Anti-glare filter with improved viewing area

A filter for reducing the glare of a viewing screen caused by ambient light and increasing the viewing area constructed of a substantially transparent sheet of material having first and second faces. The first face is substantially planar while the second face has a plurality of V-shaped grooves, each groove being formed by two walls. The geometry of the V-shaped grooves is such that ambient light enters the filter and becomes trapped therein due to internal reflection.

BACKGROUND OF THE INVENTION 
The present invention is directed generally to anti-glare devices, and more 
particularly, to a filter for transmission screens or liquid crystal 
displays (LCD) for eliminating glare caused by ambient light, including 
horizontal light rays from behind the operator, while at the same time 
increasing the vertical viewing area. 
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 reflected 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. 
U.S. Pat. Application Ser. No. 716,237, filed Aug. 20, 1976, now abandoned, 
was directed to an overlay or filter for the viewing surface of a 
television screen to reduce front face glare. The overlay is constructed 
of a sheet of transparent material having a planar back face and a front 
face having a plurality of V-shaped grooves therein such that the overlay, 
when viewed from the side, has a saw-toothed profile. The overlay is 
flexible such that a film of oil can be applied to the back surface of the 
overlay and, by pressing the overlay against the viewing surface of the 
television screen, the overlay adheres thereto. The adherence of the 
overlay to the television screen together with the film of oil 
therebetween minimizes the optical interference between the overlay and 
the television screen. Unfortunately, this overlay did not produce 
satisfactory results. The overlay produced images, known as echo images, 
which interfered with the intelligence displayed on the television screen. 
In U.S. Pat. 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 saw tooth 
forming the front surface of the overlay. It is also disclosed in the U.S. 
Pat. No. 4,165,920 that the horizontal portion of the saw tooth should be 
inclined approximately six degrees from the horizontal and the vertical 
portion of the saw tooth 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 saw tooth 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, the present inventor developed an anti-glare 
device which is described and illustrated in U.S. Pat. No. 4,473,277. The 
anti-glare device of this 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 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 saw tooth could 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 
aforementioned 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. 
In U.S. Pat. No. 4,697,881, the present inventor disclosed an anti-glare 
filter for reducing the glare of a viewing screen caused by ambient light 
coming from the ceiling area where most of the distorting light 
originates. The anti-glare filter was constructed of a substantially 
transparent sheet of material having first and second faces. The first 
face was substantially planar and the second face had a plurality of 
V-shaped grooves, each groove being formed by two walls. A first wall of 
the V-shaped groove was inclined at an angle of at least the critical 
angle for the material with respect to the first face, whereby ambient 
light entering the second face and reflecting off the first face was 
internally reflected. 
However, an anti-glare filter constructed in accordance with the teachings 
of U.S. Pat. No. 4,697,881 having an angle less than 45.degree. and an 
index of refraction of the filter material of 1.41 or lower, will allow 
ambient light from behind the CRT operator to be reflected by the front 
face of the filter into the eyes of the operator. Thus, a reflection of 
the keyboard and the operator's moving hands may seriously interfere with 
the desired picture and be most disconcerting to the operator. Another 
problem encountered with filter material having a low index of refraction, 
for example 1.35, and a critical angle of 47.8.degree. (.theta.), limits 
the viewing area above the horizontal to 42.2.degree. 
(90.degree.-.theta.), since the upper limit of the viewing area is 
determined by the extension of the sloping side of the groove. 
SUMMARY OF THE PRESENT INVENTION 
The shortcomings of the known anti-glare filters discussed above are 
overcome by the present invention wherein, according to one embodiment of 
the present invention, an anti-glare filter for reducing the glare of a 
viewing screen caused by ambient light, e.g., horizontal light rays from 
behind the operator, and simultaneously increasing the vertical viewing 
area is constructed of a substantially transparent sheet of material 
having first and second faces. The first face is substantially planar and 
the second face is constructed of a plurality of V-shaped grooves, each 
groove being formed by two walls. A first wall of the V-shaped groove is 
inclined at an angle with respect to the first face at least equal to or 
greater than a computed angle .theta. determined by the equation: 
EQU sin (angle .theta.)=I.R..times.sin (2 angle .theta.-C.A.), (1) 
where I.R. equals the Index of Refraction of the filter material, and where 
C.A. equals the Critical Angle of the filter material determined from the 
equation: 
EQU sin (critical angle)=1/I.R., (2) 
whereby ambient light entering the second face and reflecting off the first 
face is internally reflected. 
According to another aspect of the present invention, the first wall is 
inclined at an angle of at least the computed angle .theta. with respect 
to the first face whereby ambient light entering the second face, passing 
through the first face and reflecting off the viewing screen, and then 
passing back through the first face is internally reflected. 
According to another aspect of the present invention, when the anti-glare 
filter is used in conjunction with a transmission screen, the anti-glare 
filter may be curved to follow the contours of the transmission screen. 
According to another aspect of the present invention, the second wall of 
the V-shaped groove is inclined at an angle of between plus or minus 
10.degree. with respect to a plane perpendicular to the first face. The 
second wall may be coated with an opaque material or may have a roughened 
surface to further improve the optical qualities of the anti-glare filter. 
