA, JP) |
Correspondence
Address: |
STAAS & HALSEY
700 ELEVENTH STREET NW
SUITE 500
WASHINGTON
DC
20001
|
Family ID: |
14192734 |
Appl. No.: |
08/622204 |
Filed: |
March 27, 1996 |
Current U.S.
Class: |
362/620 ;
362/23.16 |
Current CPC
Class: |
G02B 6/0041 20130101;
G02B 6/0053 20130101; G02F 1/133615 20130101; F21Y 2105/00
20130101; F21V 5/02 20130101 |
Class at
Publication: |
362/31 ;
362/26 |
International
Class: |
F21V 007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 1995 |
JP |
97453/1995 |
Claims
What is claimed is:
1. A surface light source device having a primary surface light
source means with an exiting surface and two prism sheets disposed
along the exiting surface of said primary surface light source
means: each of said prism sheets having a prism face formed by an
array of V-shaped channels; the directions of arrays of said
channels of said two prism sheets being vertical to each other; the
prism faces of said two prism sheets backing against said primary
surface light source means; and said two prism sheets having
prismatic vertical angles of approximately 70.degree. to
110.degree.; and wherein light exiting from the exiting surface of
said primary surface light source means undergoes narrowing of
visual field in two dimensions in passing through said two prism
sheets.
2. A surface light source device having a primary surface light
source means with an exiting surface and two prism sheets disposed
along the exiting surface of said primary surface light source
means: each of said prism sheets having a prism face formed by an
array of V-shaped channels; directions of arrays of said channels
of said two prism sheets being vertical to each other; the prism
faces of said two prism sheets backing against said primary surface
light source means; one of said two prism sheets being located
closer to said primary surface light source means and having a
prismatic vertical angle of about 90.degree.; and the other of said
two prism sheets being located remoter from said primary surface
light source means and having a prismatic vertical angle of about
70.degree..
3. The surface light source device of claim 1 or 2, wherein said
primary surface light source means supplies directional light to
the prism sheet closer to said primary surface light source
means.
4. The surface light source device of claim 1 or 2, wherein said
primary surface light source means is equipped with a light guide
plate consisting of a light scattering guide having an exiting
surface from which directional light is emitted, and wherein said
directional light is supplied to the prism sheet closer to said
primary surface light source means.
5. A liquid crystal display having a surface light source device as
its backlighting means: said surface light source device having a
primary surface light source means having an exiting surface and
two prism sheets disposed along the exiting surface of said primary
surface light source means; each of said prism sheets having a
prism face formed by an array of V-shaped channels; the directions
of arrays of said channels of said two prism sheets being vertical
to each other; the prism faces of said two prism sheets backing
against said primary surface light source means; and said two prism
sheets having prismatic vertical angles of approximately 70.degree.
to 110.degree. ; and wherein light exiting from the exiting surface
of said primary surface light source means undergoes narrowing of
visual field in two dimensions in passing through said two prism
sheets.
6. A liquid crystal display having a surface light source device as
its backlighting means: said surface light source device having a
primary surface light source means having an exiting surface and
two prism sheets disposed along the exiting surface of said primary
surface light source means; each of said prism sheets having a
prism face formed by an array of V-shaped channels; directions of
arrays of said channels of said two prism sheets being vertical to
each other; the prism faces of said two prism sheets backing
against said primary surface light source means; one of said two
prism sheets being located closer to said primary surface light
source means and having a prismatic vertical angle of about
90.degree.; and the other of said two prism sheets being located
remoter from said primary surface light source means and having a
prismatic vertical angle of about 70.degree..
7. The surface light source device of claim 5 or 6, wherein said
primary surface light source means supplies directional light to
the prism sheet closer to said primary surface light source
means.
8. The surface light source device of claim 5 or 6, wherein said
primary surface light source means is equipped with a light guide
plate consisting of a light scattering guide having an exiting
surface from which directional light is emitted, and wherein said
directional light is supplied to the prism sheet closer to said
primary surface light source means.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention relates to techniques for narrowing
the visual field or increasing the brightness of a surface light
source device, by making use of two prism sheets. The present
invention is especially advantageously applied to backlighting of a
liquid crystal display which is observed preferentially from a
certain direction.
[0003] 2. Related Art
[0004] A well-known optical device taking the form of a sheet and
acting to modify the propagation direction characteristic of a
primary light beam supplied from one surface and to cause the beam
to exit from the other surface as a secondary light beam is
generally known as a prism sheet.
[0005] Generally, a prism sheet consists of a member in the form of
a plate made of an optical material having a surface (or prism
face) provided with a number of aligned, repeating V-shaped
channels (an array of convex and convex portions). Since this
device has a function of modifying the light beam propagation
direction having a cross-sectional area corresponding to the size
of the plasm face, the device is used, for example, to adjust the
direction of propagation of illuminating light for the backlighting
of a liquid crystal display.
[0006] FIG. 1 is a perspective view diagrammatically showing a
surface light source device using a prism sheet for this purpose.
Referring to FIG. 1, a light guide plate 1 of wedge-shaped cross
section is made of a light scattering guide. For example, the light
scattering guide is fabricated by preparing a matrix of
polymethylmethacrylate (PMMA) and uniformly dispersing a substance
of a different index of refraction in the matrix.
[0007] The thick end surface of the wedge-shaped light guide plate
1 forms an incident surface 2. A light source device (fluorescent
lamp) L is disposed near the incident surface 2. A reflector 3
consisting either of silver foil of positive reflectivity or of a
white sheet of diffuse reflectivity is disposed along one surface
(rear surface 6) of the light guide plate 1. The other surface 5 of
the light guide plate 1 forms an exiting surface 5 for taking out
light flux supplied from the light source L.
[0008] A prism sheet 4 is disposed outside this exiting surface 5.
One surface of the prism sheet 4 has a V-shaped prism face 4a, 4b.
The other surface is a flat surface 4e. If a well-known liquid
crystal panel (liquid crystal display device) is disposed further
outside the prism sheet 4, a liquid crystal display is given.
[0009] In this surface light source device, as the thickness of the
light guide plate 1 falls off with distance from the incident
surface 2, the surface light source device shows excellent
efficiency of utilization of light and excellent brightness
uniformity because of oblique surface multiple reflection effect
occurring inside the light guide plate 1. This effect based on the
shape of the light scattering guide is described in detail in
Japanese Patent Laid-Open No. 198956/1995.
[0010] Light introduced into the light guide plate 1 from the light
source device L is directed toward the thin-walled end surface 7
while being scattered and reflected inside the light guide plate 1.
During this process, the light gradually exits from the exiting
surface 5. As described later, if the diameters (generally, the
correlation distance regarding structures of nonuniform indices of
refraction) of particles having a different index of refraction and
dispersed in the light guide plate 1 are not very small, light
exiting from the exiting surface 5 has a clear direction of
preferential propagation. In other words, a collimated light beam
is taken out from the exiting surface 5. This nature is hereinafter
referred to as the "emitting directivity" or "emission of
directional light".
[0011] As discussed in detail later, this direction of preferential
propagation (the direction of the main axis of the collimated light
beam) is usually upwardly spaced about 25-30.degree. from the
exiting surface as viewed from the incident surface 2. Taking
account of this, the prior art function of modifying the direction
of propagation of the prism sheet 4 is described by referring to
FIGS. 2 and 3.
[0012] FIG. 2 is a diagram associated with the arrangement of FIG.
1, illustrating the behavior of light the cross section taken along
a direction vertical to the lamp L. "Direction vertical to the lamp
L" means "direction vertical to the long axis of the lamp L", i.e.,
direction vertical to the direction to which the incident surface
2extends". This may hereinafter be simply referred to as
"lamp-vertical direction". Similarly, "direction parallel to the
direction of the long axis of the lamp L", i.e., direction parallel
to the direction to which the incident surface 2 extends, may be
simply referred to as the "lamp-parallel direction".
[0013] The prism sheet 4 shown in FIG. 2 faces the exiting surface
5 of the light guide plate 1 with its prism face directed inward.
Preferably, the prismatic vertical angle .phi.3 made in the prism
face is about 60.degree.. Prism sheets satisfying this condition
and having a prismatic vertical angle .phi.3 of 64.degree. are
often used.
