Patent Description:
Mirrors are generally critical to automotive safety. Conventional mirrors have several limitations such as, e.g., excessive glare, limited field of view, damage susceptibility and aerodynamic drag. Electronic mirrors, which more and more enhance or replace traditional rear and side-view mirrors, significantly increase vehicle safety, comfort and convenience. <CIT>, <CIT>, <CIT> constitute prior art useful for understanding the invention.

An electronic mirror includes a liquid crystal cell, wherein the liquid crystal cell includes a first transparent electrode, a second transparent electrode, a liquid crystal layer including liquid crystal molecules arranged between the first transparent electrode and the second transparent electrode, and an AC voltage source configured to apply an alternating voltage across the liquid crystal layer between the first transparent electrode and the second transparent electrode, wherein, when a voltage is applied across the liquid crystal layer, the liquid crystal molecules in the liquid crystal layer change their orientation, and the electronic mirror is configured to apply a varying voltage across the liquid crystal layer which gradually decreases from outer areas towards the center of the liquid crystal layer such that the refraction index of the liquid crystal layer gradually varies from outer areas towards the center of the liquid crystal layer. The liquid crystal cell further includes a polarizing filter layer, wherein the second transparent electrode is arranged between the polarizing filter layer and the liquid crystal layer, and a reflective polarizing filter layer, wherein the first transparent electrode is arranged between the reflective polarizing filter layer and the liquid crystal layer.

Other systems, methods, features and advantages will be or will become apparent to one with skill in the art upon examination of the following detailed description and figures. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention and be protected by the following claims.

The arrangement may be better understood with reference to the following description and drawings.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely examples of the invention that may be embodied in various and alternative forms.

It is recognized that directional terms that may be noted herein (e.g., "upper", "lower", "inner", "outer", "top", "bottom", etc.) simply refer to the orientation of various components of an arrangement as illustrated in the accompanying figures. Such terms are provided for context and understanding of the disclosed embodiments.

Referring to <FIG>, a vehicle <NUM> is schematically illustrated. The vehicle <NUM> comprises rear and side view mirrors <NUM>, <NUM>. In the past, conventional mirrors were used for rear (class I mirrors) and side view mirrors (class III mirrors). In order to provide a certain amount of magnification, a surface of a conventional mirror may be curved. Conventional mirrors, however, are replaced more and more by electronic mirrors. Electronic mirrors comprise a display such as, e.g., a TFT (Thin film transistor) display. One or more cameras may be mounted on the vehicle <NUM> which capture images of the surroundings of the vehicle <NUM>. Such images may then be displayed on the display of the electronic mirror. Electronic mirrors provide several advantages over conventional mirrors. However, rear and side-view mirrors are safety critical components, Therefore, if the electronic mirror fails for whatever reason (e.g., failure of the cameras or the display), the mirror functionality still has to be ensured.

For this reason, a liquid crystal cell <NUM> may be arranged in front of the display <NUM>. This is schematically illustrated in <FIG>. The liquid crystal cell <NUM> comprises a first transparent electrode 34a, a second transparent electrode 34b, and a liquid crystal layer <NUM> comprising liquid crystal molecules arranged between the first transparent electrode 34a and the second transparent electrode 34b. The liquid crystal cell <NUM> further comprises an AC voltage source <NUM> (only referred to as voltage source in the following) configured to apply an alternating voltage across the liquid crystal layer <NUM> between the first transparent electrode 34a and the second transparent electrode 34b. When no voltage is applied to the liquid crystal layer <NUM>, the liquid crystal molecules are oriented in a first direction (see <FIG>). For example, the liquid crystal molecules may be arranged perpendicular to the first and second transparent electrodes 34a, 34b. This allows polarized light to be transmitted through the liquid crystal layer <NUM>.

The liquid crystal cell <NUM> further comprises a polarizing filter layer 32b, wherein the second transparent electrode 34b is arranged between the polarizing filter layer 32b and the liquid crystal layer <NUM>, and a reflective polarizing filter layer 34a, wherein the first transparent electrode 34a is arranged between the reflective polarizing filter layer 32a and the liquid crystal layer <NUM>. The polarizing filter 32b and the reflective polarizing filter 32a may be arranged such that the transmission axis of the polarizing filter 32b and the transmission axis of the reflective polarizing filter 32a are perpendicular to each other, as is indicated in <FIG>. The reflection axis of the reflective polarizing filter 32a, therefore, is arranged such that light that is polarized by the polarizing filter 32b and transmitted through the liquid crystal layer <NUM> is reflected on the surface of the reflective polarizer filter 32a. That is, when no voltage is applied across the liquid crystal layer <NUM>, the liquid crystal cell <NUM> functions in a similar way as a conventional mirror. The display <NUM> in this case is not visible through the liquid crystal cell <NUM>.

