Windshield for head-up display system

A windshield for a head-up display system has a pair of opposing major surfaces that are nonparallel to one another in selected areas. Light rays directed toward the nonparallel area of the windshield are reflected from the outer and inner major surfaces of the windshield and directed toward the eye of the observer as parallel or superimposed light rays to eliminate ghost images when viewing images projected by the head-up display system through the windshield. The interlayer blank utilized to make the windshield has a non-uniform thickness profile with a tapered thickness positioned in the selected areas of the windshield.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a windshield for a head-up display system and in 
particular to an automotive windshield functioning as the combiner for the 
head-up display system and having a wedged configuration for some selected 
portion of the windshield area, particularly in the viewing area of the 
head-up display, to eliminate double imaging and the interlayer used to 
provide the required wedged configuration. 
2A. Technical Considerations 
A head-up display system is a visual display arrangement that displays 
information to a viewer while he simultaneously views the road and objects 
outside his vehicle around and through the display. Head-up display 
systems are often incorporated into aircraft cockpits for pilots to 
monitor flight information. More recently the systems have been used in 
land vehicles such as cars, trucks and the like. The display is generally 
positioned so that the viewer does not have to glance downward to the 
vehicle dashboard and away from the viewing area in front of the vehicle 
as is required of a vehicle operator viewing vehicle operating information 
in a vehicle not having a head-up display. 
A head-up display system generally includes a display projection system, a 
collimator, and a combiner. The projection system includes a light source 
that projects operating information through the collimator which generally 
aligns the projected light rays. The collimated light is then reflected 
off the combiner, which is in the vehicle operator's field of view. In 
this manner, vehicle information such as, for example, fuel information 
and vehicle speed is displayed within the operator's field of vision 
through the windshield and permits the operator to safely maintain eye 
contact with the road and other objects outside his vehicle while 
simultaneously viewing the displayed information. The reflected images of 
the display may be focused at a position anywhere from immediately in 
front of the vehicle to optical infinity. 
Laminated windshields have been used as the combiner in a head-up display 
system to reflect a primary display image as taught in U.S. Pat. No. 
2,264,044 to Lee. However, it has been observed that a secondary image is 
reflected off the outer surface of the windshield. This secondary image is 
superimposed over but offset from the primary image and reduces the 
overall image clarity. 
It would be advantageous to have a windshield for a head-up display system 
which functions as a combiner and provides a clear display image without 
producing double images when viewing through the head-up display area, 
without distorting the view through other portions of the window not 
associated with the head-up display system, and without incorporating 
additional components with the windshield. 
2B. Patents of Interest 
U.S. Pat. No. 1,871,877 to Buckman teaches a display system having a glass 
sheet mounted on the windshield or dashboard which reflects 
instrumentation information to the vehicle operator. 
U.S. Pat. No. 2,264,044 to Lee teaches a motor vehicle having an 
illuminated speedometer display that is reflected off the inboard surface 
of the vehicle windshield. 
U.S. Pat. No. 2,641,152 to Mihalakis teaches a vehicle projection device 
wherein instrumentation information is reflected off of a reflecting 
screen on the inboard surface of the vehicle windshield. The reflecting 
surface has a satin finish and can be metal, glass, or plastic. 
U.S. Pat. No. 2,750,833 to Gross teaches an optical display system for 
eliminating double images which occur in reflector type sights such as 
those used in aircraft gun sighting installations. A collimated light beam 
is polarized and separated into two ray branches. One of the two ray 
branches is then eliminated. 
U.S. Pat. No. 3,276,813 to Shaw, Jr. teaches a motor vehicle display system 
which utilizes a highly reflective coating on the inboard surface of the 
vehicle windshield to reflect instrumentation information to the vehicle 
operator. 
U.S. Pat. No. 3,446,916 to Abel teaches an image combiner utilizing a 
portion of the aircraft window. The inner surface portion of the window is 
coated with a partially reflective film. 
U.S. Pat. Nos. 3,554,722, 3,591,261, and 3,647,285 to Harvey et al. teaches 
a double glazed glass window structure which eliminates objectionable 
fringe patterns produced in this structure when float glass of non-uniform 
thickness is utilized. The window structure includes a pair of spaced 
apart, float glass sheets one or both of which are tapered from a thick 
edge to an opposing thin edge. When both the glass sheets are tapered, the 
glass sheets are positioned such that a thick edge of one glass sheet is 
spaced from a thin edge of the opposing glass sheet. 
U.S. Pat. No. 3,697,154 to Johnson teaches an optical viewing system in 
which images formed on the screen of a cathode ray tube (CRT) are 
reflected from a curved mirror having a general aspheric surface of 
revolution to a partially reflective combiner having two nonparallel 
hyperboloid surfaces, the combiner being positioned in the normal line of 
sight of an observer such that a collimated CRT image is reflected from 
the near surface of the combiner to the observer's eyes and the combiner 
being adapted to transmit light incident from the outside so that the CRT 
display is superimposed without parallax on the real world to provide a 
head-up display. 