[0014] The refractive indices of materials of the matrix of the
light guide plate 1 are generally about 1.4 to 1.6. Where this is
taken into consideration, if light is directed to the light guide
plate 1 from the direction indicated by the arrow L', the direction
of preferential propagation of light flux exiting from the incident
surface 5 forms an angle .phi.2=about 60.degree. with respect to a
normal to the exiting surface 5. Where a PMMA matrix having an
index of refraction of 1.492 is used, the incident angle to the
exiting surface 5 to give .phi.2=about 60.degree. is 100 1=about
35.degree. according to the Snell's law. A light beam corresponding
to this direction of preferential propagation will hereinafter be
referred to as a representative light beam, which is indicated by
B1 herein.
[0015] The representative light beam B1 exiting from the exiting
surface 5 travels straight through an air layer AR which can be
regarded as having an index of refraction n.sub.0=1.0. Then, the
light enters the prism face 4a of the prism sheet 4 at an angle
(.phi.3=about 60.degree.) close to a right angle. This light beam
enters the opposite prism face 4b at a smaller percentage.
[0016] Then, the representative light beam B1 travels substantially
straight through the prism sheet 4 up to the opposite prism face 4b
and is reflected regularly. The light beam then enters the flat
surface 4e of the prism sheet 4 at an angle close to a right angle
and then goes out of the prism sheet 4. By this process, the
direction of preferential propagation of the light beam exiting
from the exiting surface 5 is changed to a direction substantially
vertical to the exiting surface 5. The whole light rays are
collected into a light beam traveling substantially in the
perpendicular direction. As a result, the angular range in which
the emitting surface is observed to be luminous is restricted. This
action is herein referred to as narrowing of the visual field. The
visual field will be quantitatively defined later.
[0017] The modified direction of preferential propagation is not
always vertical to the exiting surface 5. Rather, the direction can
be adjusted in a considerable range of angles by selection of the
vertical angle 100 3 of the prism sheet 4, selection of the
material (index of refraction) of the prism sheet 4, selection of
the material (index of refraction) of the light guide plate 1, and
so forth.
[0018] If the prism sheet 4 is so positioned that its prism face is
directed outward, the preferential propagation direction is
modified by similar prismatic action. In this case, however, the
range of preferred prismatic vertical angles is wider than the
range obtained where the prism face is directed inward. FIG. 3,
which takes the same form as FIG. 2, illustrates this. The vertical
angle .phi.4 of the prism made at the prism face is approximately
70.degree..
[0019] It is assumed that the direction of incident light lies in
the direction indicated by the arrow L'. In the same way as in the
case of FIG. 2, a representative light beam B2 corresponding to the
preferential propagation direction enters the exiting surface 5 at
an angle 100 1=about 35.degree.. Most of the incident light exits
and enters the air layer AR whose index of refraction n.sub.0=1.0.
At this time, the exit angle .phi.2 is approximately
60.degree..
[0020] The representative light beam B2 travels straight through
the air layer AR and then enters the flat surface 4e of the prism
sheet 4 obliquely. The light follows the illustrated refracted
path. Finally, the light goes out of the surface 4c of the prism
sheet 4 at an angle almost normal to the exiting surface 5. The
proportion of the light exiting from the surface 4d is smaller.
[0021] Since the path of the light after entering the flat surface
4e of the prism sheet 4 is varied by the index of refraction
n.sub.2 of the prism sheet 4 and by the prismatic vertical angle
100 4, the preferential propagation direction can be adjusted by
selecting these parameters. Because the whole light rays are
collected substantially into the vertical direction, the visual
field is narrowed in the same way as in the case of FIG. 2.
[0022] The action of modification of the propagation direction and
the action of the narrowing of the visual field of the prism sheet
disposed alone function effectively mainly in the plane vertical to
the lamp. It is known that the propagation direction is modified
less effectively in a plane parallel to the lamp L.
[0023] FIGS. 6 and 7 are graphs of data obtained by actual
measurements, demonstrating the above-described phenomenon. The
conditions under which the actual measurements were made are shown
in FIGS. 4 and 5. The fundamental portions are commonly applied to
various measurements for examples described later.
[0024] Referring first to FIG. 4, the same arrangement as that
shown in FIG. 1 is shown. A light guide plate 1 of wedge-shaped
cross section comprises a matrix of polymethylmethacrylate (PMMA)
having an index of refraction of 1.492 in which 0.08 wt % silicone
resin material is uniformly dispersed as a material having a
different index of refraction. The particle diameter of the
silicone resin material is 2.0 .mu.m. The index of refraction of
the resin material is 1.4345. The dimensions are given in the
figure.
[0025] The end surface which is thicker than the thin-walled end
portion 7 of the light guide plate 1 forms the incident surface 2.
A fluorescent lamp L having a diameter of 3 mm and taking the form
of a long tube is spaced 1.0 mm from the incident surface. A
reflective sheet R consisting of silver foil is mounted behind the
fluorescent lamp L to prevent scattering of the light. A reflector
3 disposed along the rear surface 6 of the light guide plate 1 is
made of a silver foil. A quite thin air layer having a thickness
given by .delta.1 exists between the rear surface 6 and the
reflector 3.
[0026] At the above-described particle diameter of the particles
having the different index of refraction, the light guide plate 1
causes directional light to exit from it. A collimated light beam
having a preferential propagation direction indicated by 5e exits
from the incident surface 5. Measurements were made under the
following conditions. Only the first prism sheet PS1 was located
outside the exiting surface 5. Alternatively, the first and second
prism sheets PS1 and PS2, respectively, were made to overlap each
other with the thin air layer AR of the thickness .delta.2
therebetween. Indicated by LP is a liquid crystal display panel
positioned when a liquid crystal display is to be constructed. The
liquid crystal display panel LP was not positioned during
measurements.
[0027] Only one prism sheet was positioned for the measurements
giving the results shown in FIGS. 6 and 7. The arrangement of each
prism sheet used for each measurement will be separately
described.
[0028] A luminance meter M (LS110 manufactured by Minolta Co.,
Ltd.; visual angle of 1/3.degree. closeup lens mounted). The center
point P on the outer surface (bright surface) a of the prism sheet
PS1 or PS2 was constantly viewed at a distance of 203 mm. The
direction of the line of sight b was changed around the central
point P. Under these conditions, the luminance meter M was used.
Let .phi. be the angle of the line of sight b in the cross section
that is vertical to the fluorescent lamp L (general expression of
angle .phi.2 used in FIG. 2).
[0029] FIG. 5 is a view in which the angle of the line of sight b
at the point P on the luminance meter M is generalized to three
dimensions. The definition of the angle .phi. is also illustrated.
As shown, we now consider a plane c on which the line of sight b
which views the central point P is located. The plane c is parallel
to the lamp L. The angle that this plane c makes with respect to
the normal d to the bright surface a is the above-described angle
.phi..
[0030] Let a straight line e pass through the central point P on
the plane c, the straight line e being vertical to a direction
parallel to the lamp. Let .theta. be the angle made between the
straight line e and the line of sight b. Let .beta. be the angle
that the line of sight makes to the normal line f starting from the
central point P. Let .zeta. be the angle that the projection h of
the line of sight b onto the bright surface a makes to a direction
vertical to the lamp. Since these various measurements were made
under conditions in which the direction of line of sight b can be
expressed by using only the angles .phi. and .theta., neither the
angle .beta. nor the angle .zeta. is cited.
[0031] The nomenclatures of the arrangements and postures of the
prism sheets PS1 and PS2 are defined as follows.
[0032] (1) When the prism surface provided with V-shaped channels
is directed toward the light scattering guide as shown in FIGS. 1
and 2, the V-shaped channels are referred to as facing inward. On
the other hand, when the prism face provided with the V-shaped
channels faces away from the light scattering guide, the channels
are referred to as facing outward.
[0033] (2) When the prism sheets are so arranged that the direction
of alignment of the V-shaped channels in the prism face is parallel
to the fluorescent lamp L (incident surface 2) as shown in FIGS. 1
and 2, the direction of alignment is referred to as being parallel
to the lamp or simply as being lamp-parallel. On the other hand,
when the prism sheets are so arranged that the direction of
alignment of the V-shaped channels formed in the prism face is
vertical to the fluorescent lamp L (incident surface 2), the
direction of alignment is referred to as being vertical to the lamp
or simply as being lamp-vertical.