Now referring to <FIG>, if a voltage (liquid crystal cells generally require that an alternating voltage be applied) is applied across the liquid crystal layer <NUM>, the liquid crystal molecules in the liquid crystal layer <NUM> change their orientation. For example, the liquid crystal molecules may twist and tip towards a plane that is parallel to the first and second transparent electrode 34a, 34b. If the applied voltage is larger than a threshold voltage, the liquid crystal molecules tip to such an extent that the polarized light transmitted through the liquid crystal layer <NUM> is rotated by <NUM>° and is therefore transmitted through the reflective polarizing filter 32a. The display <NUM> is then visible behind the liquid crystal cell <NUM> and contents displayed on the display <NUM> are visible through the liquid crystal cell <NUM>. When images captured by cameras on the outside of the vehicle <NUM> are displayed on the display <NUM>, a voltage that is larger than the threshold voltage may be applied to the liquid crystal layer <NUM> such that the electronic mirror function is active.

The reflective polarizer filter 32a, however, has a flat surface in order to allow the light to pass through the reflective polarized filter 32a undisturbed when a voltage is applied to the liquid crystal layer <NUM> (e-mirror function active). A flat reflective polarizer filter 32a in the reflective mode (no voltage applied to the liquid crystal layer <NUM> and e-mirror function inactive), however, does not provide any magnification.

In order to provide a magnification of the reflected images in the reflective mode, the electronic mirror according to the claimed invention is configured to apply a varying voltage across the liquid crystal layer <NUM> which gradually decreases from outer areas towards the center of the liquid crystal layer <NUM> such that the refraction index of the liquid crystal layer <NUM> gradually varies from outer areas towards the center of the liquid crystal layer <NUM>. The liquid crystal layer <NUM> generally comprises a certain refraction index. For example, the liquid crystal layer <NUM>, when no voltage is applied to the liquid crystal layer <NUM>, may have a refraction index of <NUM> or more. This refraction index, however, generally depends on several different parameters. In particular, the refraction index of the liquid crystal layer <NUM> changes as a function of the voltage applied across the liquid crystal layer <NUM>. That is, if a first voltage is applied to a first area of the liquid crystal layer <NUM> and a second voltage, which is lower than the first voltage, is applied to a second area of the liquid crystal layer <NUM>, the refractive index of the first area differs from the refractive index of the second area.

By applying a suitable voltage profile to the liquid crystal layer <NUM>, a spherical or aspherical mirror reflective index profile may be generated. The focusing power of the electronic mirror can be controlled by the variation of the electric field applied to the liquid crystal layer <NUM> and the frequency of the electric field. The birefringent properties of the liquid crystal layer <NUM> in combination with a reflective polarizing filter 32a may be used to provide variable focusing properties by dynamically changing the refractive index of the liquid crystal layer <NUM>.

Now referring to <FIG>, one example of an electronic mirror is schematically illustrated. In this example, the first transparent electrode 34a comprises a first area <NUM> which causes a voltage applied across the liquid crystal layer <NUM> to gradually decrease from areas of the liquid crystal layer <NUM> arranged adjacent to outer areas of the first area <NUM> towards areas of the liquid crystal layer <NUM> arranged adjacent to the center of the liquid crystal layer <NUM>. This can be achieved, for example, by a first area <NUM> having a high resistivity (electrical contactivity or sheet resistance) such as, e.g., 5Ω/sq (also denoted ohms square, Ω□, or Ω/□) or more. The first area <NUM> may be surrounded by a second area <NUM> of the first transparent electrode 34a, wherein the second area <NUM> is configured to apply the same voltage to all areas of the liquid crystal layer <NUM> that are arranged adjacent to the second area <NUM>. The resistivity of the second area <NUM>, for example, may be significantly lower than the resistivity of the first area <NUM>, such that the resistivity does not significantly influence the resulting voltage that is applied to the areas of the liquid crystal layer <NUM> which are arranged adjacent to the second area <NUM>. That is, the same voltage is applied to all areas of the liquid crystal layer <NUM> that are arranged below the second area <NUM> and below the edge region of the first area <NUM>. However, this voltage decreases towards the center of the first area <NUM>, due to the high resistivity of the first area <NUM>.

This is further illustrated by means of the equivalent circuit shown in <FIG> (top). The first area <NUM> can be seen as a plurality of resistors R. Each resistor R causes a voltage drop. That is, a voltage V1, V6 applied to the liquid crystal layer <NUM> in areas below the edge region of the first area <NUM> is greater than a voltage V3, V4 applied to the liquid crystal layer <NUM> in areas below a center of the first area <NUM>. The liquid crystal layer <NUM> in the equivalent circuit of <FIG> is represented by capacitors C. In <FIG> only seven resistors R are exemplarily illustrated. The first area <NUM>, however, can be seen as being represented by a much greater number of resistors R, resulting in a gradual decrease of the voltage V towards the center of the first area <NUM>. This gradual decrease of the voltage (potential difference) as a function of the position across the first area <NUM> is schematically illustrated in the upper diagram in <FIG>. As has been described above, a different voltage applied to the liquid crystal layer <NUM> results in a different refraction index. As can be seen in the lower diagram of <FIG>, the resulting phase profile is an inversion of the voltage profile.