According to another aspect of the present invention, when the anti-glare 
filter is used in conjunction with a liquid crystal display, a portion of 
the first wall is covered with an opaque material. That portion of the 
wall covered with the opaque material includes that portion of the wall 
which is furthest from the first face. 
The present invention, by relying on its unique geometry, provides an 
anti-glare filter which is both easy and inexpensive to manufacture. These 
and other advantages and benefits of the present invention will become 
apparent from the description of a preferred embodiment described 
hereinbelow.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In U.S. Pat. No. 4,697,881, it is disclosed that the angle of the sloped 
side of the grooves should be at least equal to the critical angle of the 
material of the filter in order to eliminate all reflective glare from the 
CRT and the back surface of the filter. Although, in a practical 
application, such an arrangement does achieve its purpose of eliminating 
all glare from the CRT and the back face of the filter, it leads to 
another problem. If the filter is made of a material having an index of 
refraction of 1.35, the critical angle then equals 47.8.degree. as 
calculated from equation (2). Since the upper limit of the viewing area is 
determined by the extension of the sloping sides of the grooves, the 
viewing area becomes uncomfortably limited. 
The viewing area can be increased by increasing the viewing angle and 
decreasing the critical angle. However, in doing this the overall 
efficiency of the filter is decreased in such a way that glare from 
overhead illumination may encroach upon the upper portions of the viewing 
area. However, the glare that is most disruptive is that glare resulting 
from behind the operator such as a window. Therefore, I found that one 
could decrease the critical angle enough to appreciably increase the 
viewing angle or area, but not so much as to allow light from behind the 
operator to create glare. Hence, the angle of inclination .theta. is the 
minimum angle that will cancel all glare resulting from light originating 
behind the operator (generally horizontal) in order to ensure a maximum 
viewing area with minimum glare from overhead illumination. 
The minimum angle that may be used to eliminate the most objectionable 
reflection or glare resulting from generally horizontal light from behind 
the operator or a lamp situated on his/her desk is determined as 
illustrated in FIG. 1. Referring to FIG. 1, there is illustrated the 
anti-glare filter's profile wherein the sloping sides of the grooves 
intersect the filter's vertical plane at some unknown angle of inclination 
.theta.. Also illustrated is a ray of horizontal light striking the filter 
at point A at an angle to the facet of .theta.. The light proceeds to 
points B and D, and is then reflected to points C and E, respectively. 
Since BC is parallel to FE, the angle at C is equal to the angle at E. 
It is necessary to find the angle of inclination .theta. such that the 
light ray at E strikes the surface at the critical angle and, therefore, 
will not pass through. Since the filter material selected, in accordance 
with one embodiment, has an index of refraction of 1.35, the critical 
angle is computed as above to be 47.8.degree.. 
Referring to FIG. 1, angle .theta.+angle (.theta.-b) should equal the 
critical angle or 47.8.degree.. This can be represented by the equation: 
EQU 2 angle .theta.-angle b=47.8.degree. (critical angle). (3) 
From the law of refraction: 
EQU sin (angle b)=(sin (angle .theta.))/I.R., where I.R. equal 1.35.(4) 
Substituting into the above equation: 
EQU sin (angle .theta.)=I.R..times.sin (2 angle .theta.-47.8.degree.).(5) 
Solving for angle .theta. by trial and error, angle .theta.=37.2.degree.. 
Therefore, if the filter is formed with an Angle .theta. of inclination of 
37.2.degree., the filter will eliminate glare from a window and have a 
filter with a much wider viewing area vertically above the horizontal 
(52.8.degree. as compared to 42.2.degree.) as shown in FIG. 2. In order to 
widen the vertical viewing area and eliminate the most annoying ambient 
light glare, the critical angle of inclination should be at least the 
angle .theta. determined by equation (1): 
The present invention is directed to an anti-glare filter constructed in 
accordance with the aforementioned teachings, a portion of which is shown 
in profile in FIG. 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. 
FIG. 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 constructed 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 sloping wall 18 and a second substantially horizontal wall 20. The 
sloping wall 18 is inclined at an angle of at least equal to our greater 
than the computed angle .theta. determined by equation (1). 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 of the first wall 18 with respect to the first face 14 is an 
important feature of the present invention and discussed more fully 
hereinbelow. 
The second wall 20 is shown in FIG. 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 10.degree. 
with respect to a plane perpendicular to the first face 14. 
In FIG. 1, the anti-glare filter 10 is constructed in accordance with one 
embodiment such that the first wall 18 is inclined at an angle of 
37.2.degree. (where I.R. equals 1.35) 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 a 
beam of horizontal light, where most of the distracting light originates, 
from the nearest light source such as a window or lamp directly in front 
of the operator. Assuming the index of refraction of the material 
comprising the anti-glare filter 10 is 1.35, light ray no. 1 will, upon 
entering the second face 16, be refracted at an angle of 10.6.degree.. 
The light ray no. 1, upon striking the first face 14 at B, will partially 
pass through the first face 14 and is reflected off the screen 12 at D and 
returns along path 23 until ray 1 strikes the sloping wall 18 at point E. 