[0034] It is also assumed that the vertical angles of the prism
sheets PS1 and PS2 are given by .psi.. This notation is a
generalization of .phi.3 shown in FIG. 2 or .phi.4 shown in FIG. 3.
Description of data obtained by actual measurements is hereinafter
given clause by clause. In the following description, a word
"visual angle" which may also be known as "viewing angle" is used
as an index representing the range of angles at which the bright
surface is observed to be luminous. The visual angle is defined in
a plane parallel to the lamp and also in a plane vertical to the
lamp. The value is given by such a notation that the half-value
width of the graph obtained by each measurement is located in the
center, or 0.degree. (e.g., .+-.30.degree.).
GRAPHS OF FIGS. 6 AND 7
[0035] (1) PS1: prismatic vertical angle .psi.=64.degree.; the
channels face inward, and the direction of alignment is parallel to
the lamp.
[0036] PS2: not used
[0037] (2) FIG. 6; Measurements were made under the condition
.phi.=0.degree.. At this time, the angle .theta. was scanned in the
range of from 80.degree. to +80.degree.. This scanned angle is
plotted on the horizontal axis.
[0038] FIG. 7; Measurements were made under the condition
.theta.=0.degree.. At this time, the angle .theta. was scanned in
the range of from -80.degree. to .+-.80.degree.. This scanned angle
was plotted on the horizontal axis.
[0039] (3) The luminance value is plotted on the vertical axis in
units of 1000 nt, where nt=cd/m.sup.2. The plotted luminance value
has been subjected to cosine correction to remove the factor
(proportional to the inverse of the cosine of the inclination
angle) contained in the output from the luminance meter M when the
line of sight b is inclined with respect to the bright surface a.
In the case of FIG. 6, .theta. is the inclination angle. In the
case of FIG. 7, .phi. is the inclination angle. With respect to all
other graphs, values subjected to cos corrections are employed as
plotted luminance values.
[0040] (4) Explanation; It can be seen from both graphs that the
measured peaks lie in the direction .theta.=.phi.=0.degree., i.e.,
the front of the surface light source device. For both graphs, the
whole shape assumes a hill-shaped profile which is substantially
symmetrical with respect to the peak and has feet.
[0041] However, it can be seen from the spread of both graphs that
the visual field in a plane vertical to the lamp is very narrow but
the visual field in a plane parallel to the lamp is considerably
broad. For detailed values obtained from actual measurements, refer
to Table 3 given later. In particular, in the present example using
one prism sheet, narrowing of the visual field is accomplished in
the plane vertical to the lamp but was not in the plane parallel to
the lamp.
[0042] Although the data obtained from actual measurements is
omitted for the arrangement where the prism face of the prism sheet
4 faces outward, narrowing of the visual field is also accomplished
in the plane vertical to the lamp but not in the plane parallel to
the lamp.
[0043] Many liquid crystal displays are observed preferentially
from the front direction with respect to both vertical and
horizontal directions. Back lighting applied to these liquid
crystal displays is, of course, required to accomplish narrowing of
the visual field in both vertical and horizontal directions, i.e.,
in the plane vertical to the lamp and also in the plane parallel to
the lamp. Also, even if the preferentially observed direction is
rather spread and conspicuous narrowing of the visual field is not
required, the visual field is preferably restricted to some extent
in the plane vertical to the lamp and in the plane parallel to the
lamp, for the following reason.
[0044] If illuminating light propagates in the direction to which
observation is hardly expected, i.e., in a direction in a greatly
deviating from the front direction, e.g., in a direction spaced
more than 30.degree. from the front direction, then increase in the
brightness of the surface light source device is hindered. This
deteriorates the display quality of a liquid crystal display
incorporating the light source device.
[0045] However, in the above-described well-known usage of prism
sheets, it is impossible to satisfy the requirement, i.e.,
narrowing of the visual field both in the plane vertical to the
lamp and in the plane parallel to the lamp.
[0046] Furthermore, even if intensive narrowing of the visual field
is not required, it is difficult for the prior art techniques to
accomplish considerable narrowing of the visual field in the plane
vertical to the lamp and in the plane parallel to the lamp, thereby
increasing the brightness of the surface light source device. The
former (intensive narrowing of the visual field) is hereinafter
referred to as "narrowing of the visual field". The latter
(increase of the brightness caused by a considerable narrowing of
the visual field) is hereinafter referred to as "increase of the
brightness".
OBJECT AND SUMMARY OF THE INVENTION
[0047] Accordingly, it is an object of the present invention to
accomplish narrowing of the visual field of a surface light source
device or increase of the brightness by using two prism sheets
under certain conditions. This, in turn, permits improvement of the
display quality of the liquid crystal display having a preferential
observational direction.
[0048] The present invention solves the foregoing problem by
arranging two prism sheets along the exiting surface of a primary
surface light source means in a given relation, the prism sheets
satisfying given prismatic vertical angle conditions.
[0049] Each prism face of the two prism sheets faces away from the
primary surface light source means. The prismatic vertical angle of
each prism sheet lies in the range of from about 70.degree. to
about 110.degree.. The vertical angle is so selected that when
light exiting from the existing surface of the primary surface
light source means passes through the two prismatic sheets, the
visual field is narrowed in two dimensions.
[0050] Where that of the two prism sheets which is closer to the
primary surface light source means has a prismatic vertical angle
of about 90.degree. and the remoter sheet has a prismatic vertical
angle of about 70.degree., quite desirable results arise.
Furthermore, it is desired to use a light source emitting
directional light as the primary surface light source means.
[0051] If this surface light source means is disposed as a
backlighting means behind a liquid crystal display panel, a liquid
crystal display making effective use of narrowing of the visual
field and increased brightness characteristics can be provided.
[0052] The present invention is based on the following
principle:
[0053] If two prism sheets are piled on the exiting surface of the
surface light source device in such a way that the directions of
alignment of the channels in the two sheets are vertical to each
other, the surface light source device shows narrowing of the
visual field or increase of the brightness under certain vertical
angle conditions.
[0054] In the surface light source device on which the two prism
sheets are disposed, the light intensity profile in the cross
section of the exiting light flux is preferably flat from a
practical point of view. In order to make better use of the
functions of the narrowing of the visual field or increase of the
brightness, it is desirable that the light emitted from the device
have directivity.
[0055] A light scattering guide provides a light guide plate which
is advantageously used in the surface light source device of the
present invention. The scattering characteristics of the light
scattering guide, especially the directivity of the emitted light,
or collimation of the light, are described by quoting the Debye
theory.
[0056] When light having intensity I.sub.0 travels y(cm) through a
medium (light scattering guide) and the intensity is attenuated to
I by the resulting scattering, the effective scattering irradiation
parameter E is defined by the following Eq. (1) or (2).
E[cm.sup.-1]=-[In (I/I.sub.0)]/y (1)
E[cm.sup.-1]=-(1/I).multidot.dI/dy (2)
[0057] The above equations (1) and (2) are so-called integral form
and differential form, respectively. They are equivalent in
physical meaning. The effective scattering irradiation parameter E
may also be called turbidity.
[0058] Where scattering of light occurs due to nonuniform
structures distributed in a medium, the intensity of the scattering
light under normal condition where incident light is vertically
polarized light and most of exiting light is vertically polarized
(Vv scattering) is given by 1 Vv = [ ( 4 < 2 > 3 ) / 0 6 ] 6
.infin. C ( r ) r ( 3 )
[0059] where
C=[r.sup.2 sin (vsr)]/vsr (4)
[0060] As well known, natural light is provided, the following
equation obtained by multiplying the right hand of Eq. (3) by
(1+cos.sup.2.PHI.)/2 is regarded as scattered light intensity,
taking account of Hh scattering.