In <FIG>, the voltage and phase profiles are schematically illustrated along one line through the liquid crystal layer <NUM>. As can be seen in <FIG>, the first area <NUM> may have a round shape, for example. Due to the low resistivity of the second area <NUM>, if the same voltage is applied to two opposite ends of the first transparent electrode 34a (voltage source <NUM> is coupled to two opposing ends of each of the first transparent electrode 34a and the second transparent electrode 34b), the same voltage is applied along the entire circumference of the first area <NUM> (due to the low resistivity of the second area <NUM>). The voltage decreases from each point along the circumference of the first area <NUM> towards the center of the first area <NUM>. The resulting phase profile in this case is parabolic with its maximum located at the center of the first area <NUM> which, in the present example corresponds to the center of the liquid crystal layer <NUM>. A parabolic phase profile results in the liquid crystal cell <NUM> resembling a cylindrical lens. Therefore, if an annular shaped second area <NUM> is used to surround and electrically connect the first area <NUM> to the driving potential, this results in a bowl shaped potential and a phase profile that is similar to a spherical lens. In this way, magnification can be achieved in the reflective mode of the arrangement. In order to ensure that the mirror remains in the reflective mode, the voltage applied to the liquid crystal layer <NUM> may be below the threshold voltage that would be necessary to activate the e-mirror function.

In the examples illustrated above, a varying voltage profile across the liquid crystal layer <NUM> is generated by means of a high resistivity area <NUM>. This, however, is only one example. A varying voltage profile, generally, can also be generated by any other suitable means.

As is illustrated in <FIG>, an electronic mirror <NUM> (e.g. a rear view mirror) may comprise a single liquid crystal cell <NUM>. In this way, magnification can be provided over the entire area of the electronic mirror <NUM>. As is illustrated in <FIG>, however, it is also possible that an electronic mirror <NUM> comprises a plurality of liquid crystal cells <NUM>. The liquid crystal cells <NUM> may be arranged in a regular pattern, for example. Each liquid crystal cell <NUM> can be controlled individually. That is, magnification can be provided for only some of the liquid crystal cells <NUM>. If, for example, an object is reflected in the electronic mirror <NUM> (e.g., a vehicle or motorcyclist approaching from behind) which may represent a potential danger, this object may be highlighted by magnifying it. That is, one or more, but not all of the liquid crystal cells <NUM> may be provided with a voltage that is below the threshold voltage such that magnification is provided but the liquid crystal cell <NUM> is still in the reflective mode, while other liquid crystal cells <NUM> are not provided with a voltage (OFF state, reflective mode). A position of an object that is to be highlighted by magnification on the electronic mirror <NUM> can be determined by any suitable means.

Claim 1:
An electronic mirror (<NUM>) comprises a liquid crystal cell (<NUM>), wherein the liquid crystal cell (<NUM>) is configured to be operated in a reflective mode in which external light is reflected or in a transmissive mode in which an electric mirror function is activated; the liquid crystal cell comprises:
a first transparent electrode (34a);
a second transparent electrode (34b);
a liquid crystal layer (<NUM>) comprising liquid crystal molecules arranged between the first transparent electrode (34a) and the second transparent electrode (34b); and
an AC voltage source (<NUM>) configured to apply an alternating voltage across the liquid crystal layer (<NUM>) between the first transparent electrode (34a) and the second transparent electrode (34b), wherein
when a voltage is applied across the liquid crystal layer (<NUM>), the liquid crystal molecules in the liquid crystal layer (<NUM>) change their orientation,
the electronic mirror (<NUM>) is configured to apply a varying voltage across the liquid crystal layer (<NUM>) which gradually decreases from outer areas towards the center of the liquid crystal layer (<NUM>) such that, in the reflective mode, the refraction index of the liquid crystal layer (<NUM>) gradually varies from outer areas towards the center of the liquid crystal layer (<NUM>), so as to provide a magnification effect for images reflected by the cell,
a polarizing filter layer (32b), wherein the second transparent electrode (34b) is arranged between the polarizing filter layer (32b) and the liquid crystal layer (<NUM>); and
a reflective polarizing filter layer (32a), wherein the first transparent electrode (34a) is arranged between the reflective polarizing filter layer (32a) and the liquid crystal layer (<NUM>).