Note: Descriptions are shown in the official language in which they were submitted.
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The present invention is directed generally to
anti-glare devices and more particularly to a filter for a
transmission screen or liquid crystal display (LCD) for
eliminating glare caused by ambient light.
It has long been recognized that the front face of
a transmission screen such as the cathode ray tube (CRT) of
an ordinary television screen or the CRT of a computer
terminal produces glare caused by ambient light. As a
result of this phenomena, a substantial amount of glare is
encountered when viewing, for example, a CRT in a brightly
lit room. The glare problem can be reduced by increasing
the intensity of the radiation from the CRT. However,
manufacturing CRT's capable of producing the intensity
necessary to overcome glare encountered in a brightly lit
room greatly increases the cost of the CRT. It is therefore
desirable to produce a filter or overlay which will reduce
the glare produced by ambient light.
In my U. S. Patent No. 4,165,920, a front face
glare reduction overlay including an echo reduction
improvement is disclosed. The improvement involves applying
a coating of opaque material to the peaks of the sawtooth
forming the front surface of the overlay. It is disclosed
in the '920 patent that the horizontal portion of the
sawtooth should be inclined approximately six degrees from
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the horizontal, and the vertical portion of the sawtooth
should be inclined approximately thirty degrees from the
vertical. Using these angles in conjunction with the opaque
material provides a front face glare reduction overlay which
produces satisfactory results. However, despite the
satisfactory results, it proved difficult to manufacture the
peaks of the sawtooth with a sharp point. Because the peaks
tended to have a round or lenticular shape, they were
difficult to coat with the opaque material. Light from the
transmission screen was refracted by the uncoated or
partially coated peaks in a number of different vertical
directions, thereby creating a further problem with echo
images.
In an effort to provide an overlay or filter for
the viewing surface of a transmission screen to reduce front
face glare which is both inexpensive and easy to
manufacture, I developed an anti-glare device which is
described and illustrated in U. S. Patent No. 4,473,277.
The anti-glare device of the '277 patent need not be applied
directly to the surface of the CRT but can, in fact, be
positioned a discrete distance therefrom. With the back
surface thus free of the requirement of being tightly fitted
to the transmission screen, it was discovered that the
opaque material applied to the peaks of the sawtooth could
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be eliminated if a circular polarizer or some similar filter
means was mounted to the back surface of the anti-glare
device. The anti-glare device disclosed in the '277 patent
proved to be easy to manufacture and free of echo image
problems. However, the cost of the circular polarizer
together with the time and expense required by the bonding
step which bonded the circular polarizer to the anti-glare
device increased the cost of the device.
According to the present invention, there is
provided a filter for reducing the glare of a viewing screen
caused by ambient light, the filter comprising a
substantially transparent sheet of material having first and
second faces, said first face being substantially planar and
said second face having a plurality of V-shaped grooves,
each groove being formed by two walls, and wherein one of
said walls is inclined at an angle of or above the critical
angle of said material with respect to said first face, so
that ambient light entering said second face, passing
through said first face, reflecting off said viewing screen,
and passing back through said first face is internally
reflected.
According to the present invention, there is also
provided a method for reducing the glare of a viewing screen
caused by ambient light, comprising the steps of providing a
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filter and positioning the filter in front of the said
viewing screen, wherein the filter comprises a substantially
transparent sheet of material having first and second faces,
said first face being substantially planar and said second
face havinq a plurality of V-shaped grooves, each groove
being formed by two walls, one of said walls being inclined
at an angle of at least the critical angle of said material
with respect to said first face, so that ambient light
entering said second face, passing through said first face,
reflecting off said viewing screen, and passing back trhough
said first face, is internally reflected.
In order that the invention may be fully
understood, it will now be described with reference to the
accompanying drawings in which:
Figure 1 is a side view of a portion of an
anti-glare filter constructed according to the teachings of
the present invention and a portion of a transmission
screen, together with illustrative light rays demonstrating
the operation of the present invention;
Figure 2 is a side view of a portion of an
anti-glare filter having roughened and blackened surfaces
which improve the optical qualities of the filter; and
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Figure 3 is a side view of a portion of an
anti-glare filter together with illustrative light rays
emanating from two light sources.
