Head-up display with narrow band reflective polarizer

#The present invention relates to projection displays that allow a user to see, displayed information while keeping vision in the general direction of other sources of information. The displayed information can be reflected from a window, eyepiece component, or portion thereof through which the user can also view the other sources of information.

#The term “Head-up display” (HUD) is used herein to refer to such display systems, whether employed in the window of a vehicle such as an aircraft, watercraft, or landcraft (including motor vehicles such as automobiles, trucks, and motorcycles), in smaller scale systems such as goggle lenses or helmet visors, or in other diverse applications.

#A wide variety of HUD systems are known. Commonly, a projection system is combined with a partial mirror (a partial reflector and partial window) as the final optical component for forming a projected image viewable by the user. Simultaneously, the user can view other scenes through the partial mirror. The partial mirror is an important component affecting the usability of the display. Generally, the reflectivity of the partial mirror must be sufficient to reflect light from the projector, but the partial mirror must also be sufficiently transparent to provide adequate viewing through it.

#Throughout the specification reference is made to the appended drawings, where like reference numerals designate like elements, and wherein:

#FIG. 1 is a fragmentary schematic sectional view of a PRIOR ART display system;

#FIG. 2 is a fragmentary schematic sectional view of a HUD system as described herein;

#A partial schematic view of an illustrative HUD system 10 is shown in FIG. 2. In brief, a projection system 12 is provided to direct light 14 towards a target area 16 of a window 18. A reflective polarizer 20, disposed proximate the target area 16, then reflects at least some of the projected light towards the intended viewer 22. Simultaneously, the reflective polarizer 20 and the window 18 transmit at least some light 24 from the outside environment to permit observation thereof by viewer 22.

#Projection system 12 can be a conventional system that projects a visible light beam or image, and can include known elements such as an LCD, electroluminescent panel, incandescent or phosphorescent light source, CRT, LEDs, and lenses, collimators, reflectors, and/or polarizers. The emitted light can be substantially monochromatic, polychromatic, narrow band, or broad band, but preferably overlaps at least a portion of the (visible) spectrum from about 400 to 700 nm. Significantly, the light 14 emitted towards target area 16 is substantially linearly polarized in the plane of the figure as shown. While it is understood that system 12 will emit light over a finite angular cone, only one ray of light 14 is depicted for ease of illustration. Furthermore, system 12 can also include a mechanism such as a tilting mirror or displacement means to change the angle and/or position of emitted light so as to accommodate viewers at different positions or heights.

#Light 14 travels along an illumination axis 26 and strikes a first exposed major surface 18a of window 18. As suggested previously, window 18 can comprise any of a wide variety of transparent members, and can be unitary or laminated, flat or curved (simple or compound curvature), water clear or tinted, can have focusing properties (e.g. in the case of goggles or other eyewear), and can be composed of any conventional glasses and/or plastics. For low cost and weight, the window can comprise a sheet of glass or other transparent material with plane parallel surfaces. As shown, the polarized light 14 strikes surface 18a at an acute angle θ1 with respect to an axis 28 that is normal to surface 18a at the point of entry. The normal axis 28 and illumination axis 26 define a plane of incidence that is coincident with the plane of the figure. Light 14 is polarized in the plane of incidence and thus is designated “p-polarized” light. As is known, ordinary optical materials such as glass and plastic exhibit a Brewster angle θB in air that is a function of the refractive index of the particular optical material, from roughly 55 to 60 degrees for typical window materials. Although strictly speaking θB is a function of optical wavelength due to dispersion, such effects are typically very minor and in most cases θB can be treated as constant over the visible spectrum. For light incident at the Brewster angle, any p-polarized component has zero reflectivity, while any s-polarized component (whose electric field vector is perpendicular to the plane of incidence) has a reflectivity even greater than its reflectivity at near normal incidence. P-polarized light incident at angles near θB experience nonzero but very low reflectivity. Therefore, by keeping incident angle θ1 equal to or close to θB, and ensuring that light 14 has little or no s-polarization component, no significant reflection of projection light 14 takes place at surface 18a. This avoids a “ghost” image being created at surface 18a. The degree to which θ1 can deviate from θB while still maintaining “no significant reflection” depends on many system variables and requirements. Generally, deviations of at least a few degrees of arc are generally believed permissible. In some cases deviations of greater than 5 or even 10 degrees may be acceptable, since p-pol reflectivity is still typically much reduced at such angles compared to reflectivity at near-normal incidence, particularly for angles less than θB. Note that some angular deviations from θB are unavoidable to the extent projection system 12 is uncollimated, target area 16 is extended, and surface 18a is non-flat.