Liquid crystal display illumination system

An illumination system is provided for a liquid crystal display panel. The panel 1 is placed to transmit light 2 from a light source 3 to a viewing location, the light source being an extended area source of omnidirectional light of high efficiency, such as a fluorescent tube. A light guide 6 is placed between the source and the panel, the light guide comprising a transparent input face 7 adjacent the source, a transparent output face 10 adjacent the panel, and reflective side walls 11 transverse to the input and output faces. The light guide is tapered, the input face being smaller in area than the output face. The divergence of light from the source is thereby reduced as it passes through the panel. The LCD panel then functions with an improved contrast range.

Description 
This invention relates to illumination systems for passive electro-optic 
display panels of the kind which can be used to display television 
pictures for example. The panels are passive in the sense that they do not 
generate light but modulate the brightness of incident illumination to 
provide modulated light for each of the picture elements in the picture 
displayed. More particularly the invention relates to an illumination 
system for a liquid crystal display panel wherein the panel is placed to 
transmit light from a light source to a viewing location. 
Such illumination systems are described in European Patent Application Nos. 
0,192,023A and 0,193,401A wherein projection television systems using 
liquid crystal panels are described. Both these systems use high intensity 
concentrated light sources and optical systems comprising mirrors and 
lenses to pass light through the panel. Projection lenses are then used to 
produce an enlarged image on a viewing screen. The light sources are 
therefore usually tungsten filament lamps of relatively low efficiency, 
requiring relatively high power and producing considerable heat which has 
to be dissipated as in European Patent Application No. 0,192,023A. 
The present invention is more applicable to direct view television 
apparatus and, with portability and power economy in mind, is concerned 
with the use of high efficiency light sources. Such sources are usually 
phosphor layers of extended area emitting visible light when stimulated by 
electrons or by ultraviolet radiation generated in a gas discharge, such 
as in the well known fluorescent tube. 
The light from such a phosphor layer is emitted into the full hemisphere, 
the layer appearing equally bright at all angles of view of the layer 
surface. Such light applied directly to a liquid crystal display panel 
would therefore pass through the panel in a wide range of angles. For many 
types of liquid crystal panel the desired function of modulating the 
intensity of the transmitted light for each picture element in accordance 
with a signal, is only obtained with a good range of contrast over a 
relatively small range of light angles, typically .+-.10 degrees, to the 
normal to the panel. At larger angles of incidence of the light, the 
contrast range is unacceptably degraded. 
It is an object of the invention to enable high efficiency phosphor light 
sources to be used to iluminate a liquid crystal display panel by 
transmission and yet achieve an acceptable contrast range in the image 
displayed. The invention provides an illumination system for a liquid 
crystal display panel wherein the panel is placed to transmit light from a 
light source to a viewing location, characterised in that the light source 
is an extended area source of omnidirectional light, in that a light guide 
is placed between the source and the panel, the light guide comprising a 
transparent input face adjacent the source, a transparent output face 
adjacent the panel, and reflective side walls transverse to the input and 
output faces, and in that the light guide is tapered, the input face being 
smaller in area than the output face. Light enters the input face of the 
tapered guide over a wide range of angles but reflection from the side 
walls reduces the range of angles present in the light emerging from the 
output face. Increasing the ratio of the output face area to the input 
face area is effective in reducing the range of angles, away from the 
normal to the output face, which is present in the output light. However, 
reducing the range of angles also increases the ratio of the length of the 
light guide to the linear dimensions of the output face. 
A material improvement in reduction of the relative length is obtained in 
such an illumination system characterised in that the output face 
comprises a positive lens. The effect of the lens is to refract light 
generally towards the guide axis, that is, to act as a collimating lens. 
For a given range of output angles the length of the guide can be 
materially reduced, leading to a more compact apparatus. 
The guide may be hollow and may be internally reflective. Alternatively, 
the invention may be characterised in that the guide comprises a 
transparent material of a refractive index higher than that of the 
surrounding space whereby the side walls are reflective by total internal 
reflection. In this event the output face may be convex toward the panel 
thereby forming a positive lens. 