The present invention is directed to an anti-glare
filter 10, a portion of which is shown in profile in
Figure 1. When the anti-glare filter 10 is placed in front
of a CRT or other back projection display, or in front of a
liquid crystal display (LCD), it interrupts ambient light
rays and light rays which have reflected from the face of
the CRT and blocks their exit from the front face of the
filter such that they are not seen by the operator.
Figure 1 illustrates the basic principle of the
present invention. The anti-glare filter 10 is placed in
front of the viewing screen of a CRT 12. The filter 10 is
comprised of a sheet of material having a substantially
planar first or back face 14 and a second or front face 16
having a plurality of v-shaped grooves. Each of the grooves
is formed by two walls, a first wall 18 and a second
wall 20. The first wall 18 is inclined at an angle 0 of at
least the critical angle for the material of the anti-glare
filter 10 with respect to the first face 14. This
inclination provides the anti-glare filter 10 with unique
properties which are a result of the filter's geometry. The
selection of the angle of inclination 0 of the first wall 18
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with respect to the first face 14 is an important feature of
the present invention and is discussed more fully
hereinbelow.
The second wall 20 is shown in Figure 1 as being
substantially perpendicular to the first face 14 although it
has been found that the second wall 20 may be inclined at an
angle of as much as between plus or minus ten degrees with
respect to a plane perpendicular to the first face 14.
In Figure l, the anti-glare filter 10 is
constructed such that the first wall 18 is inclined at an
angle 0 of thirty-nine degrees with respect to the first
face 14. Light ray no. 1 is the most critical light ray
which can enter the first wall 18. Light ray no. 1 is
representative of ambient light coming from the ceiling
area, where most of the distracting light originates, from
the nearest overhead light source. Assuming the index of
refraction of the material comprising the anti-glare
filter 10 is 1.6, light ray no. 1 will, upon entering the
second face 16, be refracted according to Snell's Law which
provides:
nl*sin ~i = n2*sin ~r . . . . . . . . eq (1)
where nl = the index of refraction of material
no. 1,
ai = the angle of incidence,
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n2 = the index of refraction of material no. 2,
and
~r = the angle of refraction.
For the above equation, nl equals 1 for a light
ray travelling through air, ~i equals ninety degrees, and n2
equals 1.6. Solving equation (1) for the angle of
refraction ~r yields a value of thirty-nine degrees which
is measured from a plane perpendicular to the first wall 18.
Thus, light ray no. 1, upon entering the anti-glare
filter 10, is refracted at an angle of thirty-nine degrees
which results in ray no. 1 proceeding substantially
horizontally along path 22 until striking the first face 14
substantially perpendicularly.
The light ray no. 1, upon striking the first
face 14, is reflected back along the same path 22 previously
traveled until it strikes the second face 16 at point A,
where it originally entered the anti-glare filter 10. The
angle at which the portion 22 of ray no. 1 strikes point A
of first wall 18 is thirty-nine degrees. Solving equation
(1) where nl now equals 1.6, ~i equals thirty-nine degrees,
and n2 equals 1, the angle of refraction ~r is calculated to
be ninety degrees. This indicates that this portion 22 of
ray no. 1 is the most critical ray which can escape from the
filter 10. Any portion 22 of ray no. 1 striking point A at
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an angle greater than thirty-nine degrees will be internally
reflected as shown in Figure 1.
It can be demonstrated that after ray no. 1 is
internally reflected at point A, it repeatedly strikes the
first face 14 and second wall 18 at angles greater than
thirty-nine degrees. Therefore, there will never be a
refracted ray no. 1 from these surfaces. Ray no. 1 is thus
said to be totally internally reflected such that no glare
is caused by this light ray.