U.S. Pat. No. 3,870,405 to Hedges teaches a visor for use an optical 
element in a helmet-mounted sight having inner and outer surfaces being 
sections of focal paraboloids of revolution. 
U.S. Pat. No. 3,899,241 to Malobicky, Jr. et al. teaches a windshield 
adapted for use in aircraft and includes a transparent reflective coating 
on the inboard surface in the center portion of the forward vision area to 
form a vision image receiving area. Vehicle information is reflected off 
the reflective coating to the vehicle operator. 
U.S. Pat. No. 3,940,204 to Withrington and 4,218,111 to Withrington et al. 
teach an optical display system utilizing holographic lenses. 
U.S. Pat. No. 4,261,635 to Freeman teaches a head-up display system 
including a holographic combiner positioned inboard of the vehicle 
windshield. The hologram is disposed substantially orthogonal to and 
midway along an axis between the observer's eye position and the 
projection optics so as to deviate light from an image produced by the 
projection optics to the observer eye with minimal field aberration. 
U.S. Pat. No. 4,398,799 to Swift teaches a head-up display system which 
simultaneously records the pilot's view by reflecting the outside scene 
and the projected display by reflecting the outside scene and superimposed 
display off a mirror mounted on the pilot's helmet and recording the 
reflected view with a camera mounted on the pilot's helmet. 
U.S. Pat. No. 4,613,200 to Hartman teaches a head-up display system which 
uses two parallel holographic optical elements to reflect instrumentation 
information to the vehicle operator. One of the elements is made part of 
or attached to the vehicle windshield. 
U.S. Pat. No. 4,711,544 to Iino et al. teaches a display system for a 
vehicle wherein instrumentation information is reflected off the front 
glass of the vehicle so that the image display can be formed in a desired 
position, aligned with the line of sight of the driver without obstructing 
the front sight of the driver. 
U.S. Pat. Nos. 4,787,711 and 4,892,386 and European Patent No. 229,876 to 
Suzuki et al. teach an on-vehicle head-up display device employing a 
catoptric system for a windshield glass of an automobile to project a 
display image onto an inner surface of the windshield glass, an optical 
system for letting a virtual image of the display image of the display 
means enter the windshield glass is adapted to make an angle formed by 
light beams of the virtual image entering the windshield glass less than a 
monocular resolving power and an optical means for correcting parallax of 
the light beams of the virtual image is provided between the optical 
system and the windshield glass to thus eliminate double imaging and 
binocular parallax. 
Defensive Publication No. T861,037 to Christensen teaches a tapered or 
wedged vinyl interlayer for use in laminating windshields such that the 
interlayer is thicker at the top of the windshield than at the bottom of 
the windshield in order to eliminate double vision caused by the 
windshield curvature and angle of installation. 
SUMMARY OF THE INVENTION 
The present invention provides a windshield for a head-up display system 
that reduces the degree of double imaging that occurs when a laminated 
windshield is used as the combiner in the display system. The windshield 
functions as a combiner for the head-up display system without requiring 
any additional reflecting or transmitting elements or components to be 
incorporated onto or into the windshield assembly. The windshield in the 
present invention is constructed such that selected opposing, outer major 
surfaces of the windshield are non-parallel with the opposing outer major 
surfaces oriented relative to each other so that an image, projected from 
a display source and reflected off a first major surface of the 
windshield, is substantially superimposed over or parallel to the same 
image from the display source reflected off the opposing outer major 
surface of the windshield. 
In one embodiment of the invention the windshield has a selected area that 
has the outer major surfaces of the windshield non-parallel to one 
another, and the outer major surfaces of the windshield at the remaining 
area generally parallel to one another. In this manner, the degree of 
double imaging is reduced in the area where the outer surfaces are 
non-parallel to one another, and the optical distortion in the remaining 
area is improved because the outer surfaces are parallel to one another. 
This may be accomplished by having one or both of the glass plies and/or 
the sheet of interlayer material partially tapered in thickness such that 
when the plies and sheet are joined together, the outer major surfaces of 
the windshield in the selected area are nonparallel to one another with 
the remaining area of the outer major surfaces of the windshield generally 
parallel to one another in the area other. 
The present invention also provides an interlayer blank for use with a 
glass ply in a vehicle windshield for a head-up display system. The blank 
is polyvinylbutyral and includes a first portion with a predetermined 
thickness profile and a second portion with a tapered thickness profile 
different from the predetermined profile. The interlayer may be provided 
by casting the interlayer to the desired configuration, extruding the 
desired cross sectional thickness, or differentially stretching the 
interlayer to the desired shape or through a combination of these 
processes. In one particular embodiment of the invention, interlayer 
having a desired varying thickness profile is produced by initially 
forming the interlayer with a thickness profile greater than the desired 
profile and subsequently differentially stretching the interlayer to 
reduce the thickness of the interlayer such that its thickness corresponds 
to the desired thickness profile. 