Ivh=Vv(1+cos.sup.2.PHI.)/2 (5)
[0061] where .lambda..sub.0 is the wavelength of incident light,
.nu.=(2.pi.n)/.lambda..sub.0, and s=2sin (.PHI./2). Indicated by n
is the index of refraction of the medium. .PHI. is the scattering
angle. <.eta..sup.2> is the squares mean of fluctuations of
the dielectric constant in the medium. In the following
description, .tau. may be used, assuming that
<.eta..sup.2>=.tau.. .gamma.(r) is known as a correlation
function. This correlation function .gamma.(r) is given by
.gamma.(r)=exp(-r/a) (6)
[0062] According to the Debye theory, if refractive index
nonuniform structures of a medium are dispersed and divided into
phase A and phase B with an interface, then the following
relational formulas (7) and (8) hold regarding the correlation
function .gamma.(r), the correlation distance a, and the squares
mean of fluctuations of the dielectric constant .tau..
a[cm]=(4V/S).multidot..phi.A.phi.B (7)
.tau.=.phi.A.phi.B(nA.sup.2-nB.sup.2).sup.2 (8)
[0063] If the nonuniform structures of the refractive index can be
regarded as consisting of a spherical interface of radius R, then
the correlation distance a is given by
a[cm]=(4/3)R(1-.phi.A) (9)
[0064] Eq. (6) concerning the function .gamma.(r) is used. Natural
light is provided, effective scattering irradiation parameter E is
calculated, based on Eq. (5) and the formula (6) for correlation
function. The result is given below.
E=[(32a.sup.3.tau..pi..sup.4)/.lambda..sub.0.sup.4].multidot.f(b)
(10)
[0065] where
f(b)=[{(b+2)/.sup.2/b.sup.2(b+1)}-{2(b+2)/b.sup.3.multidot.ln
(b+1)] (11)
b=4.nu..sup.2a.sup.2 (12)
[0066] It can be seen from the relations described thus far that
the correlation distance a, the squares mean of fluctuations of the
dielectric constant .tau., and the effective scattering irradiation
parameter E are mutually dependent on each other.
[0067] In FIG. 8, the correlation distance a is plotted on the
horizontal axis, while the squares mean of fluctuations of the
dielectric constant .tau. is plotted on the vertical axis. Curves
expressing conditions under which the effective scattering
irradiation parameter E is constant are drawn about E=50 cm.sup.-1
and E=100 cm.sup.-1.
[0068] Generally, light scattering guides having large values of E
tend to have large scattering power. Light scattering guides having
small values of E tend to have small scattering power. E=0
cm.sup.-1 indicates a transparent state in which no scattering
occurs at all. Therefore, the following general rule holds: a light
scattering guide having a small value of E is suitable for a
surface light source device having a wide luminous portion;
Conversely, a light scattering guide having a large value of E is
suitable for a surface light source device having a narrow luminous
portion. Taking this rule into account, the range E=0.45 cm.sup.-1
to 100 cm.sup.-1 provides a rough standard of the effective
scattering irradiation parameter E preferable for applications to
backlighting of ordinary size.
[0069] On the other hand, the correlation distance a is deeply
concerned with the direction characteristics of scattered light in
individual scattering phenomena inside the light scattering guide.
That is, as can be estimated from the forms of Eqs. (3)-(5), light
scattering inside the light scattering guide generally has forward
scattering nature. The magnitude of the forward scattering is
varied by the correlation distance a.
[0070] FIG. 9 is a graph illustrating this regarding two values of
a. In the figure, the horizontal axis indicates the scattering
angle .PHI., it being noted that the direction of propagation of
incident rays is given by .PHI.=0.degree.. The vertical axis
indicates the intensity of scattered light when natural light is
assumed. That is, it indicates the value obtained by normalizing
Eq. (5) with respect to .PHI.=0.degree., or Vvh(.PHI.)/Vvh(0).
[0071] As is drawn also in FIG. 9, a=0.13 .mu.m corresponding to
2R=0.2 .mu.m in particle size converted by Eq. (9) is provided, the
graph of the normalized scattering intensity is a slowly decreasing
function regarding .PHI.. On the other hand, a=1.3 .mu.m
corresponding to 2R=2.0 .mu.m in particle size converted by Eq. (9)
is provided, the graph of the normalized scattering intensity shows
a rapid falling in a range of small .PHI..
[0072] In this way, scattering caused by minute refractive index
nonuniform structures inside a light scattering guide essentially
has forward scattering nature. As the correlation distance a
decreases, the intensity of the forward scattering nature weakens,
and the range of angles of scattering for one scattering process
tends to widen. Conversely, as the correlation distance a
increases, the forward scattering property tends to become
stronger.
[0073] The discussion thus far has been held on individual
scattering phenomena themselves due to refractive index nonuniform
structures distributed in the light scattering guide. In order to
evaluate the direction characteristics of light actually exiting
from the exiting surface of the light scattering guide, it is
necessary to take account of both total reflection phenomenon at
the exiting surface and transmittivity (the rate at which light
escapes from the light scattering guide) when light exits from the
surface.
[0074] As already described by referring to FIG. 2, if the incident
angle (assuming that the direction of a normal to the exiting
surface lies at 0.degree.) from inside the light scattering guide
onto the exiting surface is greater than the critical angle
.alpha.c determined by the indices of refraction of media inside
and outside the light scattering guide, no light exit (escape) to
the outside (air layer).
[0075] For PMMA (having an index of refraction of 1.492) which is a
representative material capable of being used in the matrix of the
novel light scattering guide, .alpha.c=42.degree.. Other materials
give similar values. Representative materials of the matrix of the
light scattering guide are listed in Tables 1 and 2.
1TABLE 1 Refractive Category Name of Polymer Index MA 1. PMMA
[polymethyl methacrylate] 1.49 2. PEMA [polyethyl methacrylate]
1.483 3. Poly(nPMA) 1.484 [poly-n-propyl methacrylate] 4.
Poly(nBMA) 1.483 [poly-n-butyl methacrylate] 5. Poly(nHMA) 1.481
[poly-n-hexyl methacrylate] 6. Poly(iPMA) 1.473 [polyisopropyl
methacrylate] 7. Poly(iBMA) 1.477 [polyisobutyl methacrylate] 8.
Poly(tBMA) 1.463 [poly-t-butyl methacrylate] 9. PCHMA
[polycyclohexyl methacrylate] 1.507 XMA 10. PBzMA [polybenzyl
methacrylate] 1.568 11. PPhMA [polyphenyl methacrylate] 1.57 12.
Poly(1-PhEMA) 1.543 [poly-1-phenylethyl methacrylate] 13.
Poly(2-PhEMA) 1.559 [poly-2-phenylethyl methacrylate] 14. PFFMA
[polyfurfuryl methacrylate] 1.538 A 15. PMA [polymethyl acrylate]
1.4725 16. PEA [polyethyl acrylate] 1.4685 17. Poly(nBA)
[poly-n-butyl acrylate] 1.4535 XA 18. PBzMA [polybenzyl acrylate]
1.5584 19. Poly(2-CIEA) 1.52 [poly-2-chloroethyl acrylate]
[0076]
2TABLE 2 Refractive Category Name of Polymer Index AC 20. PVAc
[polyvinyl acetate] 1.47 XA 21. PVB [polyvinyl benzoate] 1.578 22.
PVAc [polyvinyl phenyl acetate] 1.567 23. PVClAc 1.512 [polyvinyl
chloroacetate] N 24. PAN [polyacrylonitrile] 1.52 25.
Poly(.alpha.MAN) 1.52 [poly-.alpha.-methyl acrylonitrile] .alpha.-A
26. PMA(2Cl) 1.5172 [polymethyl-.alpha.-chloroacrylate- ] St 27.
Poly(o-ClSt) 1.6098 [poly-o-chlorostyrene] 28. Poly(p-FSt) 1.566
[poly-p-fluorostyrene] 29. Poly(o, p-FSt) 1.475 [poly-o-,
p-diflurostyrene] 30. Poly(p-iPSt) 1.554 [poly-p-isopropyl styrene]
31. PSt [polystyrene] 1.59 C 32. PC [polycarbonate) 1.59
[0077] Since scattering occurring inside a light scattering guide
generally shows forward scattering nature as mentioned above, in
the case in which the incident surface is located at a side of the
exiting surface as in the arrangement of FIG. 1, primary scattered
light produced travelling through the incident surface and meeting
nonuniform structures hardly satisfies the above-described critical
angle conditions.
[0078] In other words, most of light exiting from the exiting
surface after satisfying the critical angle conditions is
considered to have experienced multiple scattering inside the light
scattering guide and reflections off reflectors disposed at or
close to the interface behind the light scattering guide.