The light ray reflected at point B will strike the sloping wall 18 at 
point C. The angles at which the paths 24 and 25 of ray no. 1 strike point 
C and E on sloping wall 18 is 37.2.degree. each. Any portion of ray no. 1 
striking points C or E at an angle equal to or greater than 37.2.degree. 
will be internally reflected as shown in FIG. 1. 
It can be demonstrated that after ray no. 1 is internally reflected at 
points B, C or E, it repeatedly strikes the first face 14 and sloping wall 
8 at angles greater than 37.2.degree.. Therefore, there will never be a 
refracted ray no. 1 from these surfaces Ray no. 1 is thus said to be 
internally reflected such that no glare is caused by this light ray. 
If the angle of inclination .theta. of the sloping wall 18 is less than the 
computed angle .theta. using Equation (1), light ray no. 1 will strike 
first face 14 at an angle less than angle (.theta.-b). Thereafter, 
reflected ray no. 1 will not strike the sloping wall 18 at an angle 
greater than or equal to the computed angle such that total internal 
reflection will not be achieved. Conversely, if the angle of inclination 
of the sloping wall 18 is greater than the computed angle .theta., ray no. 
1 will be reflected off the first face 14 and strike the sloping wall 18 
at an angle greater than the computed angle .theta. 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 of the sloping wall 18 with respect to the first face 14 be at 
least the computed angle of inclination .theta. for the material of which 
the anti-glare filter 10 is constructed. 
Listed below are the various materials which may be used to construct the 
anti-glare filter 10 of the present invention: Polymethylacrylate, 
polyethylacrylate, polytutylacrylate, polyethoxyethylacrylate, poly (2 
methoxyethyl) acrylate, poly (2 bromo sec. butyl) acrylate, poly (2 bromo 
phenyl) acrylate, poly (2 chloromethyl) acrylate, polyacrylonitrile, 
polymethylmethacrylate, polyethylmethacrylate, poly butyl methacrylate, 
poly (t-butyl) methacrylate, polychclohexyl methacrylate, poly 
(2-hydroxyethyl) methacrylate, poly (2-phenoxyethyl) methacrylate, poly 
phenylmethacrylate, poly (o-chloro) styrene, poly (2.6 dichloro) styrene, 
poly (O-methoxy) styrene, polyacetal, poly (n-benzyl) methacrylamide, poly 
(N-butyl) methacrylamide, polyvinyl chloride, polyvinyl fluoride, 
polyvinylidene chloride, polyvinyl acetate, polyvinyl carbazole, polyvinyl 
isobutyl ether, polyvinyl alcohol, poly (n-vinyl) phthalimide, polyallyl 
phthalate, polyester-styrene, poly (o-tolyl) methacrylate, poly carbonates 
(bisphenol), poly (N-2, phenethy) methacrylamide, polystyrene, zinc crown 
glass, higher dispersion crown glass, light flint glass, heavy flint 
glass, and heaviest flint glass. 
Another feature of the present invention is an anti-glare filter wherein 
the angle of inclination .theta. according to Equation (1) of the first 
sloping wall 18 is chosen to be above the critical angle with respect to 
the first face 14 such that ambient light entering the front 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. 
FIG. 2 shows that the viewing area is dependent on the critical angle or 
the computed angle of inclination based on Equation (1). The filter 10, 
made of a material having an index of refraction of 1.35 has a critical 
angle of 47.8.degree.. The upper limit of the viewing area is determined 
by the extension of the first sloping wall 18. The viewing area angle is 
therefore 90.degree.-47.8.degree. (critical angle)=42.2.degree.. 
Substituting the computed angle of inclination according to Equation (1) 
of 37.2.degree. for the critical angle of a material having an index of 
refraction of 1.35 increases the viewing area angle to 52.8.degree. 
(90.degree.-37.2.degree.=52.8.degree.), as illustrated in FIG. 2. Using 
the inclination angle .theta. based on Equation 1 of a filter material 
having an index of refraction of 1.4 or less, increases the viewing area 
angle 10.6.degree.. 
There are three ways to compensate for the light rays which escape from the 
anti-glare filter 10 through second wall 20. First, the second wall 20 may 
be covered with an opaque material which will absorb light. A second 
approach is to provide the second wall 20 with a roughened surface. 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 
well as increasing the contrast of the intelligence displayed on the CRT. 
Another factor which should 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 should also be 
curved to be substantially parallel to the curvature of the CRT. That is, 
the anti-glare filter 10 should be curved or bent around its vertical and 
horizontal axes. Unless this is done, the rows or lines of the text 
appearing on the face of the CRT will appear to be arched. 
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 40 mils and an index of 
refraction of 1.5. The filter is approximately 10 inches by 10 inches with 
80 horizontal V-shaped grooves per inch. Each groove is comprised of a 
horizontal wall 20 extending approximately 7 1/2 mils into the material 
and an inclined wall 18 having a slope of 30.91.degree. with respect to 
the first face 14. 
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 notorious 
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 an 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 not 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. 
It will be understood that the embodiment described 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.