By solving equation (1) for various other angles
of inclination 0 of the first wall 18, it can be
demonstrated that the critical angle for a given material
must satisfy the following equation:
sin (critical angle) = l/index of refraction of
material. . . . eq(2)
If the angle of inclination 0 of the first wall 18
is less than the critical angle, light ray no. 1 will not
strike the first face 14 perpendicularly. Thereafter,
reflected ray no. 1 will not strike the first wall 18 at an
angle greater than or equal to the critical angle such that
total internal reflection will not be achieved. Conversely,
if the angle of inclination 0 of the first wall 18 is
greater than the critical angle, ray no. 1 will be reflected
off the first face 14 and will strike the first wall 18 at
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an angle greater than the critical angle such that the
reflected ray no. 1 will be totally internally reflected.
Thus, it is necessary for proper operation of the present
invention that the angle of inclination 0 of the first
wall 18 with respect to the first face 14 be at least
substantially equal to the critical angle for the material
of which the anti-glare filter 10 is constructed.
Listed below are the critical angles of various
materials which may be used to construct the anti-glare
filter 10 of the present invention.
MATERIAL CRITICAL ANGhE
polymethylacrylate 42.53
polyethylacrylate 42.92
polytutylacrylate 43.01
polyethoxyethylacrylate 42.83
poly (2 methoxyethyl) acrylate 43.12
poly (2 bromo sec. butyl) acrylate 40.43
poly (2 bromo phenyl) acrylate 38.34
poly (2 chloromethyl) acrylate 41.23
polyacrylontitrile 41.47
polymethylmethacrylate 42.16
polyethylmethacrylate 42.33
poly butyl methacrylate 42.4
poly (t-butyl) methacrylate 43.09
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polycyclohexyl methacrylate 41.59
poly (2-hydroxyethyl) methacrylate 41.41
poly (2-phenoxyethyl) methacrylate 39.96
poly phenylmethacrylate 34.82
poly (o-chloro) styrene 38.4
poly (2.6 dichloro) styrene 37.99
poly (O-methoxy) styrene 38.87
polyacetal 41.47
poly (n-benzyl) methacrylamide 38.78
poly (N-butyl) methacrylamide 41.36
polyvinyl chloride 40.53
polyvinyl fluoride 38.68
polyvinylidene chloride 38.68
polyvinyl acetate 42.97
polyvinyl carbazole 36.36
polyvinyl isobutyl ether 43.58
polyvinyl alcohol 41.81
poly (n-vinyl) phthalimide 38.13
polyallyl phthalate 41.21
polyester-Styrene 40.5
poly (o-tolyl) methacrylate 39.54
poly carbonates (bisphenol) 39.12
poly (N-2, phenethy) methacrylamide) 39.01
polystyrene 38.93
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zinc crown glass 41.24
higher dispersion crown glass 41.14
light flint glass 39.41
heavy flint glass 37.31
heaviest flint glass 31.94
Returning now to Figure 1, light ray no. 1, upon
striking the first face 14, is partially reflected back upon
itself as discussed above while a portion 24 of the light
ray no. 1 passes through the first face 14. The portion 24
of light ray no. 1 passing through the first face 14
impinges upon the face of the CRT 12 at point B. Because
the portion 24 of ray no. 1 strikes the CRT 12
perpendicularly, it is reflected back upon itself, through
first face 14, and back to point A where it strikes the
first wall 18 at the critical angle of thirty-nine degrees.
Thus, not only is the portion 22 of light ray no. 1 which is
reflected from the first face 14 totally internally
reflected, but the portion 24 of ray no. 1 is also totally
internally reflected after is is reflected from the CRT 12.
In practice, however, most CRT'S are somewhat
curved about a horizontal axis as shown in Figure 1. Thus,
the farther from point B light rays are when they impinge
upon the CRT, the greater the reflected angle. Consider
light ray no. 2 which enters the first face 16 at point C
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and is reflected at an angle of thirty-nine degrees such
that it travels substantially parallel to the portion 22 of
ray no. 1. When ray no. 2 strikes the first face 14, a
portion (not shown) of the ray is reflected back upon itself
and internally reflected just as in the case of ray no. 1.