When the glass plies and the interlayer having the tapered section are 
assembled and laminated to form a unitary structure, the opposing major 
surfaces of the laminate are non-parallel in the area of the tapered 
section and offset at a predetermined angle in the vicinity of the tapered 
section. The windshield in use is positioned relative to a display system 
such that the images generated by light rays from the display source 
reflected off the non-parallel opposing major surfaces of the windshield 
or laminate are substantially superimposed over or parallel to each other 
such that double imaging is reduced if not eliminated, while the remaining 
sections of the windshield have the outer major surfaces substantially 
parallel to one another to eliminate or minimize optical distortion of 
objects viewed through the remaining area of the windshield. 
The invention also contemplates making the windshields of the instant 
invention by joining glass sheets and a sheet of interlayer material to 
provide a windshield having the outer surfaces parallel in a first 
predetermined area and nonparallel in a second predetermined area.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to the elimination of double imaging in a 
head-up display system that uses an automotive windshield as the combiner, 
but it should be appreciated that the present invention may be used in any 
type of combiner having a laminate construction where double imaging is to 
be eliminated. 
With reference to FIG. 1, head-up display 10 system includes a motor 
vehicle windshield 12, an image source 14 and a projection assembly 16, 
preferably mounted immediately beneath the upper surface of the vehicle 
dashboard 18 and positioned between the image source 14 and windshield 12. 
Light rays emanate from the image source 14 and are projected onto the 
windshield 12, which operates as a combiner as will be discussed later, 
and reflected into the field of vision of the vehicle operator or observer 
20. The light rays projected onto the windshield 12 are collimated so as 
to create a virtual image in front of the car, preferably at about 5 to 50 
feet (3 to 15 meters) in front of the windshield 12. 
Although not limiting in the present invention, the image source 14 
preferably is a transmissive liquid crystal display (LCD) that is 
adequately illuminated to project information carrying light rays through 
the projection assembly 16 onto the windshield 12 at a location within the 
vehicle operators direct line of sight while permitting peripheral viewing 
of the road and objects outside of the vehicle as the operator or observer 
20 monitors the display. It is contemplated that alternative viewing 
locations will also provide an effective head-up display for the vehicle 
windshield. The displayed image (not shown) may include numerical or 
graphical symbols including for example, vehicle speed, fuel level, engine 
RPMs, temperature, and warning symbols. 
The following discussion will be directed towards the use of a prior art 
windshield which incorporates an interlayer having a constant thickness, 
as the combiner in a head-up display system which projects an image a 
finite distance in front of the windshield. In particular, referring to 
FIG. 2, windshield 30 represents a windshield assembly with the opposing 
inner and outer major surfaces of the windshield being parallel to each 
other for the full length of the windshield, i.e. from the top edge to the 
opposite edge and between the side edges. The windshield 30 includes outer 
glass ply 32 bonded to inner glass ply 34 by an interlayer material 36. 
Because the thickness of the interlayer 36 is fairly uniform and the 
opposing major surfaces of each glass ply are substantially parallel to 
each other, i.e., inner surface 37 of the ply 32 is parallel to its outer 
surface 38 and inner surface 39 of the ply 34 is parallel to its outer 
surface 40, the outer major surface 38 of the glass ply 32 is parallel to 
the outer major surface 40 of the glass ply 34 after the glass plies 32 
and 34 and interlayer 36 are laminated together to form a unitary 
structure. Although not limiting in the present invention, for the 
purposes of illustration, surfaces 38 and 40 are assumed to be planar in 
the following discussion. However, the surfaces may be non-planar, as will 
be discussed later. 
With continued reference to FIG. 2, a light ray A from image source 14 is 
directed along line 42 and a portion of the light ray A is reflected off 
surface 40 of the ply 34 along line 44 to eye 46 of the vehicle operator 
20. Additional light rays from the source 14 are directed along additional 
lines. For example, light ray B is directed along line 48 and a portion of 
the light ray B is reflected off the surface 40 along line 50 toward the 
vehicle operator 20. However, the ray B along the line 50 is not directed 
to the eye 46 as shown in FIG. 2 so it will not be detected by the 
observer 20. 
A portion of the light ray B which is directed along line 48 will enter the 
windshield assembly 30 and be refracted along line 52. The angular 
difference between the lines 48 and 52 depends on the angle of refraction 
as the light ray passes through the air and into the glass ply 34. The 
angle of refraction in turn depends, in part, on the angle at which ray B 
is incident on the surface 40 and the relative densities of the air and 
the glass. The ray B passes through the windshield assembly 30 and a 
portion of the light ray B is reflected off the surface 38 of the ply 32 
along line 54. It is assumed that the refractive index of interlayer 36 is 
essentially the same as that of the glass plies 32 and 34 so that the 
light rays are not redirected as they pass through the windshield assembly 
30 along lines 52 and 54. 
