Holographic Rear Window Stoplight

1. Field of the Invention

The present invention relates to a high mounted or rear window stoplight that is mounted within the line of sight of the driver of the vehicle when he views the rear of the vehicle through the rearview mirror and, more particularly, to a substantially transparent holographic rear window stoplight that can be mounted on the rear window of a vehicle without obscuring the vision of the driver.

2. Description of Related Art

Currently, United States automobiles must be provided with a high mounted, centered brake light or stoplight to maximize the visibility of an indicator to drivers following the subject vehicle. Usually, these stoplights are now mounted above or below an optimum position and provide a partial obscuring of the line of sight or viewing angle of the driver through his rearview window. The optimum position being ideally located at the driver's eye level for maximum visibility. Such a location, however, obscures the rear viewing angle of the driver of the vehicle. The configuration and position of the mounting of these high mounted, centered brake lights also detract from the aesthetic design characteristics of the vehicle.

While the use of hologram images having a plurality of diffraction gratings is generally known in the prior art such as disclosed in U.S. Pat. Nos. 3,940,204, 4,447,128 and 4,487,472, there is still a demand in the prior art to provide a new and improved rear stoplight assembly for vehicles.

An improved rear stoplight assembly capable of being mounted within the field of view of the rearview window of a vehicle without obscuring the vision of a driver is provided. A transmission hologram member capable of being positioned adjacent the rear window, such as being laminated onto the surface of the rear window, can be illuminated by a light projected at an appropriate angle and in an appropriate wavelength band characteristic of a stoplight color, onto the hologram member. The light source, for example, can be mounted on the rear deck below the rear window and out of the line of sight of the driver to prevent any obscuring of the vision of the driver.

In one embodiment, the hologram member supports a holographic image with a characteristic of responding to a predetermined relative angle of impact of the source of light to reflect that light to the rear of the vehicle about a substantially horizontal plane parallel to the vehicle support surface. The hologram member itself is substantially transparent to the driver and permits him to view objects to the rear of the car without obscuring his vision even when activated to provide a stoplight effect. The hologram member, however, is capable of providing a stoplight effect when illuminate at a predetermined angle relative to the source of light. Preferably, the reflection hologram member comprises a transparent flexible substrate that can be laminated by an optical cement to the rear surface of the rear window. A double film layer of a recording material, such as a dichromated gelatin or a poly-N-vinyl carbazole film, is bonded to the substrate for providing a pair of diffraction gratings or fringes to provide a folded optical path for the transmission of the stoplight light through the hologram. By folding the optical path, exterior light is prevented from being reflected directly forward into the vision of the driver. Thus a substantially transparent stoplight is provided within the viewing angle of the driver to provide a centered stoplight effect without any distraction to the actual driver.

In another embodiment, the hologram member can be constructed so that the reference beam enters through the edge of the hologram member. Thus, edge illumination will provide the stoplight effect and internal reflection towards the driver by ambient lighting will be avoided.

The hologram member can be multiple layered with each layer carrying a predetermined diffraction grating or it can be of a single layer with a complex diffraction grating that can be activated by different wavelengths of light and/or incidence angles to provide, selectively, either a stoplight effect and/or predetermined indicia, such as a caution or emergency warning.

The foregoing and other objects, advantages and features of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.

In the accompanying drawings:

FIG. 1 shows the schematic of a conventional stoplight arrangement on a vehicle that obscures a portion of the view of the driver through his rearview mirror;

FIG. 2 discloses a single layer transmission hologram stoplight;

FIG. 3 discloses a cross-sectional view of a portion of FIG. 2;

FIG. 4 shows an alternative embodiment with a dual reflection hologram stoplight configuration;

FIG. 5 shows a cross-sectional view of the dual reflection hologram stoplight and rearview window;

FIG. 6 shows a schematic of a brakelight circuit;

FIG. 7 shows a double layer edge lit hologram stoplight;

FIG. 8 shows a basic method of constructing the edge lit hologram of FIG. 7; and

FIG. 9 shows a single holographic layer edge lit hologram.

In the following description, like parts are designated by like reference numbers through the several drawings.

The following description is provided to enable any person skilled in the optical and automotive fields to make and use the invention, and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art since the generic principles of the present invention have been defined herein specifically to provide a relatively economical and easily manufactured hologram rear window stoplight assembly.