A portion 26 of ray no. 2 also passes through the first
face 14 and strikes the CRT at point D. Because point D is
displaced thirteen degrees from point B, a portion 28 of ray
no. 2 is reflected at an angle of twenty-six degrees from
the face of the CRT with respect to the portion 26 of ray
no. 2. When the portion 28 of ray no. 2 strikes the first
face 14, it is refracted according to Snell's Law at an
angle of sixteen degrees. Proceeding at such an angle, ray
no. 2 will strike the first wall 18 at an angle greater than
the critical angle such that ray no. 2 will be totally
internally reflected.
Because the radius of curvature increases as you
move farther and farther from point B, the angle of the
light rays reflected off the surface of CRT above point B
becomes greater and greater such that all such light rays
will be totally internally reflected.
The situation is different with respect to the
bottom half of the CRT 12. Light ray no. 3 enters the first
face 16 at point E and travels substantially horizontally
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until the light ray no. 3 strikes the first face 14 of the
anti-glare filter 10. A portion of the ray no. 3 will be
reflected back upon itself (not shown) and a portion 30 of
ray no. 3 will pass through the first face 14. The portion
of the ray 30 passing through the first face 14 will be
reflected off the CRT 12 at an angle of twenty-six degrees.
This reflected portion 32 of the ray no. 3 will strike the
first face 14 and be refracted at an angle of sixteen
degrees. This refracted ray will strike the first wall 18
at an angle less than the critical angle and will therefore
pass through the anti-glare filter and be visible by the
user.
This situation can be cured by increasing the
angle of inclination 0 of the first wall 18 above the
critical angle for the material or, alternatively,
maintaining the angle while changing the material to a
material having a higher index of refraction. If the
difference between the angle of slope of the CRT 12 and the
vertical at the point F is "an, then the increase ( ~ ) in
the angle of inclination 0 of the first wall 18 above the
critical angle of the material is provided by the following
equation:
( ~ ) = (a/index of refraction of the material)
. . . . . eq.(3)
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For the illustrated case, "a" equals thirteen
degrees and the index of refraction of the material equals
1.6. Solving the equation yields a value for ~ of 8.08.
If the anqle of inclination 0 of the first wall 18 is
increased by 8.08 as shown by the dotted line in Figure 1,
then ray no. 3' enters the second face 16 at point G. Ray
no. 3' passes through the first face 14 and strikes the face
of the CRT 12 perpendicularly such that ray no. 3' is
reflected back upon itself. Upon passing back through the
first face 14, ray no. 3' strikes the first wall 18 at an
angle which is equal to the critical angle such that ray
no. 3 is totally internally reflected.
Thus, another feature of the present invention is
an anti-glare filter wherein the angle of inclination 0 of
the first wall 18 is chosen to be above the critical angle
with respect to the first face 14 such that ambient light
entering the second face 16, passing through the first
face 14, reflecting off the CRT 12, and passing back through
the first face 14, is totally internally reflected.
As shown in Figure 2, ambient light that has
entered the anti-glare filter 10 and has been entrapped
therein due to total internal reflection, can nonetheless
still cause a distraction as illustrated by ray no. 4. Ray
no. 4 enters the anti-glare filter 10 through first wall 18,
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is reflected off the first face 14, and is then reflected
off the first wall 18 so as to strike second wall 20. Ray
no. 4 strikes the second wall 20 perpendicularly such that
the ray passes through the second wall 20 and out of the
anti-glare filter 10. Ray no. 4 follows the path shown in
Figure 2 and is partially reflected back to the observer's
eye causing the CRT 12 to have an over-all frosted
appearance.