[0079] Accordingly, if attention is paid only to the light
satisfying the critical angle conditions, the forward scattering
nature which is an attribute of individual scattering phenomena
will be weakened considerably and hence the distribution of the
propagation direction of light will be considerably spread. As a
result, the direction characteristics of light exiting from the
light scattering guide are greatly depending on the angle
dependence of the transmittivity (rate of escape) at the existing
surface for the light satisfying the critical angle conditions.
[0080] It is known that the interface transmittivity is generally
quite low where the critical angle conditions are narrowly
satisfied. For example, in the case of an acrylic resin-air
interface, the interface transmittivity for P-polarized light is
about 40%, and the interface transmittivity for S-polarized light
is about 20%. However, if the incident angle is lower than the
critical angle by some amount, then the interface transmittivity
increases rapidly. If the incident angle is less than the critical
angle by 5 to 10.degree. or more, the transmittivity is almost
constant. For instance, in the case of an acrylic resin-air
interface, the interface transmittivity is more than 90% for
P-polarized light and more than 85% for S-polarized light.
[0081] According to an estimate based on the foregoing, the light
entering at incident angles of about 35.degree. to the exiting
surface contributes most to light emission from the exiting surface
of the light scattering guide. If the considerations given in
connection with FIGS. 2 and 3 and refraction at the exiting surface
are taken into consideration, the light entering the exiting
surface at incident angles of about 35.degree. makes exiting angles
of about 60.degree. to the normal to the exiting surface. It is to
be noted that the index of refraction of the light scattering guide
is in general around 1.5.
[0082] Eventually, roughly speaking, is light going out of the
existing surface of the light scattering guide has directivity in a
direction upwardly spaced about 30.degree. from the exiting
surface.
[0083] It is to be noted that if the correlation distance a becomes
too small, the forward scattering nature itself weakens. As a
result, even if only primary scattering is taken into account,
scattering light is produced over a wide range of directions. This
lowers the directivity of the outgoing light. Roughly estimated,
correlation distance a greater than 0.01 .mu.m hardly gives such a
phenomenon conspicuously. A preferable range is a >0.05 .mu.m.
This is considered as the condition under which the light
scattering guide has emitting directivity.
[0084] In the present invention, light flux going out of the light
guide plate consisting of a light scattering guide which imparts
directivity to the outgoing light is supplied to two prism sheets.
As a consequence, a surface light source device accomplishing
narrowing of the visual field or increased brightness is
provided.
[0085] Generally, the manner in which prism sheets are disposed has
the following degrees of freedom.
[0086] (1) It is possible to select either the inward arrangement
of the channels or outward arrangement of the channels for each of
the prism sheets PS1 and PS2 (see FIG. 4). The latter arrangement
is necessary for the present invention.
[0087] (2) Orientations of the prism sheets PS1 and PS2. That is,
it is possible to select either the lamp-parallel orientation or
lamp-vertical orientation. In the present invention, where the
prism sheet PS1 is placed parallel to the lamp, the prism sheet PS2
is placed vertical to the lamp.
[0088] Conversely, where the prism sheet PS1 is placed vertical to
the lamp, the prism sheet PS2 is placed parallel to the lamp.
[0089] With respect to the prismatic vertical angle conditions, the
vertical angles of both prism sheets are selected in the range from
about 70.degree. to 110.degree. . However, as can be understood
from the various examples given below, the degree of accomplishment
of the narrowing of the visual field and increase of the brightness
varies considerably depending on the combination of the prismatic
vertical angle conditions of the prism sheets. Therefore, it is
desired to flexibly select the specific prismatic vertical angle
conditions, taking account of the characteristics required for the
surface light source device or a liquid crystal display using a
backlighting arrangement making use of the surface light source
device, as well as data obtained from actual measurements, the data
being given in the various examples.
[0090] The present invention is hereinafter described in further
detail by referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0091] FIG. 1 is a perspective view of a conventional surface light
source device using prism sheets;
[0092] FIG. 2 is a diagram associated with the arrangement shown in
FIG. 1, illustrating the behavior of light in a cross section taken
along a direction vertical to a lamp L;
[0093] FIG. 3 is a diagram associated with the arrangement shown in
FIGS. 1 and 2, in which a prism sheet 4 is inverted and its prism
face is directed outward, for illustrating behavior of light;
[0094] FIG. 4 is a cross-sectional view of the basic structure of a
surface light source device according to each example of the
invention, also showing the arrangement of the device adopted when
measurements are made with a luminance meter for various examples
of the invention and referential examples;
[0095] FIG. 5 is a view illustrating three-dimensional arrangement
adopted when measurements are made with a luminance meter for
various examples of the invention and referential examples;
[0096] FIG. 6 is a graph showing results of measurements of
emitting characteristics in a plane parallel to a lamp for a
referential example in which a prism sheet having a prismatic
vertical angle .phi.=64.degree. is used singly and the channels are
directed inward;
[0097] FIG. 7 is a graph showing results of measurements of
emitting characteristics in a plane vertical to a lamp for a
referential example in which only a prism sheet having a prismatic
vertical angle .phi.=64.degree. is used and the channels are
directed inward;
[0098] FIG. 8 is a graph showing curves expressing conditions in
which the effective scattering irradiation parameter E is kept
constant for cases of E=50 cm.sup.-1 and E=100 cm.sup.-1, in which
the correlation distance a is plotted on the horizontal axis and
the dielectric constant fluctuation squares mean .tau. is plotted
on the vertical axis;
[0099] FIG. 9 is a graph illustrating the manner in which the
forward scattering property of a light scattering guide changes
with the correlation distance a;
[0100] FIG. 10 is a graph showing the results of measurements of
the emitting characteristics of Example 1 in a plane parallel to a
lamp;
[0101] FIG. 11 is a graph showing the results of measurements of
the emitting characteristics of Example 1 in a plane vertical to a
lamp;
[0102] FIG. 12 is a graph showing the results of measurements of
the emitting characteristics of Example 2 in a plane parallel to a
lamp;
[0103] FIG. 13 is a graph showing the results of measurements of
the emitting characteristics of Example 2 in a plane vertical to a
lamp;
[0104] FIG. 14 is a graph showing the results of measurements of
the emitting characteristics of Example 3 in a plane parallel to a
lamp;
[0105] FIG. 15 is a graph showing the results of measurements of
the emitting characteristics of Example 3 in a plane vertical to a
lamp;
[0106] FIG. 16 is a graph showing the results of measurements of
the emitting characteristics of Example 4 in a plane parallel to a
lamp;
[0107] FIG. 17 is a graph showing the results of measurements of
the emitting characteristics of Example 4 in a plane vertical to a
lamp;
[0108] FIG. 18 is a graph showing the results of measurements of
the emitting characteristics of Example 5 in a plane parallel to a
lamp;
[0109] FIG. 19 is a graph showing the results of measurements of
the emitting characteristics of Example 5 in a plane vertical to a
lamp;
[0110] FIG. 20 is a graph showing the results of measurements of
the emitting characteristics of Example 6 in a plane parallel to a
lamp;
[0111] FIG. 21 is a graph showing the results of measurements of
the emitting characteristics of Example 6 in a plane vertical to a
lamp;
[0112] FIG. 22 is a graph showing the results of measurements of
the emitting characteristics of Example 7 in a plane parallel to a
lamp;
[0113] FIG. 23 is a graph showing the results of measurements of
the emitting characteristics of Example 7 in a plane vertical to a
lamp;
[0114] FIG. 24 is a graph showing the results of measurements of
the emitting characteristics of Example 8 in a plane parallel to a
lamp;
[0115] FIG. 25 is a graph showing the results of measurements of
the emitting characteristics of Example 8 in a plane vertical to a
lamp;
[0116] FIG. 26 is a graph showing the results of measurements of
the emitting characteristics of Example 9 in a plane parallel to a
lamp;
[0117] FIG. 27 is a graph showing the results of measurements of
the emitting characteristics of Example 9 in a plane vertical to a
lamp;
[0118] FIG. 28 is a graph showing the results of measurements of
the emitting characteristics of Example 10 in a plane parallel to a
lamp;
[0119] FIG. 29 is a graph showing the results of measurements of
the emitting characteristics of Example 10 in a plane vertical to a
lamp;
[0120] FIG. 30 is a graph showing the results of measurements of
the emitting characteristics of Example 11 in a plane parallel to a
lamp; and
[0121] FIG. 31 is a graph showing the results of measurements of
the emitting characteristics of Example 11 in a plane vertical to a
lamp.
