Electro-optic mirror with contrasting display

An electro-optic display system includes a display patterned on at least one of the conductive layers of an electro-optic device. Voltages can be applied to individual display segments of the display to create a contrast with the rest of the electro-optic medium to enable information to be displayed. In a preferred embodiment this display is incorporated into a rearview mirror of a vehicle to display information during day and night conditions.

BACKGROUND OF THE INVENTION
 The present invention relates to an electro-optic display system that is
 especially useful for mirrors in the automotive vehicle environment.
 Rearview and sideview mirrors for a vehicle such as an automobile are
 standard equipment which allow the driver to observe surrounding traffic.
 Although such mirrors are very helpful, they can become a nuisance at
 night due to glare reflected into the eyes of the driver by the headlights
 of following vehicles. To solve this problem, in the past rearview mirrors
 were adjustable to two positions to provide day and night modes. The day
 mode position operates as a normal mirror, reflecting substantially all of
 the light to the driver. In the night mode position, the mirror is tilted
 to reflect only a portion of the incoming light, thus reducing
 objectionable headlight glare. Although this type of mirror alleviates the
 problem, it must be either manually adjusted by the driver or an expensive
 motor drive and sensing system is employed.
 The two-position adjustable rearview mirror can be replaced by an
 automatically adjusting electro-optic mirror, the structure and operation
 of which are known in the art. An electro-optic mirror has an
 electro-optic medium, the ability of which to transmit light can be
 increased or attenuated depending on the strength of an applied electrical
 field. The electro-optic medium typically consists of either
 electrochromic or polymer dispersed liquid crystal (P.D.L.C.) material. If
 electrochromic material is used, the medium is normally in a clear state
 and is darkened by application of an electric field. If P.D.L.C. material
 is used, the medium is normally in a darkened state and is cleared by
 application of an electric field. The electro-optic medium is located in
 the mirror between front and rear transparent conductive layers to which
 the electric field is applied. A reflective layer is positioned behind the
 rear conductive layer such that its reflective surface faces forwardly
 towards the electro-optic medium. The mirror so formed includes an
 electrical control circuit to automatically adjust the transmissivity of
 the medium depending on the intensity of light sensed by photodetectors.
 If a photodetector senses "day" conditions, then, depending on the type of
 electro-optic material used, an electric field is or is not applied across
 the electro-optic medium such that the medium is in a clear state so that
 substantially all of the light is reflected by the mirror surface to the
 driver. However, if the photodetector senses "night" conditions and a
 second photodetector detects glare from a following vehicle, then,
 depending on the type of electro-optic material used, an electric field is
 or is not applied across the electro-optic medium such that the medium is
 selectively darkened, attenuating its ability to transmit light, such that
 undesired glare is substantially reduced.
 It is known in the prior art to equip electro-optic mirrors with display
 systems to display information to the driver such as the time of day,
 ambient temperature, compass headings, and the like. Frequently, such
 mirror displays use illuminated displays that are viewed through the
 electro-optic medium or through a window created in the electro-optic
 medium. An alternate approach is disclosed in U.S. Pat. No. 5,189,537 to
 O'Farrell. The display system disclosed by this patent has an indicia
 defined by dielectric material positioned between the electro-optic medium
 and a conductive layer such that the electric field in the vicinity of the
 indicia is minimized. This causes the contrast between the indicia and the
 electro-optic medium to be maintained regardless of whether the mirror is
 in the day or night mode. This display system is limited, however, because
 it only provides a permanent, unchangeable display of the indicia pattern.
 Thus, there exists a need for an electro-optic mirror display system
 capable of contrastingly displaying changing information, in either day or
 night conditions, and without the use of an illuminated display.
 SUMMARY OF THE INVENTION
 The electro-optic mirror display system of the present invention is capable
 of displaying changing information, in either day or night conditions,
 without the use of an illuminated display. The present invention is
 embodied in an electro-optic device having an electro-optic medium located
 between front and rear transparent conductive layers. In one embodiment,
 the electro-optic device is a mirror with a reflective layer positioned
 behind the rear conductive layer with its reflective surface facing
 forwardly towards the electro-optic medium. At least one, and preferably
 both of the conductive layers of this electro-optic device are patterned
 to define display elements. A display voltage source is coupled to
 individual display elements defining the display and creates an electric
 field of the same or opposite polarity of the electric field applied
 across the rest of the electro-optic medium in order to provide a
 contrasting display. This enables information displayed to be visible to
 the driver during "day" or "night" conditions of operation of the
 electro-optic mirror.
 BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a fragmentary front elevational view of a rearview mirror for a
 vehicle such as an automobile and which embodies the present invention;
 FIG. 2 is a greatly enlarged fragmentary vertical cross-sectional view of
 the internal structure of the electro-optic mirror shown in FIG. 1;
 FIG. 3 is an enlarged fragmentary front elevational view of display
 patterns on the conductive layers of the electro-optic mirror display of
 the present invention showing the electrical circuit therefor in block
 form;
 FIG. 4 is a front elevational view of the rearview mirror display of the
 present invention shown operating in the day-activated mode; and
 FIG. 5 is a front elevational view of the rearview mirror display of the
 present invention shown operating in the night-activated mode.

DETAILED DESCRIPTION OF THE EMBODIMENT
 The electro-optic mirror display system of the present invention is capable
 of displaying any type of information, in either day or night conditions.
 Examples of information that may be displayed are time, temperature,
 compass headings, or paging messages. The display system is coupled to and
 works in conjunction with external electrical circuits that are capable of
 generating and supplying electrical signals including such information for
 display. The electro-optic display system of the present invention can be
 employed in any electro-optic device; however, one especially useful
 embodiment of the display system is for mirrors in the automotive
 environment. Within this specific environment, the preferred location for
 the display system is the rearview mirror of a vehicle which is positioned
 to the upper right of the driver and is in a convenient location for the
 driver to view displayed information.
 FIG. 1 shows a vehicle 10 such as an automobile. Mounted to the windshield
 12 of the vehicle is a rearview mirror assembly 14 embodying the present
 invention. Assembly 14 includes a mirror housing 16 connected to the
 windshield 12 or roof 13 of a vehicle by a conventional mounting bracket
 18. Housing 16 supports an electro-optical mirror 20 with a mirror viewing
 surface 22 which is adjustably positioned such that the driver can see the
 reflection of vehicles travelling behind the vehicle.
 The mirror assembly 14 of the present invention is an electro-optic mirror
 20 which, as seen in FIG. 2, includes an electro-optic medium 15,
 consisting of either electrochromic or polymer dispersed liquid crystal
 (P.D.L.C.) material, positioned between and attached to the first sides
 17a and 19a of a front conductive layer 17 and a rear conductive layer 19,
 respectively. Any clear conductive material such as indium tin oxide (ITO)
 can be used for the conductive layers 17 and 19, the conductive layers
 being approximately fifteen hundred angstroms thick if ITO material is
 used. The thickness of the electro-optic medium is typically 50 to 150
 microns if electrochromic material is used. If P.D.L.C. material is used,
 the thickness is typically eight to twelve microns. For purposes of the
 following operational description of the present invention, it is assumed
 that electrochromic material is used for electro-optic medium 15 such that
 the medium is normally in a clear state and is darkened by application of
 an electric field.
 A front optically transparent substrate, or plate, 21 such as glass is
 attached to the second side 17b of the front conductive layer 17. The
 front surface of substrate 21 provides the mirror viewing surface 22
 facing the driver. A rear optically transparent substrate, or plate, 23
 such as glass has a first side 23a attached to the second side 19b of the
 rear conductive layer 19. The front and rear optically transparent
 substrates 21 and 23, respectively, can be made of glass or any optically
 clear plastic material. Attached to the second or outer side 23b of the
 rear substrate 23 is a reflective layer 25 having its reflective surface
 26 facing forwardly toward the electro-optic medium 15 and the mirror
 viewing surface 22 to define the mirror so formed. The reflective layer 25
 is typically of metallic material such as aluminum, chromium, stainless
 steel, or platinum. The electro-optic mirror includes an electrical
 control circuit 30 with output conductors 32 and 34 coupled to conductive
 layers 17 and 19 for supplying a control voltage to the layers 17 and 19
 for changing the light transmissivity of the medium 15. Circuit 30 is
 conventional, and integrally includes a pair of photodetectors located in
 the mirror housing 16 and mirror itself to measure the ambient light and
 separately glare from the headlights of a trailing vehicle.