The illumination system in accordance with the invention may be further 
characterised in that the side walls are curved in planes transverse to 
the input and output faces. The shape of the side walls can be chosen 
either to reduce the divergence of the output light or, in conjunction 
with the positive lens, to shorten the guide for a given divergence. Parts 
of a guide wall may be convex and other parts concave in some possible 
designs of guide. To obtain efficient reduction in divergence angles and 
reasonably sharp cut-off angles, special optical design techniques are 
employed. These special optical design principles of the tapered guide of 
the present invention are identical to those of the non-imaging 
concentrators of radiation designed, for example, for use as concentrators 
of solar radiation. The only difference between the guide and the 
concentrator being the reversed direction of the radiation. The design 
principles are covered in a book entitled "The optics of nonimaging 
concentrators" by W. T. Welford and R. Winston, Academic Press 1978 in 
chapters 4, 5 et seq., and will not be further referred to here except to 
give examples of guide shapes obtained for various values of the design 
parameters. 
The invention may be further characterised in that a plurality of 
illumination systems are assembled so that the output faces of the light 
guides are adjacent to one another to cover the whole area of an extended 
liquid crystal display panel. If the output faces are identical to one 
another they may be arranged in regular rows and columns. Rectangular 
output and input faces may be used, though other shapes, such as 
triangular or hexagonal faces may be used. 
For high efficiency conversion of electrical energy to light, the 
illumination system may be characterised in that the source comprises a 
phosphor layer energised to produce light. The phosphor layer may be 
energised by ultra-violet radiation from a gas discharge in a fluorescent 
tube, making use of a light source of high brightness and high 
reliability. For high efficiency in use of light from the phosphor layer 
in the case where a plurality of light guides are used, the illumination 
system may be characterised in that feeder light guides are provided one 
each for the tapered light guides, each feeder guide coupling an area of 
the source to the input face of the associated tapered guide. 
To improve viewing conditions the illumination system may be characterised 
in that a light-diffusing screen is placed adjacent to the liquid crystal 
display panel on the side remote from the light guide. The light emerging 
from the liquid crystal display panel is then spread and is visible from a 
reasonably wide range of angles. A displayed television picture, for 
example, can then be seen by several people sitting side-by-side.

Referring to FIG. 1, an embodiment of an illumination system for a liquid 
crystal display panel is shown in accordance with the invention. A liquid 
crystal display panel 1 is placed to transmit light 2 from a light source 
3 to a relatively distant viewing location, not shown, off to the left of 
FIG. 1. The panel is shown divided into sixteen separate pixels (picture 
elements) 4, largely for clarity in the drawing. The panel may in 
practice, at one extreme, comprise an array of pixels equal in number to 
those in a normal television picture. At the other extreme, the panel may 
comprise a single pixel, a television picture being assembled from an 
array of such pixels, each with its own illumination system. In between 
these extremes the panel may be proportional to provide a subset of pixels 
of the whole television picture, an array of such panels being assembled 
to provide all the pixels of the T.V. frame. 
The light source 3 comprises a fluorescent tube having an extended area of 
phosphor energised by ultra-violet radiation from a mercury gas discharge 
in the well known manner. The phosphor is very nearly a lambertian 
emitter, that is to say the phosphor on the inside of the glass envelope 5 
appears equally bright at all angles of view to the surface. The 
cylindrical envelope of the tube then appears as a bar of light of uniform 
brightness across its width. Each point on the phosphor surface therefore 
emits light omnidirectionally. A light guide 6 is placed between the 
source 3 and the panel 1. In this example the guide 6 may consist of a 
transparent plastics material, such as polymethylmethacrylate (PMMA), and 
has a transparent input face 7 either placed directly in contact with the 
tube 3 or coupled by a parallel sided guide 8 to the cylindrical surface 9 
of the fluorescent tube. The guide 6 has a transparent output face 10 
adjacent the panel and covering the area of panel 1. In this example the 
guide is shown having a square cross-section. Guide 6 is tapered, the 
input face 7 being smaller in area than the output face 10. In this 
example, the linear dimension of the output face is four times that of the 
input face so that the input face area is one sixteenth of the output face 
area. The walls 11 of the guide are polished smooth and are reflective to 
light inside the guide by virtue of the total internal reflection (T.I.R). 