Referring to FIG. 1, a vehicle 2 has a conventional rear window 4 mounted within an aperture in the vehicle body for the rear window. A high mounted, central stoplight assembly 6 is mounted on the rear deck at a position below the driver's eye level but still blocking a portion of the rearview window of the driver. As can be appreciated, the awkward structure of the conventional high mounted brake light is relatively unaesthetic to an observer's eyes. Additionally, the stoplight is relatively small and not positioned at the eye level of the following drivers. Other variations of this conventional rear light have been utilized in different manufacturer's vehicles.

To resolve these problems, a unique transmission hologram 8 is proposed in FIG. 2 in combination with a hidden light source capable of projecting light at a specific angle for interfacing with the transmission hologram and subsequent diffraction to the rear of the vehicle. The transmission hologram 8 can comprise a flexible transparent substrate surface, such as mylar, that can be laminated onto the interior surface of the rear window 6. The actual holographic layer can be about 30 microns in thickness. An illumination light source 10 can be mounted in the trunk of the vehicle or at least in a low position on the rear window deck so that it is out of the line of sight or at least out of the viewing angle of the driver. The light source 10 could be connected in series with the brakelight circuit of FIG. 6 to be activated by the brake foot pedal 26 of the vehicle.

The hologram 8 includes a laminated film 20 that has been developed with a holographic image having a characteristic of directing light of a particular wavelength range and angular incidence to the rear of the vehicle as shown in FIG. 2 and FIG. 3. For example, the holographic image can be responsive to an angle of 60.degree. to 120.degree. to direct the light substantially rearward. Diffraction gratings having recorded holographic fringes can be developed in the film 8 for providing a direction of light from the light source 10 to extend approximately parallel in a horizontal plane from the rear window surface of the vehicle. In this regard, the particular angular visibility requirement of a high mounted stoplight, e.g., the field angle of view about a central optical axis (plus or minus 45.degree.), can be determined during the construction of the holographic image. The scale of FIG. 3 is exaggerated for illustrative purposes of the present invention.

The holographic procedures for forming the hologram of the present invention can be found in known reference works, such as "Optical Holography" by Collier et al., Academic Press, New York, N.Y. (1971). Additionally, the holographic image can be developed to be particularly sensitive to only a predetermined bandwidth of the illumination source. As is well known in the holographic field, the hologram can be made to operate over a broader wavelength band giving increased brightness for a broadband source, but trading off increased coloration provides a lower phototopic see-through capability.

As can be appreciated, predetermined indicia images can also be shown in the holographic image, for example, flashing messages in emergency situations can be included that can be activated by light of a different wavelength and/or incidence of impact with the hologram 8. While the advantages of the present invention in providing a relatively transparent transmission hologram 8 centered in the rear window 6 of a vehicle for projecting the stoplight or brake light rearward from the illumination source 10 is achieved in the embodiment of FIG. 2, a potential problem can exist in that exterior light such as sun rays 12 can, at certain angles, be diffracted into the vehicle to create a glare in the angle of view of the driver. Certain rearview windows particularly in sports car models are provided with sun shutters, blinds or shields that could prevent this from occurring.