A portion of the light ray B leaves the windshield assembly 30 at the 
surface 40 of the glass ply 34 where the direction of the light is again 
changed due to the difference in the refraction index between the 
windshield assembly 30 and the air, as discussed earlier, and directed 
along line 56 to the operator's eye 46. Because the light rays A and B 
received by the eye 46 from the image source 14 are along two different 
lines, i.e. lines 44 and 56, which are convergent toward one another to 
the eye of the observer 20 rather than parallel, the observer 20 will 
perceive two offset images where in fact there is only one image source 
14. The first image 58, or virtual image, is the image seen by the 
observer 20 from the portion of the light ray A directed along line 44. 
The second image 60 is the image seen by the observer 20 from the portion 
of the light ray B directed along line 56. 
When viewing both images, the virtual image 58 will appear brighter than 
the second image 60 because a greater portion of the light from the image 
source 14 which was initially directed along line 42 will be directed 
along line 44 as compared to the amount of light initially directed along 
line 48 and which is finally directed along line 56 to the observer 20. 
This condition of seeing two offset images is commonly referred to as 
double imaging, or ghost imaging, and results when the outer surfaces of 
the windshield assembly, i.e. surfaces 38 and 40 of windshield assembly 
30, are parallel to each other. Stated another way, double imaging occurs 
when the light rays A and B, projected from image source 14 onto parallel 
surfaces 38 and 40, are directed toward the eye of the observer along 
non-parallel lines, i.e., lines 44 and 56 which converge toward each other 
to the eye 46 of the operator 20. 
In order to reduce the amount of double imaging in the windshield assembly 
30, the present invention may be used to modify the windshield structure 
by adjusting the spaced relationship of the surfaces 38 and 40 relative to 
one another such that the portion of the light rays A and B directed to 
the eye of the observer are superimposed over or parallel to one another. 
Referring to FIG. 3, windshield 130 includes outer glass ply 132 bonded to 
inner glass ply 134 by an interlayer material, e.g., a thermoplastic 
material 136 along inner major surfaces 137 and 139 of plies 132 and 134, 
respectively. The interlayer 136 is fabricated in such a way as to 
gradually taper in thickness from top to bottom of the windshield with the 
thicker section at the top edge as reviewed in FIG. 3. 
As a result of the "wedged" shape section of the interlayer, when the 
windshield components are assembled and laminated, outer major surface 138 
of glass ply 132 and outer major surface 140 of glass ply 134 will be 
non-parallel. It has been found that by controlling the amount by which 
the glass plies 132 and 134 of the windshield 130 are offset from each 
other, the double imaging encountered when using a windshield as shown in 
FIG. 2 as a combiner can be reduced in a manner to be discussed below. The 
actual wedge angle X required to reduce the double imaging depends, in 
part, upon the thickness of the windshield, the windshield materials, and 
the relative positions and orientations of the image source 14, the 
windshield 130 and the vehicle operator 20. Although not limiting in the 
present invention, the interlayer 136 can be cast in place to provide the 
desired wedge angle X. As an alternative, the interlayer 136 can be 
extruded or differentially stretched in any convenient manner known in the 
art, such as that disclosed in U.S. Pat. No. 4,201,351 to Tolliver and 
U.S. Pat. No. 4,554,713 to Chabel, which teachings are incorporated herein 
by reference, or cast or extruded and subsequently differentially 
stretched as will be discussed later in more detail, to achieve the 
desired wedge angle. 
With continued reference to FIG. 3, the discussion will now be directed to 
the elimination or minimization of double images or ghost images using a 
windshield that has wedge or taper, i.e., nonparallel outer surfaces. 
Light ray A.sup.1 from the image source 14 is directed along line 142 and 
a portion of the light ray A.sup.1 is reflected off the surface 140 along 
line 144 to the eye 46. Another portion of the light ray B.sup.1 directed 
along line 148 is reflected from the surface 140 along line 150 such that 
it is not seen by the observer 20 as was discussed for the portion of the 
light ray B reflected from the surface 40 along line 50 of the prior art 
arrangement shown in FIG. 2. The remaining portions of the ray B.sup.1 are 
refracted through the glass plies 132 and 134 and the interlayer 136 along 
line 152 and refracted as it leaves the assembly 130 to the observer's eye 
46 along line 144 in a similar manner as that discussed for the ray B 
shown in FIG. 2. However, unlike the windshield assembly 30 in FIG. 2 
wherein the line 56 from the ray B is along a different orientation than 
line 44 from ray A, in the present invention as shown in FIG. 3, the wedge 
angle X is such that the refracted light from light ray B.sup.1 exits the 
assembly 130 along the line 144, i.e., parallel or superimposed over the 
light from ray A.sup.1 reflected off the surface 140 of the glass ply 134 
also moving along the line 144. As a result, the image viewed by the 
observer 20 resulting from light rays A.sup.1 and B.sup.1 are superimposed 
over or parallel to each other so that there is viewed only a single image 
158. 