[0122] Where a well-known liquid crystal display or liquid crystal
display panel is disposed outside the prism sheet 4, a liquid
crystal display construction is completed.
PREFERRED EMBODIMENTS OF THE INVENTION
[0123] To briefly describe the features of the embodied examples of
the invention, the examples are classified into group I and group
II. The graphs of FIGS. 10-31 are described clause by clause. Group
I (Examples 1-7) mainly intends clear narrowing of the visual
field. Increase of the brightness is also at least considerably
accomplished as a result of the narrowing of the visual field.
[0124] Group II (examples 8-11) mainly intends to increase the
brightness. Narrowing of the visual field is also accomplished at
least to some extent.
[0125] The conditions of all the examples, including the
measurement conditions and excluding the direction of arrangement
of two prism sheets, orientations (either inward or outward) and
vertical angle conditions, have been already described in
connection with FIG. 4, which shows the essential structure of the
surface light source device of each example. Obviously, where the
surface light source device is applied to backlighting arrangement
for a liquid crystal display, a liquid crystal display panel LP is
disposed outside two prism sheets, although during measurements, no
liquid crystal display panel LP is disposed.
[0126] These items common to these examples will not be repeatedly
described below. The meanings of the vertical and horizontal axes
of the graphs and the method of plotting brightness values (i.e.,
adoption of cosine corrected values) have been already described in
connection with the graphs of FIGS. 6 and 7 for referential
examples.
GRAPHS OF FIGS. 10 and 11
Example 1 (Group I)
[0127] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and parallel to the lamp.
[0128] PS2; prismatic vertical angle .psi.=70.degree. ; the
channels are directed outward and vertical to the lamp.
[0129] (2) FIG. 10; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0130] FIG. 11; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0131] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=.phi.=0.degree., i.e., in
front of the surface light source device. For each graph, the whole
shape is roughly symmetrical about the peak and shows a hill-shaped
profile having feet. However, small ridged portions exist around
.theta.=.+-.60.degree. in a plane parallel to the lamp.
[0132] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp and the visual field in the
plane vertical to the lamp are both very narrow. That is, in the
present example, it is obvious that narrowing of the visual field
has been accomplished both in the plane parallel to the lamp and in
the plane vertical to the lamp. It cannot be said that the levels
of the brightness of the peaks are especially high. However, these
levels are higher than the levels of the referential examples shown
in FIGS. 6 and 7 in each of which only one prism sheet is employed.
Consequently, considerable increase of the brightness has been
achieved.
GRAPHS OF FIGS. 12 AND 13
Example 2 (Group I)
[0133] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and vertical to the lamp.
[0134] PS2; prismatic vertical angle .psi.=70.degree.; the channels
are directed outward and parallel to the lamp.
[0135] (2) FIG. 12; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0136] FIG. 13; At .theta.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0137] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=.phi.=0.degree., i.e., in
front of the surface light source device. For each graph, the whole
shape is roughly symmetrical about the peak and shows a hill-shaped
profile having feet. However, small ridged portions exist around
.phi.=.+-.60.degree. in a plane vertical to the lamp.
[0138] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp and the visual field in the
plane vertical to the lamp are both very narrow. That is, in the
present example, it is obvious that narrowing of the visual field
has been accomplished both in the plane parallel to the lamp and in
the plane vertical to the lamp. The levels of the brightness of the
peaks are higher than the level of Example 1. That is, it is
obvious that increase of the brightness has been achieved.
GRAPHS OF FIGS. 14 AND 15
Example 3 (Group I)
[0139] (1) PS1; prismatic vertical angle .psi.=70.degree.; the
channels are directed outward and parallel to the lamp.
[0140] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and vertical to the lamp.
[0141] (2) FIG. 14; At .psi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0142] FIG. 15; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0143] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=.phi.=0.degree., i.e., in
front of the surface light source device. For each graph, the whole
shape is roughly symmetrical about the peak and shows a hill-shaped
profile having feet. However, small ridged portions exist around
.theta.=.+-.50.degree. to .+-.80.degree. in the plane parallel to
the lamp. Also, small ridged portions are present around
.phi.=.+-.40.degree..
[0144] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp is very narrow and that the
visual field in the plane vertical to the lamp is less narrow. That
is, in the present example, it is obvious that narrowing of the
visual field has been accomplished in the plane parallel to the
lamp. Similarly, it is obvious that narrowing of the visual field
in the plane vertical to the lamp has been accomplished. The levels
of the brightness of the peaks are comparable to the level of
Example 2. That is, it is obvious that increase of the brightness
has been achieved.
GRAPHS OF FIGS. 16 AND 17
Example 4 (Group I)
[0145] (1) PS1; prismatic vertical angle .psi.=70.degree.; the
channels are directed outward and vertical to the lamp.
[0146] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and parallel to the lamp.
[0147] (2) FIG. 16; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0148] FIG. 17; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0149] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=.phi.=0.degree., i.e., in
front of the surface light source device. For each graph, the whole
shape is roughly symmetrical about the peak and shows a hill-shaped
profile having feet. However, small ridged portions exist around
.theta.=.+-.40.degree. in the plane parallel to the lamp. Also,
small ridged portions are present around .phi.=.+-.50.degree. to
.+-.90.degree..
[0150] It can be seen from the spread of each graph that the visual
field in the plane parallel to the lamp and the visual field in the
plane vertical to the lamp are both considerably narrow. That is,
in the present example, it is obvious narrowing of the visual field
has been accomplished both in the plane parallel to the lamp and in
the plane vertical to the lamp. The levels of brightness of the
peaks are high, in the same way as in Example 1. Consequently,
considerable increase of the brightness has been achieved.
GRAPHS OF FIGS. 18 AND 19
Example 5 (Group I)
[0151] (1) PS1; prismatic vertical angle .psi.=70.degree.; the
channels are directed outward and parallel to the lamp.
[0152] PS2; prismatic vertical angle .psi.=100.degree.; the
channels are directed outward and vertical to the lamp.
[0153] (2) FIG. 18; At .theta.=0.degree., .theta. is scanned from
-80.degree. to +8020 .
[0154] FIG. 19; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0155] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=0.degree., .phi.=+5.degree.,
i.e., roughly in front of the surface light source device. For each
graph, the whole shape is roughly symmetrical about the peak and
shows a hill-shaped profile having feet. However, the brightness
levels outside .theta.=.+-.50.degree. in the plane parallel to the
lamp are very low. Small ridged portions exist around .phi.=-4020
in the plane vertical to the lamp.
[0156] It can be seen from the spread of each graph that the visual
field in the plane parallel to the lamp and the visual field in the
plane vertical to the lamp are both considerably narrow. That is,
in the present example, it is obvious that narrowing of the visual
field has been accomplished both in the plane parallel to the lamp
and in the plane vertical to the lamp. The levels of brightness of
the peaks are comparable to the level of Example 1. Consequently,
considerable increase of the brightness has been achieved.
GRAPHS OF FIGS. 20 AND 21
Example 6 (Group I)
[0157] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and parallel to the lamp.
[0158] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and vertical to the lamp.
[0159] (2) FIG. 20; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0160] FIG. 21; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0161] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=0.degree., .phi.=10.degree.,
i.e., peaks are observed roughly in front of the surface light
source device. For each graph, the whole shape is roughly
symmetrical about the peak and shows a hill-shaped profile having
feet. However, small ridged portions exist around
.theta.=.+-.70.degree. in the plane parallel to the lamp. Small
ridged portions exist around .theta.=.+-.40.degree. in the plane
vertical to the lamp.
[0162] It can be seen from the spread of each graph that the visual
field in the plane parallel to the lamp and the visual field in the
plane vertical to the lamp are both considerably narrow. That is,
in the present example, it is obvious that narrowing of the visual
field has been accomplished both in the plane parallel to the lamp
and in the plane vertical to the lamp. The levels of brightness of
the peaks are very high. That is, in the present example, increase
of the brightness has been achieved to a very great extent.
GRAPHS OF FIGS. 22 AND 23
Example 7 (Group I)
[0163] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and vertical to the lamp.
[0164] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and parallel to the lamp.