The refractive index of PMMA is 1.495 so that the critical angle for 
T.I.R. is 42 degrees, light incident on the walls internally at angles to 
the normal greater than 42 degrees being totally reflected back into the 
guide. The parallel sided guide 8, used to feed light from the source to 
the input face is also polished smooth on all its faces and is in optical 
contact with the input face 7 so that there are no refraction or 
reflection effects at the interface. The input face 9 of guide 8 will 
typically be in touching but not optical contact with the cylindrical 
surface of the tube. The input face 9 is shown curved to fit the 
cylindrical envelope 5 in the FIG. 1 embodiment, but it could equally well 
be flat. In order to ensure a sealed interface between the guide and tube 
to exclude scattering or absorbing contaminants, the guide may optionally 
be optically bonded to the tube. In this case, a proportion of the light 
entering the guide escapes from the guide at first incidence because it is 
less than the critical angle. A light absorbing jacket, not shown, may 
optionally be provided around the tapered guide, but not in optical 
contact with it, to remove stray light. 
The end 12 of the guide 6 is shown as flat and optically bonded to a 
plano-convex lens 13, which may possibly be of the same material as the 
guide, in which case the guide and lens may be made as one piece. The 
action of the lens 13 is to provide a general collimating effect on 
divergent light emerging from the guide, reducing the length of the guide. 
The action of the tapered guide in reducing the divergence of light 
entering the small input face, passing down the guide, and leaving by the 
large output face is covered in detail, with the light direction exactly 
reversed, in the book by Welford and Winston cited in the preamble of this 
specification. Therein the action of non-imaging radiation concentrators 
is analysed in detail. In these concentrators fairly highly collimated 
radiation, usually direct sunshine, is taken in over a large area 
(analogous to our output face) and concentrated in a highly divergent form 
on a desirably much smaller area radiation collector (analogous our input 
face). The collector is either a heat collector in which it is desired to 
produce as high a temperature as possible or is a photovoltaic collector 
which is desirably of small area in view of the high relative cost of 
these devices. However the divergence reducing action of the tapered guide 
can be seen from the fact that a ray from the small input face passing 
down the guide at an angle to the guide axis will strike a reflective side 
wall which is itself inclined away from the guide axis because of the 
designed taper and will therefore be reflected back into the guide at a 
smaller angle to the guide axis than it had before reflection. In 
contrast, in a parallel sided guide no change of a ray angle magnitude to 
the guide axis occurs. One reflection may be sufficient to reduce the 
divergence to the desired extent. 
With many types of liquid crystal displays it is desirable that the 
divergence of the light passing through the liquid crystal layer should be 
no greater than .+-.15 degrees, and preferably .+-.10 degrees, to obtain a 
full contrast range. This is a fact well known from the operation of the 
layer in attenuating light passing through it to a controlled extent. 
The desired maximum divergence is determined by the ratio of the dimensions 
of the input and output faces. If the light guide is to have the ideal 
characteristic of sharp cut-off angles, the profile of the guide is 
determined in relation to the power of the positive lens on the output 
face. FIG. 2 shows an XY coordinate system which will be used to describe 
the profiles of some examples of guides. In these examples the profiles 
are in two parts, the first part extending from X=0 at the input face 7 to 
X=X.sub.1, the second part extending from X=X.sub.1 to X=X.sub.2 at the 
output face 10 with a smooth transition between the two parts at 
X=X.sub.1. The output face 10 can have a substantially spherical or 
cylindrical radius of curvature R. It could also be compounded of two 
cylindrical surfaces superposed with their cylinder axes at right angles. 
The guide is symmetrical about the X axis, each positive Y ordinate having 
an equal negative Y ordinate. 
FIG. 3 shows ray paths in a tapered light guide having a positive lens 13 
on the output face. The ratio of the linear dimension of the output face 
to the input face is four to one, as it is in FIG. 4 which shows a guide 
with a flat output face. In FIG. 3 rays 30, 31 and 32 originate from three 
points 33, 34 and 35 respectively on the input face having entered at the 
critical angle, that is, as the greatest angle to the axis 39 that is 
possible for them. These rays are reflected off the first part of the 
profile and all emerge at approximately 15 degrees to the axis after 
passing through lens 13. Rays from any part of the input face at smaller 
angles to the axis but directed at a wall are reflected at least once off 
some part of the wall to emerge at 15 degrees or less to the axis, rays 36 
and 37 being examples of rays from point 35, ray 37 being incident on the 
wall with a larger angle of incidence than ray 36. After passing through 
lens 13 rays 36 and 37 emerge at approximately 15 degrees to the axis 39. 