It should be appreciated that in a windshield assembly, the surfaces 138 
and 140 of the glass plies 132 and 134, respectively, are often not planar 
but rather have a curved configuration. However, the amount of relative 
curvature in the windshield assembly 130 within the small area used as the 
combiner is relatively small so that the area within the combiner portion 
of the windshield 130 is nearly planar. Furthermore, if required due to 
excessive curvature of the windshield within the combiner area, the image 
from the image source 18 can be distorted, for example by incorporating 
additional lens arrangements (not shown) into the projection assembly 16 
(shown only in FIG. 1) to account for the curvature of the windshield 
surfaces. 
In one particular embodiment of a windshield construction as illustrated in 
FIG. 3, the windshield 130 includes 0.090 inch (2.3 mm) thick glass plies 
and two polyvinylbutryl interlayer plies. Each interlayer ply is 
originally 0.020 inches (0.05 mm) thick and is differentially stretched so 
that each interlayer ply has a taper of approximately 0.003 inches (0.076 
mm) over a 36 inch (91 cm) interlayer width for a combined thickness 
differential of approximately 0.006 inches (0.152 mm) from the top to the 
bottom edge (as viewed in FIG. 3) when incorporated into the windshield 
130. It should be obvious to one skilled in the art that the two 
interlayer plies may be replaced by a single ply having the required wedge 
configuration. Referring to FIG. 1, it has been observed that a windshield 
of this construction, mounted in a vehicle at an installation angle Y of 
approximately 30.degree. from the horizontal with an angle of incidence Z 
between the windshield 12 and image source 14 of approximately 65.degree. 
significantly reduces the amount of double imaging in a head-up display 
system as compared to a conventional windshield having a non-wedged 
configuration. 
Although the windshield configuration 130 as shown in FIG. 3 includes two 
glass sheets each having generally parallel opposing major surfaces and a 
tapered interlayer ply, based on the teachings of this disclosure, it is 
obvious to one skilled in the art that other windshield configurations can 
be used to provide a wedged windshield configuration similar to that shown 
in FIG. 3. In particular, referring to FIG. 4, one or both of the glass 
plies 232 and 234 may be provided with a taper such that when the assembly 
230 is laminated to form a unitary structure using a non-stretched 
interlayer 236, opposing surfaces 234 and 236 of the windshield 230 are 
non-parallel and are oriented relative to each other so as to eliminate 
the double imaging. It is further contemplated that one or more tapered 
interlayers may be used in combination with one or more tapered glass 
plies so that the final laminated assembly provides a required windshield 
construction having the configuration required to reduce double imaging. 
In other words, the windshield configuration as taught herein to reduce 
double imaging in a head-up display system is not directed to providing 
glass plies and a sheet of interlayer that does or doesn't have a taper as 
long as after the plies are secured about the interlayer the outer 
surfaces of the windshield are nonparallel so that the portions of the 
light rays A.sup.1 and B.sup.1 directed toward the eye of the observer are 
parallel or superimposed over one another along the line 144 as shown in 
FIG. 3. 
It is an object of this invention to provide a windshield configuration 
wherein the outer surfaces of the windshield are non-parallel only in the 
head-up display area of the windshield. In this manner, the problem of 
double imaging of the display is reduced or eliminated while at the same 
time optical distortions associated with viewing objects through wedged 
glass or a wedged windshield is eliminated. More particularly, it has been 
noted that the use of non-tapered or non-wedged areas of laminated glass, 
i.e., outer surfaces of the laminated windshield being substantially 
parallel to one another for the upper portion of a windshield as mounted, 
with the tapered or wedged areas, i.e. a partial wedge, at or near the 
bottom of the windshield as mounted, improves the optics of the 
windshield. More particularly, a vehicle operator viewing an object such 
as a traffic light at a 45.degree. angle through an upper area of the 
windshield encounters less double vision of objects viewed through a 
non-tapered or non-wedged windshield than a tapered or wedged windshield. 
However, a vehicle operator viewing an object such as a head-up display at 
a 90.degree. angle through the bottom portion of the windshield will 
observe a double image for the reason discussed above. Thus, a windshield 
having a partial wedge area such as the windshield types shown in FIGS. 5 
and 6 would be advantageous over a windshield of the type shown in FIG. 3 
that has a taper or wedge from the top edge of the windshield to the 
opposite or bottom edge. 