However, to provide a more universal hologram rearview stoplight, an embodiment of two separate holograms shown in FIG. 4 is preferred to resolve this problem of exterior light being diffracted into the vehicle. As with the first embodiment, a substrate of a substantially transparent flexible configuration can be laminated onto the interior surface of the rearview window with the optical cement 18. Actually, the hologram can be laminated onto the exterior surface of the window 4 and still be operable, but this would expose it to the weather. Referring to FIG. 5, a cross-sectional view of the window 4 and dual reflection hologram 14 is disclosed. The substrate 16 is adhered to the surface of the rear window 4 by a layer of optical cement 18. A first hologram film layer 20 is deposited on the substrate 16 and a second hologram film layer 22 can be subsequently deposited on top of the first hologram film layer 20. The hologram film layers 20 and 22 can be formed of conventional film coatings, such as poly-N-vinyl carbazole or a dichromated gelatin, such as disclosed in known reference books, e.g., "Dichromated Gelatin For The Fabrication Of Holographic Optic Elements" by Change et al., Applied Optics, Vol. 18, pgs. 2407-17 (1979). Finally, an abrasion resistant and moisture protection layer is deposited on the second hologram film layer 22, such as a film silicon dioxide or a silicon nitride. A series of parallel fringes are formed as a diffraction grating F1 in the first hologram layer 20, and a second series of parallel fringes are developed as a second diffraction grating F2 at a predetermined angle to the first diffraction gratings in the second hologram layer 22. As can be seen by the schematic illustration of light coming from the light source in FIG. 5, the effect is to provide a folded optical path (similar to a roof prism effect) to the desired brake light so that it is transmitted on the desired optical axis to the rear of the vehicle. The practical effect of this double reflection off of the gratings is to eliminate the possibility of overhead light reflection from the sun entering the car as can be seen in FIG. 4. Thus, this multiple layer hologram configuration not only can replace the current high mounted brake light required in all new United States automobiles but it also prevents outside light from being reflected back into the car, as would be the case with the simple single hologram approach of the first embodiment. Additionally, since the hologram is essentially transparent to the driver, particularly with a narrow bandwidth transmission capability, it can be mounted in a central optimum position at the eye level of the driver on the rear window for maximum visibility to the rear vehicles. FIG. 7 discloses a composite hologram that can serve both as a stoplight and as providing a warning message, such as an emergency or caution signal. The hologram member 30 includes double holographic layers 32 and 34 that are mounted on a flexible transparent substrate 36. The substrate 36 can be glued to the interior surface of a rear window 38 by a layer of optical cement 40. An abrasive resistant protection layer 42 can be provided on the interior surface of layer 34. A series of parallel fringes are formed as a diffraction grating in the first holographic layer 32 to direct a reference light source 44 that is coupled, e.g., through a fiber optic coupling 46 directly to the lower edge of the layer 32. Thus, the first holographic layer 32 can direct red light from the light source 44 to the rear of the vehicle to serve as a stoplight.

The second holographic layer 34 can be constructed to contain an image and or indicia in the form of a diffraction grating such as a caution or warning indicator. Again, an appropriate light source 45 can be coupled to the lower edge of the layer 34 through a fiber optic coupling 47 to reconstruct the image.

In another embodiment of the invention, the holographic layers 32 and 34 can be selectively activated independent of each other with external illumination sources, such as 10 of FIG. 3, that differ in either wavelength or incident angle.

As can be appreciated, it is within the ordinary skill of this art to provide two or more separate diffraction gratings in the same common holographic layer that can be independently activated by different wavelengths. Thus, the embodiment of FIG. 7 can be accomplished equivalently with a single holographic layer if selectivity is based on wavelength or incident angle difference. Finally, if images or messages are contained within a holographic layer then the physical envelope of the windshield should be taken into consideration during the construction of the hologram to compensate for any distortion, for example, the object source can be distorted to compensate for the effect of the window curvature, or the holographic layer can be physically defined in the same position that it will be mounted on the window to permit the development of the diffraction gratings to take into account the non-planar configuration.

FIG. 8 discloses a construction of a single layer hologram 50 wherein a source light or object image is directed at a holographic layer 52. A Reference Light is directed along the length of the holographic layer in a fashion analogous to a wave guide. The interference pattern of the Source Light and the Reference Light is developed as the diffraction grating in the holographic layer.

FIG. 9 represents a practical application of the embodiment of FIG. 8 mounted in a rear deck 78 adjacent the rear window. Such a mounting is of particular advantage in realizing an avoidance of internal light reflection towards the driver and in saving space since a fiber optic link can accommodate a remote light linkage with the hologram 60. The hologram 60 includes the antiabrasive coating 62, the holographic layer 64 and the transparent substrate 66 that is attached by optical cement 68 to the interior of the rear window 70. The light source 72 transmits a replication of the Reference Light through a fiber optic link 74 to a optical coupler 76 that insures a proper light distribution into the edge of the holographic layer 64.

As a result, a red light is appropriately transmitted to the rear while the driver still has full vision through the rear window including through the holographic layer 64.

These hologram members can be relatively inexpensive in mass production with substantially little weight addition to the vehicle. They also do not occupy any substantial space in the interior of the vehicle. Finally, they do not mar the aesthetic design of the vehicle thereby permitting the vehicle designers to provide an ideal compliance with the federal regulations on high mounted brake lights while rendering the brake light relatively invisible when not in use to both the exterior observer and, even when in use, invisible to the driver inside the vehicle. As can be appreciated, the present invention can be provided as either a retrofit kit to modify an existing vehicle or provided as a manufactured improved rear window.

Various modifications to the above described invention may be readily apparent to those skilled in the optical and automotive fields in view of the above described generic concepts. Therefore, the scope of the present invention should be interpreted solely from the following claims.