Any ray from the input face not incident upon a wall is incident directly 
on the lens 13 and emerges at an angle of less than 15 degrees on one side 
or the other of the axis 39. For clarity only rays on one side of the axis 
are shown, rays on the other side being a mirror image of those shown. 
In FIG. 4 the rays from corresponding points have the same units numeral as 
the rays in FIG. 3. It will be seen that the absence of the positive lens 
on the output face necessitates a considerably longer guide in relation to 
the input and output face dimensions. 
There now follows five particular examples of numerically defined profiles. 
The first two examples show how the length of the light guide is dependent 
on the radius of the curved output face. They have identical input and 
output apertures, and are made of the same material. Dimensions are given 
in arbitrary units, since the guides would be scaled up or down in size to 
suit the particular application. 
EXAMPLE 1 
An angle reducing light guide with maximum output angle of .+-.15 degrees 
in which the input aperture is 2.5, the output aperture is 10.0, and 
length is 29.58. 
The input face is flat, and the output face is curved with a radius of 40.0 
convex to the outside. The light guide is made of a material which has a 
refractive index of 1.495, such as PMMA or an appropriate glass. 
The profile of the curved side surfaces of the light guide is given by the 
equation: 
EQU y=A.sub.0 +A.sub.1 x+A.sub.2 x.sup.2 +A.sub.3 x.sup.3 +A.sub.4 x.sup.4 
+A.sub.5 x.sup.5 +A.sub.6 x.sup.6 
where x is the distance along the axis of the light guide measured from the 
flat input face, y is the height of the profile above this axis, and 
A.sub.0 to A.sub.6 are coefficient as given below. 
The profile is in two sections with a smooth transition at the joint. 
From x=0 to x=3.987, the coefficients are 
##EQU1## 
From x=3.987 to x=29.268, the coefficients are 
##EQU2## 
EXAMPLE 2 
An angle reducing light guide similar to Example 1, with maximum output 
angle of .+-.15 degrees in which the input aperture is 2.5, and the output 
aperture is 10.0, but with a much reduced length of 16.65. 
The input face is flat, and the output face is curved with a radius of 8.0 
convex to the outside. The light guide is made of a material which has a 
refractive index of 1.495, such as PMMA or an appropriate glass. 
The profile of the curved side surfaces of the light guide is given by the 
same equation as in Example 1, the coefficients being given below. 
The profile is in two sections with a smooth transition at the joint. 
From x=0 to x=3.765, the coefficients are 
##EQU3## 
From x=3.765 to x=14.894, the coefficients are 
##EQU4## 
The next example shows how smaller output angles may be obtained and how 
other materials may be used. The dimensions given are smaller to indicate 
that a large number of very small light guides may be used with consequent 
reduction of the depth of the system. It is possible to have one light 
guide for each picture element, light feed arrangements being shown later. 
EXAMPLE 3 
An angle reducing light guide with maximum output angle of .+-.10 degrees 
in which the input aperture is 0.1667, the output aperture is 1.0, and 
length is 2.965. 
The input face is flat, and the output face is curved with a radius of 2.0 
convex to the outside. The light guide is made of a material which has a 
refractive index of 1.57, such as polycarbonate or an appropriate glass. 
The profile of the curved side surfaces of the light guide is given by the 
equation as before, the coefficients being given below. 
The profile is in two sections with a smooth transition at the joint. 
From x=0 to x=0.2966, the coefficients are 
##EQU5## 
From x=0.2966 to x=2.901, the coefficients are 
##EQU6## 
The next example has similar output angles to Example 3, and also shows how 
an output lens of different material may be used. The dimensions given are 
larger to indicate that a small number of large light guides may be used 
with consequent increase in the depth of the system. It is possible to 
have one light guide for the whole picture. 
EXAMPLE 4 
An angle reducing light guide with maximum output angle of .+-.10 degrees, 
in which the input aperture is 33.3, the output aperture is 200, and 
length is 253.5. 
The main part of the light guide has flat input and output faces, and is 
made of a material which has a refractive index of 1.495, such as PMMA or 
an appropriate glass. A plano-convex lens with a radius of 150 on the 
convex surface, and made of a glass or plastic material with refractive 
index 1.75, is cemented or moulded to the output face. 
The profile of the curved side surfaces of the light guide is given by the 
equation as before, the coefficients being given below. 
The profile is in two sections with a smooth transition at the joint. 