In the following discussions regarding FIGS. 5 and 6, the left hand side of 
windshields 330 and 430 as viewed in FIGS. 5 and 6, respectively, is the 
top end of the windshield as mounted in a vehicle and the right side as 
viewed in FIGS. 5 and 6 is the opposite or bottom end of the windshield as 
mounted. If a shade band (shown only in FIG. 7) of a type well known in 
the art were incorporated into windshields 330 and 430, it would be 
positioned at the left side or top end of the windshield. With reference 
to FIG. 5, the windshield 330 has glass plies 332 and 334 secured to each 
other about interlayer 336. The windshield 330 has a partial wedge by 
providing outer surface 338 of the glass ply 332 nonparallel to the outer 
surface 340 of glass ply 334 in the mid-area 337 and the outer surfaces 
338 and 340 of the glass plies 332 and 334, respectively, in the upper 
area 342 and lower area 344 parallel to one another. This may be 
accomplished by providing the glass plies 332 and 334 with a constant 
thickness throughout their length, i.e. from the top edge to the bottom 
edge of the windshield, and width, i.e. from one side to the other side 
(only one side shown in FIG. 5 and 6), and an interlayer 336 with a 
predetermined non-uniform thickness profile to position the desired wedge 
in the windshield at the required location. More specifically, the 
portions of interlayer,336 in the top and bottom areas 342 and 344, 
respectively, of the windshield 330 are provided with a constant thickness 
and the portion of the interlayer in the mid-area 337 are provided with a 
tapered thickness. 
A windshield of the type shown in FIG. 5 may be constructed using a sheet 
336 of polyvinylbutyral that has a constant thickness in area 342 of about 
0.034 to 0.040 inches (0.086 to 0.102 cm), a constant thickness in area 
344 of about 0.027 to 0.030 inches (0.069 to 0.076 cm), and varying 
thickness in area 337 that decreases, preferably uniformly, from the 
constant thickness in area 342 to the constant thickness in area 344. As 
an example, windshield 330 may include a pair of 0.090 inch (2.3 mm) thick 
glass plies 332, 334. The glass plies would each have a length from top to 
bottom of approximately 45 inches (114.3 cm). A sheet 336 of 
polyvinylbutyral preferably having a shape as shown in FIG. 7 would be cut 
to fit between the glass plies. The polyvinylbutyral sheet 336 would have 
a constant thickness of 0.034 inches (0.086 cm) from the top edge to a 
distance of 25 inches (63.5 cm) therefrom (top area 342); a taper defined 
by a thickness of 0.034 inches (0.086 cm) at a point spaced 25 inches 
(63.5 cm) from the top edge of the windshield and a thickness of 0.030 
inches (0.076 cm) at a point 37 inches (94 cm) from the top of a 
windshield (mid area 337), and a constant thickness of 0.030 inches (0.076 
cm) from a point 37 inches (94 cm) from the top edge to the bottom edge 
(bottom area 344). 
FIG. 6 illustrates another embodiment of the invention directed to partial 
wedging of the windshield. The windshield 430 shown in FIG. 6 has a pair 
of glass plies 432 and 434 about an interlayer 436. The thickness of the 
glass plies and interlayer are selected to provide the windshield with a 
lower area 437 having outer surface 438 and 440 nonparallel to one another 
and upper area 439 having outer surfaces 438 and 440 of the windshield 
parallel to one another. In FIG. 6, this arrangement may be achieved by 
using glass plies 432 and 434 of constant thickness and a sheet of 
interlayer material 446 having a non-uniform thickness profile that has a 
constant thickness at its upper portion and a taper at its lower portion. 
A windshield of the type shown in FIG. 6 may be constructed using a sheet 
436 of polyvinylbutyral that has a constant thickness in area 439 of about 
0.034 to 0.040 inches (0.086 to 0.102 cm) and varying thickness in area 
437 that decreases, preferably uniformly, from the constant thickness in 
area 439 to a thickness along its lower edge of about 0.027 to 0.030 
inches (0.069 to 0.076 cm). As an example, the windshield 430 may include 
a pair of 0.090 inch (2.3 mm) thick glass plies 432, 434. The glass plies 
would each have a length of 45 inches (114.3 cm). A sheet 436 of 
polyvinylbutyral preferably having a shape shown in FIG. 7, would be cut 
to fit between the glass plies. The sheet 436 would have a constant 
thickness of 0.038 inches (0.097 cm) from the top edge to a distance of 25 
inches (63.5 cm) therefrom (upper area 439) and a taper defined by a 
thickness of 0.038 inches (0.097 cm) at a point spaced 25 inches (63.5 cm) 
from the top edge to a thickness of 0.030 inches (0.076 cm) at the bottom 
edge (lower area 437). 
Referring to FIG. 1, it has been observed that windshields of the 
construction discussed above and shown in FIGS. 5 and 6 mounted in a 
vehicle at an installation angle Y of approximately 30.degree. with an 
angle of incident Z between the mid-section 337 of the windshield 330 or 
the lower area 437 of the windshield 430 and the image source 14 of 
approximately 65.degree. significantly reduces the amount of double 
imaging in a head-up display system as compared to the prior art 
windshield having a non-wedged configuration while maintaining the optical 
properties of the prior art windshield in the upper and lower areas 342 
and 344 of the windshield 330 and the upper area 439 of the windshield 
430. 
