Liquid crystal displays (LCDs) find wide usage in applications such as air vehicle cockpits, automobile dashboards, etc. Such LCDs generally include a liquid crystal layer sandwiched between a pair of polarizers and a pair of electrodes. Voltage is selectively applied by the electrodes across the liquid crystal (LC) layer so as to selectively portray an image to the viewer by way of the front polarizer.
FIG. 1(a) illustrates prior art liquid crystal display 3 in an air vehicle cockpit 5 occupied by pilot 7 and copilot 9. In prior art cockpit (or automobile dashboard) applications such as this, situations commonly arise where a portion of the image 13 emitted from the active matrix liquid crystal display (AMLCD) 3 is reflected at point 11 off of cockpit canopy 13 toward copilot 9, while another portion of the image is transmitted through canopy 13. Air vehicle cockpit 5 as shown in FIG. 1(a) is designed so as to house copilot 9 directly behind and above pilot 7 as illustrated. Thus, when a portion 15 of the image 13 from LCD 3 is reflected off of canopy 13 at reflection point 11, it can be seen by copilot 9 when the copilot is looking out of the cockpit. The other portion or remainder of the image is transmitted through the canopy as shown at 14. This reflection 15 from point 11 is, of course, undesirable in view of the fact that copilot 9 typically wishes to locate targets and the like exterior the aircraft and does not wish to be interfered with by such display reflections. Needless to say, it is undesirable to have the pilot's or copilot's vision obstructed during flight operations due to display image reflections off of the canopy.
The above-referenced display reflection problems inside of aircraft cockpits are most severe at night when reflections 15 exceed the intensity of the light or environment outside of the cockpit, but are also present during daylight hours. The majority of undesirable reflections 15 come from the interface between the air inside of the cockpit and the material (e.g. glass or plastic) of canopy 13. Typically, the index of refraction of cockpit canopy 13 is about 1.5. Depending upon the shape of the canopy, reflections 15 may be concentrated in convex curved areas or mirror-like in flat canopy areas.
In view of the above, it is clear that there exists a need in the art for a liquid crystal display and method of implementation for reducing display reflections off of exterior mediums such as canopies of aeronautic cockpits.
FIG. 1(b) is a reflectance versus incident angle .THETA..sub.i graph from "Optics" by Eugene Hecht, 1987. As shown in FIG. 1(b), for an incoming unpolarized wave made up of two incoherent orthogonal polarization states, only the component polarized normal (R.sub.perpendicular) to the incident plane and therefore parallel to the surface of the medium will be reflected. The incident plane is defined by the line connecting the reflection point on the exterior medium and the display front surface and the medium surface normal The component (R.sub.parallel) parallel to the plane of incidence is not reflected and is transmitted through the medium as shown at 14. The particular angle of incidence for which this situation occurs is designated by .THETA..sub.p and is referred to in the art as the polarization angle or Brewster's angle, whereupon .THETA..sub.p +.THETA..sub.t =90.degree.. Therefore, from Snell's Law, EQU n.sub.i Sin.THETA..sub.p =n.sub.t Sin.THETA..sub.t
and the fact that .THETA..sub.t =90.degree.-.THETA..sub.p, it follows that EQU n.sub.i Sin.THETA..sub.p =n.sub.t Cos.THETA..sub.p
and EQU Tan.THETA..sub.p =n.sub.t /n.sub.i.
This is known as Brewster's Law. The above parameters and equations are defined in "Optics", by Eugene Hecht, the disclosure of which is incorporated herein by reference.
Thus, when the incident light ray or beam is in air (n.sub.i =1) and if the transmitting medium (i.e. exterior medium such as canopy 13 or the like) is glass, in which case n.sub.t =1.5, the polarization angle .THETA..sub.p is about 56.degree.. Similarly, this concept may be exemplified by the situation when an unpolarized beam strikes the surface of a pond (n.sub.t =1.33 for water) at an angle of 53.degree., the reflected beam will be completely polarized with its E-field perpendicular to the plane of incidence or, if you like, parallel to the water's surface.