[0165] (2) FIG. 22; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0166] FIG. 23; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0167] (3) Explanation; It can be seen from both graphs that peaks
are measured in the direction .theta.=0.degree., .phi.=5.degree.,
i.e., peaks are observed roughly in front of the surface light
source device. For each graph, the whole shape is roughly
symmetrical about the peak and shows a hill-shaped profile having
feet. However, small ridged portions exist around
.theta.=+40.degree. in the plane parallel to the lamp. Small ridged
portions exist outside .phi.=.+-.60.degree. in the plane vertical
to the lamp.
[0168] It can be seen from the spread of each graph that the visual
field in the plane parallel to the lamp and the visual field in the
plane vertical to the lamp are both considerably narrow. That is,
in the present example, narrowing of the visual field in the plane
parallel to the lamp is clear. Narrowing of the visual field in the
plane vertical to the lamp is clearer. The levels of brightness of
the peaks are very high. That is, in the present example, increase
of the brightness has been achieved to a very great extent.
GRAPHS OF FIGS. 24 AND 25
Example 8 (Group II)
[0169] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and vertical to the lamp.
[0170] PS2; prismatic vertical angle .psi.=100.degree.; the
channels are directed outward and parallel to the lamp.
[0171] (2) FIG. 24; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0172] FIG. 25; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0173] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=0.degree., .phi.=10.degree.,
i.e., peaks are observed roughly in front of the surface light
source device. For each graph, the whole shape is roughly
symmetrical about the peak and shows a hill-shaped profile having
feet. However, a flat portion is observed around the peaks
(.theta.=around 0.degree. to .+-.15.degree.) in the plane parallel
to the lamp. Small ridged portions exist around .phi.=+70.degree.
in the plane vertical to the lamp.
[0174] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp is considerably narrow but
the visual field in the plane parallel to the lamp is not very
narrow. That is, in the present example, narrowing of the visual
field in the plane vertical to the lamp is clear but narrowing of
the visual field in the plane vertical to lamp is not very clear.
The levels of brightness of the peaks are very high. It is obvious
that increase of the brightness has been accomplished.
GRAPHS OF FIGS. 26 AND 27
Example 9 (Group II)
[0175] (1) PS1; prismatic vertical angle .psi.=90.degree.; the
channels are directed outward and parallel to the lamp.
[0176] PS2; prismatic vertical angle .psi.=100.degree.; the
channels are directed outward and vertical to the lamp.
[0177] (2) FIG. 26; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0178] FIG. 27; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0179] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=5.degree., .phi.=10.degree.,
i.e., peaks are observed roughly in front of the surface light
source device. For each graph, the whole shape is roughly
symmetrical about the peak and shows a hill-shaped profile having
feet. However, flat portions are observed around the peaks
(.theta.=around 0.degree. to .+-.10.degree.) in the plane parallel
to the lamp. Small ridged portions exist around .phi.=+70.degree.
in the plane parallel to the lamp.
[0180] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp is considerably narrow but
the visual field in the plane parallel to the lamp is not very
narrow. That is, the characteristics of the present invention are
very similar to those of Example 8. Narrowing of the visual field
in the plane vertical to the lamp is considerably clear but
narrowing of the visual field in the plane vertical to lamp is not
very clear. The levels of brightness of the peaks are very high. It
is obvious that increase of the brightness has been
accomplished.
GRAPHS OF FIGS. 28 AND 29
Example 10 (Group II)
[0181] (1) PS1; prismatic vertical angle .phi.=100.degree.; the
channels are directed outward and vertical to the lamp.
[0182] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and parallel to the lamp.
[0183] (2) FIG. 28; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0184] FIG. 29; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +80.degree..
[0185] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=-5.degree.,
.phi.=+10.degree., i.e., peaks are measured roughly in front of the
surface light source device. For each graph, the whole shape is
roughly symmetrical about the peak and shows a hill-shaped profile
having feet. However, a flat portion is observed around the peaks
(.theta.=around 0.degree. to .+-.15.degree.) in the plane parallel
to the lamp. A rather great ridged portions exist around
.phi.=+70.degree. in the plane parallel to the lamp.
[0186] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp is considerably narrow but
the visual field in the plane parallel to the lamp is not very
narrow. That is, the characteristics of the present invention are
also very similar to those of Example 8. Narrowing of the visual
field in the plane vertical to the lamp is considerably clear but
narrowing of the visual field in the plane vertical to lamp is not
very clear. The levels of brightness of the peaks are very high. It
is obvious that increase of the brightness has been
accomplished.
GRAPHS OF FIGS. 30 AND 31
Example 11 (Group II)
[0187] (1) PS1; prismatic vertical angle .psi.=100.degree.; the
channels are directed outward and parallel to the lamp.
[0188] PS2; prismatic vertical angle .psi.=90.degree.; the channels
are directed outward and vertical to the lamp.
[0189] (2) FIG. 30; At .phi.=0.degree., .theta. is scanned from
-80.degree. to +80.degree..
[0190] FIG. 31; At .theta.=0.degree., .phi. is scanned from
-80.degree. to +8020 .
[0191] (3) Explanation; It can be seen from both graphs that peaks
are observed in the direction .theta.=0.degree., .phi.=+10.degree.,
i.e., peaks are observed roughly in front of the surface light
source device. For each graph, the whole shape is roughly
symmetrical about the peak and shows a hill-shaped profile having
feet. However, a flat portion is observed around the peaks
(.theta.=around 0.degree. to .+-.15.degree.) in the plane parallel
to the lamp. Small ridged portions exist around
.phi.=.+-.70.degree. in the plane parallel to the lamp.
[0192] It can be seen from the spread of each graph that the visual
field in the plane vertical to the lamp is considerably narrow but
the visual field in the plane parallel to the lamp is not very
narrow. That is, the characteristics of the present invention are
also very similar to those of Example 8. Narrowing of the visual
field in the plane vertical to the lamp is considerably clear but
narrowing of the visual field in the plane vertical to lamp is not
very clear. The levels of brightness of the peaks are very high. It
is obvious that increase of the brightness has been
accomplished.
[0193] The results of verifications of the performance, or
narrowing of the visual field and increase of the brightness, of
the eleven examples have been described thus far using data
obtained by actual measurements. Similar measurements were also
made under various prismatic vertical angle conditions. The results
of the measurements are listed in Table 3, together with
referential examples shown in the graphs of FIGS. 6 and 7.
[0194] This Table 3 shows configurations of prism sheets, the
characteristics in the plane parallel to the lamp, the
characteristics in the plane vertical to the lamp, data about the
visual angle and, in the final column, total evaluations.
[0195] The evaluations were made based on the referential examples
of FIGS. 6 and 7 in which a prism sheet having a prismatic vertical
angle of 64.degree. is singly disposed the inward. Narrowing of the
visual field and increase of he brightness have been evaluated in
terms of 4 levels (.circleincircle., O, .DELTA., x). The meanings
of the symbols are as follows.
[0196] .circleincircle.: Conspicuous narrowing of the visual field
has been achieved compared with the referential examples. Also,
increase of the brightness is clear.
[0197] O: The level of achievement of narrowing of the visual field
is secondary to the levels indicated by ;. Sufficient increase of
the brightness has been accomplished.
[0198] .DELTA.: Greater increase of the brightness is noticed than
those of the referential examples. However, it cannot be said that
narrowing of the visual field is conspicuous.
[0199] x: Either increase of the brightness or narrowing of the
visual field is unsatisfactory and, totally saying, the performance
is not worth highly mentioning.
3TABLE 3 PERFORMANCE LIST OF FLUX COLLIMATION SURFACE LIGHT SOURCE
DEVICE WITH PRISM SHEET ARRANGEMENT PRISM LAMP-PARALLEL
LAMP-PERPENDICULAR SHEET ARRANGEMENT CHARACTERISTICS
CHARACTERISTICS 1st 2nd PEAK PEAK ANGULAR PEAK PEAK ANGULAR VERT.
ANG. VERT. ANG. LUMINANCE ANGLE FIELD LUMINANCE ANGLE FIELD
PAR./PERP. PAR./PERP. (nt) (deg) (deg) (nt) (deg) (deg) EVALUATION
64.degree. PAR. NONE 4516 0 .+-.44.4 4506 0 .+-.18.4 -- (INWD)
70.degree. PAR. 70.degree. PERP. 3535 -5 .+-.23.5 3416 -5 .+-.29.3
x 70.degree. PAR. 80.degree. PERP. 4353 0 .+-.29.5 5162 +5 .+-.21.3
.DELTA. 70.degree. PAR. 90.degree. PERP. 5882 0 .+-.20.8 6081 0
.+-.18.9 .largecircle. 70.degree. PAR. 100.degree. PERP. 4806 0
.+-.23.6 5615 +5 .+-.21.8 .largecircle. 70.degree. PAR. 110.degree.