In the windshield construction of the type shown in FIGS. 5 and 6, the 
non-tapered sections, e.g., areas 342 and 344 of the windshield 330 of 
FIG. 5 and area 449 of the windshield 430 of FIG. 6, there will be 
minimal, if any, optical distortion because the outer surfaces of the 
windshield in those portions are parallel. 
As discussed earlier, interlayers 336 and 436 may be formed by casting, 
extrusion, or differential stretching or a combination of these processes. 
It should be appreciated that differential stretching of the interlayer 
and subsequent cutting of the interlayer into blanks for incorporation 
into the windshield, results in an interlayer blank 500 having a 
peripheral configuration that is generally quadrilateral in shape with two 
opposing acurate sides 502 and 504 as shown in FIG. 7. It should further 
be appreciated that the stretching also facilitates the use of interlayer 
having a shade band 506, as is widely used in the art. More particularly, 
interlayer 500 is generally supplied as a continuous ribbon or web of 
material having straight, longitudinally extending opposing edges and a 
shade band incorporated into the web along and generally parallel to an 
upper edge of the web. The differential stretching operation stretches the 
bottom edge 504 of the interlayer more than the top edge 502 so that the 
initially opposing straight edges are formed into opposing arcuate edges. 
This stretching may be accomplished in a continuous manner by passing the 
continuous web over a series of shaping rolls. For example, the web is 
moved successively over a cylindrical heating roll to heat the interlayer 
prior to stretching, a conical heating roll to differentially stretch the 
interlayer to a desired contour, and a conical cooling roll to maintain 
the web's stretched configuration, as disclosed in U.S. Pat. No. 
4,554,713. As an alternative, the interlayer web may be stretched in a 
batch fashion by wrapping a predetermined amount of interlayer around an 
adjustable cylindrical roll and expanding portions of the roll to form a 
conical surface with the circumference of the expanded roll at the end 
corresponding to the bottom of the interlayer being greater than the 
circumference at the opposite end, i.e. the end corresponding to the top 
of the interlayer, as disclosed in U.S. Pat. No. 4,201,351. During this 
stretching operation, the thickness of the interlayer is reduced, with 
this reduction being larger at the bottom of the interlayer since it is 
stretched a greater amount than the top portion. 
As a result, if the sheet 336 is initially formed so that it has a constant 
thickness in areas 342 and 344, and the sheet is differentially stretched, 
as discussed above, to a shape a shown in FIG. 7, sheet 336 in areas 342 
and 344 would not have a constant thickness but rather have a slight taper 
as shown in FIG. 8. Although not limiting in the present invention, it is 
expected that the taper would be on the order of 0.001 to 0.003 inches 
over a 25 inch distance (0.025 to 0.076 mn over 63.5 cm). It is noted that 
this rate of taper, i.e. the change in thickness over a given length, is 
less than the rate of taper in areas 337 and 437 of interlayers 336 and 
436, respectively, which correspond to the display areas of the 
windshield. For example, interlayer 536 of the type shown in FIG. 8 would 
have a thickness that decreases in upper area 542 from about 0.034 to 
0.040 inches (0.086 to 0.102 cm) along upper edge 502 to 0.031 to 0.039 
inches (0.079 to 0.099 cm), decreases in mid-area 537 from about 0.031 to 
0.039 inches (0.079 to 0.099 cm) to 0.028 to 0.030 inches (0.071 to 0.076 
cm), and decreases in lower area 544 from about 0.028 to 0.030 inches 
(0.071 to 0.076 cm) to 0.027 to 0.029 inches (0.069 to 0.074 cm) along 
lower edge 504. If desired, to compensate for this tapering, the sheet 336 
in FIG. 5 may be initially formed to a thickness profile such that after 
differential stretching, areas 342 and 344 each have a constant thickness. 
More particularly, the interlayer 336 may be initially cast or extruded 
with areas 342 and 344 having a slightly tapered thickness profile, with 
their thickness increasing from the top edge towards the bottom edge of 
the interlayer as shown in FIG. 9 so that after differential stretching, 
their profiles will have a constant thickness. For example, interlayer 636 
of the type shown in FIG. 9 would have a thickness that increases in upper 
area 642 from about 0.034 to 0.040 inches (0.086 to 0.102 cm) along upper 
edge 602 to 0.035 to 0.043 inches (0.089 to 0.109 cm), decreases in 
mid-area 637 from about 0.035 to 0.043 inches (0.089 to 0.109 cm) to 0.028 
to 0.035 inches (0.071 to 0.089 cm), and increases in lower area 644 from 
about 0.028 to 0.035 inches (0.071 to 0.089 cm) to 0.029 to 0.036 inches 
(0.074 to 0.091 cm) along lower edge 604. 