For linearly polarized light having its E-field parallel to the plane of incidence, the "amplitude reflection coefficient" is defined as r.sub.parallel =(E.sub.or /E.sub.oi).sub.parallel, that is, the ratio of the reflected to incident electric field amplitudes. Similarly, when the electric field is normal to the incident plane, r.sub.perpendicular =(E.sub.or /E.sub.oi).sub.perpendicular. The corresponding irradiance ratio (the incident and reflected beams have the same cross-sectional area) is known as the "reflectance", and since irradiance is proportional to the square of the amplitude of the field, EQU R.sub.Parallel =r.sup.2.sub.parallel
and EQU R.sub.perendicular =r.sup.2.sub.perpendicular
Squaring the appropriate Fresnel equations results in ##EQU1##
The reflectance for linearly polarized light with E or a polarization direction parallel to the plane of incidence vanishes and the beam is completely transmitted when the angle of incidence .THETA..sub.i equals Brewster's angle .THETA..sub.p. This is shown in FIG. 1(b) where n.sub.t =1.5 and Brewster's angle .THETA..sub.p equals about 56.degree., so that the reflectance of R.sub.parallel is substantially zero when .THETA..sub.i substantially matches .THETA..sub.p or Brewster's angle. FIG. 1(b) is a plot for the particular case where n.sub.i =1 and n.sub.t =1.5. The middle curve corresponds to incident natural light. As shown, when the polarization direction of incoming linearly polarized light is substantially perpendicular to the plane of incidence at Brewster's angle of about 56.degree., substantial reflection results at reflection point 11. However, at Brewster's angle .THETA..sub.p of about 56.degree., when the direction of polarization of incoming light is substantially parallel to the plane of incidence (i.e. parallel to the Y-axis), the result is substantially no reflectance and nearly complete transmission at point 11. As shown by the middle curve in FIG. 1(b), when the polarization direction is mixed, reflection from point 11 results.
FIG. 1(c) illustrates incident light 16 being reflected at point 11 off of canopy 13. Reflection 15 and transmission 14 define angles .THETA..sub.r and .THETA..sub.t with the Y-axis (normal to the surface of medium 13) respectively. The angle of incidence .THETA..sub.i is also shown in FIG. 1(c).
U.S. Pat. No. 4,025,161 discloses a liquid crystal display device including a quarter wavelength retarder disposed on the front of the display so that the front polarizer is between the liquid crystal material and the retarder. The quarter wavelength retarder together with the front polarizer form a circular polarizer in the '161 patent. Unfortunately, if the liquid crystal display of the '161 patent were positioned in place of display 3 in FIG. 1(a), undesirable image reflection 15 would still occur in a substantial amount because the circularly polarized display image incident upon reflection point 11 would not be substantially polarized parallel to the plane of incidence and therefore reflection 15 toward copilot 9 would result thus inhibiting the copilot's view. In other words, circularly polarized light incident upon canopy 13 at reflection point 11 (even when .THETA..sub.i =.THETA..sub.p) in FIG. 1(a) would be reflected at 15 toward viewer 9, this, of course, being undesirable.
U.S. Pat. No. 4,266,859 discloses a liquid crystal display of the reflective dichroic type, including upper and lower quarter wavelength retarders. Again, the LCD of the '859 patent suffers from the same drawbacks as those discussed above with respect to the '161 patent.
In view of the above, it is clear that there exists a need in the art for a liquid crystal display designed so as to reduce image reflection off of exterior mediums (e.g. automotive windshields or cockpit canopies) at a reflection point and a method of implementing same. There also exists a need in the art for a liquid crystal display which when arranged in an appropriate manner, causes the image emitted toward a reflection point on the exterior medium to have a polarization direction substantially parallel to the plane of incidence at the reflection point and .THETA..sub.i to substantially match .THETA..sub.p so that substantially no reflection results and the image is mostly transmitted through the medium instead of being reflected thereby.
It is a purpose of this invention to fulfill the above-described needs, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.