There are three ways to compensate for the light
rays which escape from the anti-glare filter 10. First, the
second wall 20 may be covered with an opaque material which
will absorb light. This is illustrated in conjunction with
ray no. 5 which follows a path similar to ray no. 4 but is
absorbed by the second wall 20 such that no reflected light
reaches the observer. A second approach is to provide the
second wall 20 with a roughened surface. This is shown in
conjunction with eay no. 6. Ray no. 6 follows a path
similar to ray no. 4 but, upon striking second wall 20, is
refracted in numerous directions by the roughened surface
such that a miniscule portion of the light ray may be
reflected back to the observer. A third approach is to
provide a slight tint in the anti-glare filter 10 which will
cut down on the light rays reflected back to the observer as
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well as increasing the contrast of the intelligence
displayed on the CRT.
Another factor which must be considered in
designing an anti-glare filter is the fact that in the case
of a CRT, where the face of the CRT is curved either
spherically or cylindrically, the anti-glare filter must
also be curved to be substantially parallel to the curvature
of the CRT in the horizontal plane. That is, the anti-glare
filter lO must be curved or bent around its vertical axis.
Unless this is done, the rows or lines of text appearing on
the face of the CRT will appear to be arched. This
phenomena is explained in conjunction with Figure 3.
In Figure 3, light source 34 is closer to the
anti-glare filter lO than light source 36. Light ray no. 7
follows the illustrated path and emerges from the second
face 16 of the anti-glare filter lO at point A. Light ray
No. 7 then travels from point A to an observer's eye 38. If
light ray no. 8 from the second light source 36 were to
appear at the same vertical height as that of light ray
no. 7 from the first light source 34, it too would have to
emerge at point A. However, as shown in Figure 3, ray no. 8
follows a path which passes the eye 38 of the observer well
above eye-level and is not seen. The ray which the eye 38
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of the observer does see is illustrated by ray no. 9 which
emerges at point B, well below point A.
Thus, it is apparent that the farther a point is
from the anti-glare filter 10, the lower it appears to be to
an observer. In the case of a CRT, along any horizontal
line, a point at the extreme right or left of the ~RT is
farther from a flat anti-glare filter 10 than a point at the
center and appears to come from a point lower than does a
point at the center. This causes a straight line on the CRT
to be arched when viewed through a flat filter. Thus,
bending the filter such that it is substantially parallel to
the face of the CRT is necessary to overcome this phenomena.
A typical anti-glare filter 10 constructed
according to the teachings of the present invention is, for
example, constructed of a modified acrylic known in the
trade as DR having a thickness of forty mils and an index of
refraction of 1.49. The filter is approximately ten inches
by ten inches with eighty horizontal V-shaped grooves per
inch. Each groove is comprised of a horizontal wall 20
extending approximately twelve and one-half mils into the
material and an inclined wall 18 having a slope of
forty-five degrees with respect to the first face 14. The
filter 10 is then placed an arbitrary distance from the
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viewing screen, with the distance being chosen to achieve
the best overall viewing results.
The anti-glare filter 10 disclosed herein is
particularly advantageous when used in conjunction with a
liquid crystal display (LCD~. Because LCD's depend on
reflected ambient light for visibility and the liquid
crystal is enclosed between two layers of glass, the
surfaces of which are notorius glare producers, the
anti-glare filter 10 of the present invention is extremely
well-suited for LCD's. If a filter, such as a circularly
polarized medium, is placed in front of a LCD to cut out
surface glare, it also cuts out much of the ambient light on
which the LCD depends for visibility and the display becomes
unreadable. However, some filtering is needed since in many
situations there is so much ambient light coming from so
many different directions that the display is all but
useless. The anti-glare filter 10 of the present invention
does not depend on tint or polarization and therefore allows
all the ambient light to enter. At the same time, however,
the anti-glare filter 10 does completely cancel all specular
glare from the glass surfaces. Also, since most LCD's are
flat, there is no curvature complication to contend with as
in the case of CRT's. Therefore, it is very advantageous to
use the present invention in conjunction with LCD's.
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It will be understood that the embodiment
describeæ herein is merely exemplary and that a person of
ordinary skill in the art may make many variations and
modifications without departing from the spirit and scope of
the invention. All such modifications and variations are
intended to be included within the scope of the invention as
defined in the appended claims.