PERP. 4106 +5 .+-.32.2 6048 +10 .+-.17.8 .DELTA. 70.degree. PERP.
70.degree. PAR. 2983 -5 .+-.40.1 3099 -5 .+-.25.7 x 70.degree.
PERP. 80.degree. PAR. 3829 0 .+-.25.5 4235 +5 .+-.27.8 x 70.degree.
PERP. 90.degree. PAR. 5310 0 .+-.20.2 5537 0 .+-.22.6 .largecircle.
70.degree. PERP. 100.degree. PAR. 4753 0 .+-.25.9 5279 +5 .+-.24.0
.DELTA. 70.degree. PERP. 110.degree. PAR. 3591 +5 .+-.28.4 4855 +10
.+-.25.3 .DELTA. 80.degree. PAR. 90.degree. PERP. 4348 -5 .+-.29.3
5897 +10 .+-.25.9 .DELTA. 80.degree. PERP. 90.degree. PAR. 4704 +5
.+-.32.7 6183 +10 .+-.22.0 .DELTA. 90.degree. PAR. 70.degree. PERP.
5335 0 .+-.16.5 5407 0 .+-.17.8 .circleincircle. 90.degree. PAR.
80.degree. PERP. 4070 +5 .+-.31.1 5564 +10 .+-.21.5 .DELTA.
90.degree. PAR. 90.degree. PERP. 6021 0 .+-.23.2 7297 +10 .+-.20.0
.largecircle. 90.degree. PAR. 100.degree. PERP. 5148 +5 .+-.27.1
6838 +10 .+-.22.5 .DELTA. 90.degree. PAR. 110.degree. PERP. 3929
+15 .+-.34.4 6100 +15 .+-.19.1 .DELTA. 90.degree. PERP. 70.degree.
PAR. 5951 0 .+-.18.1 6180 0 .+-.15.8 .circleincircle. 90.degree.
PERP. 80.degree. PAR. 4399 0 .+-.28.0 5866 +10 .+-.24.2 .DELTA.
90.degree. PERP. 90.degree. PAR. 6327 0 .+-.23.4 7278 +5 .+-.19.3
.largecircle. 90.degree. PERP. 100.degree. PAR. 5573 0 .+-.28.1
7079 +10 .+-.21.2 .DELTA. 90.degree. PERP. 110.degree. PAR. 4154 0
.+-.30.1 6345 +15 .+-.22.3 .DELTA. 100.degree. PAR. 90.degree.
PERP. 5195 -5 .+-.27.2 6751 +10 .+-.21.7 .DELTA. 100.degree. PERP.
90.degree. PAR. 5631 0 .+-.27.0 7183 +10 .+-.21.3 .DELTA.
100.degree. PERP. 100.degree. PAR. 4478 0 .+-.31.0 6269 +15
.+-.22.4 .DELTA. 110.degree. PAR. 90.degree. PERP. 4080 +10
.+-.30.2 6019 +15 .+-.24.8 .DELTA. 110.degree. PERP. 90.degree.
PAR. 4314 +10 .+-.35.0 6598 +15 .+-.19.5 .DELTA.
[0200] The results of these total evaluations are consistent many
matters described thus far and sufficiently support the validity of
the conditions (especially, for prism vertical angle .phi.) for
narrowing of the visual field or increase of the brightness.
[0201] The primary light beam source for supplying primary light
beam to the prism sheets is preferably a surface light source means
comprising one lamp and a wedge-shaped light scattering guide with
emitting directivity as used in the examples. It is to be noted
that the primary light beam source is not always limited to this
means. For example, long tubular lamps may be disposed along the
end surfaces of the both sides a flat guide plate with emitting
directivity. In this case, the output characteristics will
correspond to superposed characteristics of a pair of single-light
source means.
[0202] In any of the described examples, if a liquid crystal
display panel is disposed in the passage of light flux produced
from the novel surface light source device, it is obvious that a
liquid crystal display having a viewing screen either observed at a
narrow visual angle or having high brightness is constructed. In
this case, it is desired to select the characteristics of the
narrowed visual field or increased brightness of the backlighting
arrangement according to the desired characteristics of the liquid
crystal display.
[0203] Finally, the materials of the prism sheets and of the light
scattering guide used in the present invention and the method of
fabricating them are described. Various polymer-based materials may
be used for the prism sheets and for light scattering guide used in
the present invention. Representative examples include PMMA
(polymethyl-methacrylate), PSt (polystyrene) and PC (polycarbonate)
as listed in Tables 1 and 2.
[0204] Since the prism sheets are usually transparent, their
material can be employed as it is. The V-shaped channels which give
desired prismatic vertical angles may be formed by well-known
plastic film molding techniques.
[0205] The light scattering guide may be fabricated from a polymer
material by the following method, whether scattering power is
imparted to the prism sheets or not.
[0206] One method employs a molding process including kneading two
or more polymers.
[0207] In particular, two or more polymer materials of different
refractive indices are mixed, heated, and kneaded together. The
polymer materials can take any desired form. Industrially available
polymer materials may be in the form of pellets. The kneaded
material in a liquid phase is injected into the mold of an
injection molding machine under a high pressure. The material is
cooled and solidified. The molded light scattering guide is taken
out of the mold. As a result, the light scattering guide has a
shape conforming to the shape of the mold.
[0208] The kneaded polymers of different refractive indices
solidify without being completely mixed, thereby rendering local
concentrations nonuniform (fluctuated) to give a uniform scattering
power. If the kneaded material is injected into the cylinder of an
extrusion molding machine and extruding the material in a
conventional manner, then molded article can be obtained.
[0209] The combinations of these polymers and their ratios of
mixture can be selected in a very wide choice. They may be
determined by taking account of the refractive index difference,
the strength of refractive index nonuniform structures a produced
through the molding process, and the properties (e.g., the
scattering irradiation parameter E, the correlation distance a and
the dielectric constant fluctuation squares mean .tau.) of the
refractive index nonuniform structures. Representative usable
polymer materials are listed in Tables 1 and 2.
[0210] Another method of forming a material of the light scattering
guide is to uniformly disperse a particle material different in
refractive index to produce a refractive index difference (more
than 0.001) in a polymer material.
[0211] One available method of uniformly dispersing a particle
material is known as suspension polymerization. In particular, the
particle material is mixed into a monomer and undergoes
polymerization while suspended in a hot water. As a result, a
polymer material in which the particle material is uniformly
dispersed can be obtained. If this material is molded, a light
scattering guide of desired shape can be manufactured.
[0212] Furthermore, light scattering guides of various
characteristics can be manufactured by executing suspension
polymerization for various particle materials and monomers (various
combinations of particle concentrations, particle sizes, and
refractive indices) to prepare various kinds of materials and then
by selectively blending and molding them. Through an optical adding
of a polymer containing no particle material, the particle
concentration can be easily controlled.
[0213] A further available method of uniformly dispersing a
particle material is to knead a polymer materials and particle
material. Also in this case, kneading and pelletization processes
are carried out with various combinations of particle materials and
polymers (various combinations of particle concentrations, particle
diameters, refractive indices, and so on). These materials are
selectively mixed and then light scattering guides are molded. In
this way, light scattering guides of varied characteristics can be
obtained.
[0214] The aforementioned method of mixing polymers and the method
of adding particle materials can be combined. For example, a
particle material may be added to polymers of different refractive
indices while they are mixed and kneaded. Since these methods are
well known per se, detailed conditions for manufacturing and so
forth are not described.
[0215] As described in detail thus far, according to the present
invention, narrowing of the visual field of a surface light source
device or increase of the brightness can be accomplished by using
two prism sheets under certain conditions. Utilizing this, the
viewing screen of a liquid crystal display is made brighter.
Particularly, the present invention affords a surface light source
device adapted for the backlighting arrangement of a liquid crystal
display which is preferentially observed from directions in a
narrow angular range.
* * * * *