From x=0 to x=54.92, the coefficient are 
##EQU7## 
From x=54.92 to x=232.2, the coefficients are 
##EQU8## 
An examination of the profiles in Example 2 shows that they approximate to 
linear profiles over most of their length. It is therefore possible to 
design a light guide which is of much simpler construction, but with a 
performance slightly inferior to a light guide which has the ideal shape. 
The most noticeable effect will be that the cut-off of the output angle 
will not be so sharp. 
EXAMPLE 5 
An angle reducing light guide similar to Example 2, with nominal output 
angle of .+-.15 degrees, in which the input aperture is 2.5, and the 
output aperture is 10.0, and with a length of 16.65. 
The input face is flat, and the output face is curved with a radius of 8.0 
convex to the outside. The light guide is made of a material which has a 
refractive index of 1.495, such as PMMA or an appropriate glass. 
The profile of the side surfaces of the light guide is given by the 
simplified equation: 
EQU y=A.sub.0 +A.sub.1 x 
where x is the distance along the axis of the light guide measured from the 
flat input face, y is the height of the profile above this axis, and 
A.sub.0 and A.sub.1 are coefficients as given below. 
The coefficients for the profile are 
##EQU9## 
All the light guides in the examples given have been solid transparent 
bodies, using total internal reflection to produce reflective side walls. 
To give a light weight structure, the guides can alternatively be hollow 
shapes with relatively thin walls coated on the inside with a reflective 
layer. The input and output faces then have no interface surface, except 
when a positive lens is attached over the output face. 
In the event that a television picture is displayed, the output light 
divergence of .+-.15 degrees or less may not render the picture visible 
over a sufficient range of angles, particularly in the horizontal plane, 
to accommodate a number of viewing persons seated in a row in front of the 
picture. A diffusing screen may then be placed adjacent to the liquid 
crystal display panel on the side remote from the light guide. A simple 
diffuser such as a ground glass surface may spread the output light in the 
vertical plane, towards the ceiling and floor of the room where it is not 
required and with the consequence that the brightness of the picture seen 
by the viewers in the horizontal plane is reduced. A vertically oriented 
lenticular lens system may be preferable with a separate vertical 
cylindrical lens for each column of picture elements or for a number of 
columns of picture elements. The angle of light spread in the horizontal 
plane can then be controlled with little spread in the vertical plane. 
However, in other applications the limited divergence of light output from 
the guides may not be a disadvantage and may even be turned to advantage. 
In applications where there is only one viewer, such as the head-up 
display in a single seat aircraft cockpit, the limited divergence would be 
no disadvantage. In bank terminals, cash-points, and the like, the limited 
divergence would assist in providing a display restricted to viewing by 
the user only. 
FIG. 5 shows a rectangular array 50 of 108 square output face tapered light 
guides 51 arranged in a 12.times.9 array to cover an extended area liquid 
crystal display panel having an aspect ratio of 4:3 as required for a 
television picture. Thus each guide illuminates one ninth of the height or 
about 67 lines of a nominal 625 line picture. 
FIG. 6 shows a small part of the array of FIG. 5 with a feeder light guide 
arrangement 80 for feeding each of a column A of tapered guides with light 
from a common source. A column of four adjacent guides 81, 82, 83 and 84 
are shown fed by a closed-packed assembly of four parallel-sided 
square-section solid feed guides 85, 86, 87 and 88 respectively. Feed 
guide 85 is straight with a 45 degree face 89 opposite the input face of 
guide 81. Light 90 fed into guide 85 passes by T.I.R. down the guide and 
is reflected, also by T.I.R., by face 89 into the guide. Guide 86 is also 
initially straight and is close packed alongside guide 85 to which it may 
or may not be bonded in optical contact. Spreading of light 90 between the 
parallel parts of the feed guides may be of assistance in equalising the 
light inputs to the guides. A solid, mechanically robust light feeder 
guide assembly may be desirable. Guide 86 has a swan-neck portion 91 to 
bring its output end into line with the second guide 82 of the column. 
Feeder guides 87 and 88 are very similar, differing only in being 
progressively longer and having more accentuated swan-neck portions. 
FIG. 6 shows a second column B of four tapered guides fed by a set of four 
feeder guides identical to guides 85, 86, 87 and 88. The input ends of the 
eight guides are arranged in a line to face a planar light source. 