Similarly, if sheet 436 in FIG. 6 initially has a constant thickness in 
area 439 and is differentially stretched, the interlayer in area 439 will 
have a slight taper as shown in FIG. 10. For example, interlayer 736 of 
the type shown in FIG. 10 would have a thickness that decreases in upper 
area 739 from about 0.034 to 0.040 inches (0.086 to 0.102 cm) along upper 
edge 702 to 0.031 to 0.039 inches (0.079 to 0.099 cm) and decreases in 
lower area 737 from about 0.031 to 0.039 inches (0.079 to 0.099 cm) to 
0.027 to 0.030 inches (0.069 to 0.076 cm) along lower edge 704. If 
desired, sheet 436 of FIG. 6 may be formed to an initial thickness profile 
such that after differential stretching, area 439 of sheet 436 has a 
constant thickness. More particularly, the interlayer 436 may be initially 
cast or extruded with area 439 having a slightly tapered thickness 
profile, with its thickness increasing from the top edge towards the 
bottom edge of the interlayer so that after differential stretching, its 
profiles will have a constant thickness as shown in FIG. 11. For example, 
interlayer 836 of the type shown in FIG. 11 would have a thickness that 
increases in upper area 839 from about 0.034 to 0.040 inches (0.086 to 
0.102 cm) along upper edge 802 to 0.035 to 0.043 inches (0.089 to 0.109 
cm) and decreases in lower area 837 from about 0.035 to 0.043 inches 
(0.089 to 0.109 cm) to 0.029 to 0.036 inches (0.074 to 0.091 cm) along 
lower edge 804. 
It should be further appreciated that even with windshield configurations 
where interlayer areas 542 and 544 shown in FIG. 8 and area 739 shown in 
FIG. 10 have a slight taper, any optical distortion in these areas due to 
the outer surfaces of the windshield being non-parallel would be minimal. 
Refereing to FIG. 7, the stretching operation also forms the shade band 506 
along arcuate path so that when the interlayer 500 is positioned between 
two glass plies and the assembly is laminated to form a windshield, the 
shade band 506 remains generally parallel with the upper edge of the 
windshield. In this manner, when the windshield is installed in a vehicle, 
the shade band 506 will have a generally horizontal orientation. 
Cast or extruded interlayer may also be made to provide the same blank 
peripheral contour as shown in FIG. 7, with or without the contoured shade 
band. 
It is obvious that multiple sheets of polyvinylbutyral interlayer may be 
used in place of the single sheets 336, 436, 536, 636, 736 and 836 
provided that when they are used in combination, the resulting 
multi-layered interlayer has the same thickness profiles as discussed 
above with respect to a single interlayer blank. 
As can now be appreciated, the partial wedging of the windshields 330 and 
430 can be attained in any convenient manner. For example, one or more of 
the glass plies may have varying thicknesses and/or the interlayer may 
have varying thicknesses in the wedged sections, i.e. area 337 of the 
windshield 330 and area 437 for the windshield 430. The requirement in the 
practice of the invention is that when the windshield is assembled using 
the glass plies and interlayer, selected areas of the windshield have the 
outer surfaces nonparallel to one another while the other areas have the 
outer surfaces parallel or nearly parallel. As used herein and as can now 
be appreciated the term "outer surface parallel" does not require the 
surfaces to be perfectly parallel to one another but have the degree of 
parallelism that is usually observed in the prior art windshields to which 
this invention is directed. The invention also includes windshield 
configurations where the outer major surfaces of the windshield in the 
non-display areas are not parallel but are angularly offset a small amount 
which is less than the offset in the display area, as discussed above. 
Further, the invention was discussed with the windshield having the 
tapered area extending from side to side; however, as can now be 
appreciated only the area in front of the observer may have the partial 
wedge with the remaining area of the windshield having parallel outer 
surfaces. 
Further from the foregoing discussion, it can now be appreciated that the 
invention may be practiced by tapering selected sections of a sheet of 
interlayer, by providing a piece of tapered interlayer over a sheet of 
interlayer having a constant thickness, by providing a glass ply having 
selected portions tapered by adding a piece of tapered glass to a glass 
ply having a constant thickness and/or by removing glass, e.g., by 
grinding and polishing a selected section of the glass. 
Still further, the invention may be practiced using colored glass of the 
type known in the art, e.g. such as that taught in U.S. Pat. No. 
4,792,536, may be used on windshields having environmental coatings, e.g., 
of the type taught in U.S. Pat. No. 4,610,771, may be used on heatable 
windshields, e.g., of the type taught in U.S. Pat. No. 4,820,902 or may be 
used with a coating on any surface of the glass plies at least in the area 
of the windshield having nonparallel outer surfaces to enhance the image, 
e.g., coatings of the type taught in U.S. Pat. No. 3,899,241, which 
teachings of these patents are hereby incorporated by reference. 
The forms of this invention shown and described in this disclosure 
represent illustrative preferred embodiments and various modifications 
thereof. It is understood that various changes may be made without 
departing from the scope of the invention as defined by the claimed 
subject matter which follows.