FIG. 7 shows the first two guides 92, 93 of a four guide column extension 
to column A of FIG. 6. The respective feeder guides 94 and 95 are stacked 
immediately above and parallel to guides 85 and 86 respectively. The 
swan-neck portion 96 is provided to lower the guide 94 onto the input face 
of tapered guide 92. The swan-neck portion 97 not only brings the output 
end of the feeder guide into line with column A but also lowers it to meet 
the input face of tapered guide 93. The feeder guides are preferably in 
optical contact with, and could be mechanically bonded to, their 
respective tapered guides. This avoids an optical interface where 
scattering and loss of light by contaminants might occur and also provides 
mechanical robustness. 
FIG. 8 shows an arrangement for coupling a bundle 100 of close-packed 
feeder guides to a fluorescent tube 101. The end 102 of the bundle is 
formed into a polished cylindrical surface to fit the glass envelope 103 
of the tube and is butted against it. Except in the area of the tube 
facing bundle end 102, the fluorescent tube has a reflective layer 104 to 
reflect light back into the tube. After scattering, some of this reflected 
light will emerge from the tube opposite bundle end 102 increasing the 
quantity of light entering the feeder guides. 
FIG. 9 shows a cross section of an alternative form of fluorescent tube 
light source. The envelope 105 of the tube is made of quartz, or other 
material transparent to the 365 nm line of the mercury gas discharge. The 
reflective layer 104 is present in the same position and extent as in the 
FIG. 8 tube. However, there is no phosphor on the inside of the tube 
opposite the reflective layer, only opposite the bundle end 102 at 106. 
Thus light is only produced immediately adjacent to the bundle end, the 
reflective layer 104 acting to concentrate ultra-violet radiation on the 
phosphor. Light absorption losses are therefore much reduced as compared 
to the FIG. 8 tube. The power consumption of the FIG. 9 tube can therefore 
be reduced for a given phosphor brightness. 
FIG. 10 shows the feeder guides of FIG. 9 separated, 98, each being normal 
to the phosphor layer 106 so that the illumination conditions at the guide 
inputs are more nearly identical. 
FIG. 11 shows a front view of part of a close-packed array of hexagonal 
cross-section light guides. The hexagonal output faces 60 are 
close-packed, the input faces 61 being separated but still arranged in 
columns. Thus a feeder guide arrangement as shown in FIGS. 6 and 7 can be 
used to feed the hexagonal guides, the only changes being that each feeder 
guide would be hexagonal in cross-section, and alternate columns would 
have different length feeder guides. 
Desirably, it should be possible to display a colour television picture. In 
this event each pixel is a cluster of three sub-pixels, red, blue and 
green. If the picture is viewed at angles away from the normal to the 
panel, the relative brightnesses of the three sub-pixels in a pixel should 
not change as a function of viewing angle, otherwise the hue of the pixel 
will change as a function of angle. The absolute brightness of the pixel 
may well fall off with increasing angle of view. It is important, 
therefore, that the horizontal light diffusion polar diagram should be 
well controlled and equal for the three sub-pixels of any pixel. FIG. 12 
shows a tapered guide 70 illuminating one or more pixels or sub-pixel 
panel area 71. A positive or negative power lens array 72 is provided 
beyond the panel 71, the figure showing the cross-section of a cylindrical 
lens accepting light from a vertical column of panels 71 normal to the 
plane of the drawing. The light diffusion polar diagram can be controlled 
via the choice of the lens power. Being a lens rather than a scattering 
element, there is much lower loss of light by scattering, the light being 
directed only where it is required. 
FIG. 13 shows a tapered guide 130 feeding a single pixel having three 
sub-pixels 131, 132, 133 with positive lenticular lenses 134, 135, 136 
respectively, the sub-pixels being the red, blue and green parts of a 
colour pixel. Light passing through each sub-pixel is brought to a focus 
immediately in front of the lens and diverges thereafter. 
Returning to the array of FIG. 5 and the feeder guide arrangement of FIGS. 
6 and 7, it is desirable that a single source of light should illuminate 
the whole panel. Ageing of the light source will then not produce 
differences in brighnesses between pixels. But a column of tapered guides 
could be split into two halves and a separate feeder guide arrangement be 
provided for the two halves, each fed from its own light source. A more 
compact display panel with fewer assembly problems would result.