 A typical electro-optic mirror having an electrochromic medium operates as
 a normal mirror when one of the photodetectors senses "day" ambient light
 driving conditions, with no electric field applied across the
 electrochromic medium. However, when the photodetector senses "night"
 driving conditions such that the ambient light is less than a
 predetermined threshold, and the second photodetector facing rearwardly
 senses light from the headlights of a trailing vehicle, the control
 circuit applies an electric field across the conductive layers 17 and 19
 causing the electrochromic medium to darken and to transmit less light,
 thus reducing the amount of glare reflected to the driver by surface 26
 under nighttime conditions. The display system of the present invention
 finds application within this typical electro-optic mirror structure and
 environment.
 FIG. 3 shows in detail one pair of seven-segment displays 40 which are
 employed for the information display of the preferred embodiment of the
 present invention. Several such pairs of seven-segment displays are
 employed to provide the information desired in a given application. The
 size of the displays can be varied as desired as can their location on
 mirror surface 22 as long as they do not interfere with the use of this
 mirror 20 as a rearview mirror. Each display 40 also can have a desired
 pattern other than the traditional seven-segment array. Displays 40 are
 patterned on both the front conductive layer 17 and the rear conductive
 layer 19 of mirror 20, with each segment 44 of each display 40 defined by
 a non-conductive border 42. The patterning of each conductive layer is
 such that each segment 44 of each display 40 is aligned with a
 corresponding segment of a display on the other conductive layer to form
 matched pairs of displays 40 which align to form display 60 (FIGS. 4 and
 5) when viewed by the driver via mirror viewing surface 22. Borders 42
 also define thin conductors 46 (about 0.002 inch) for coupling each
 display element or pad 44 to a display driver 50.
 There are several methods by which to pattern the non-conductive borders or
 paths 42 into the conductive layers 17 and 19. The preferred method is to
 use a laser to cut the pattern. Alternative methods include chemically
 etching the conductive layers or masking the adjoining optically
 transparent substrates before the conductive layers are applied to them.
 Non-conductive paths 42 are shaped to form individual display elements 44
 that form a typical seven-segment display 40. In FIG. 3, the
 non-conductive paths 42 form all of the display segments 44 of the pair of
 displays 40, but formation of only four segments are shown for the sake of
 simplicity. The number and shape of the display elements can be varied
 depending on the type and amount of information that is to be displayed to
 the vehicle operator. In a preferred embodiment, the clock display 60
 shown in FIGS. 4 and 5 is made up of four spaced seven-segment displays 40
 on each conductive layer forming four aligned pairs of displays, with a
 pair of dots between the hour and minute digits.
 Display elements 44 are rectangular segments of conductive material of the
 conductive layers 17 and 19 that are electrically isolated from each other
 as well as the rest of the conductive layers because they are enclosed
 within or outlined by the non-conductive paths 42. Each display element 44
 of the matched pairs of displays 40 is individually coupled by conductors
 46 to a display driver 50, shown in block form in FIG. 3, which supplies
 variable voltages to the elements. The conductive lines 46 are of limited
 width, and preferably are approximately 0.002 inch wide so as not to be
 visible to the user. FIG. 3 shows these conductors 46 greatly enlarged for
 purpose of illustrating this patterning of the conductive layers 17 and
 19. A source 70 of information to be displayed is coupled to the display
 driver 50 which also receives day or night control signals from circuit 30
 via conductors 36 and 38. The display driver 50 provides approximately one
 volt maximum signals to aligned segments 44 of matched pairs of displays
 40 which creates either no electric field or an electric field across the
 aligned segments that is of the same or opposite polarity as that provided
 across the rest of mirror 20 in order to provide a contrasting display
 such as time display 60 under all light and glare conditions, as described
 in detail below. Circuit 50 may include a conventional integrated circuit
 display driver of the type used in LED display systems and voltage
 regulators such as an LM 317 circuit to provide the desired signal levels
 to the display from the vehicle's power system. The source of information
 70 may be any desired source in addition to the clock shown in FIGS. 4 and
 5. One example is a digital compass of the type disclosed in U.S. Pat. No.
 4,953,305.
 FIG. 4 shows the rearview mirror 20 implementing the electro-optic mirror
 display system in its day-activated mode. In this mode of operation, the
 photodetector of circuit 30 senses light which prevents an electric field
 from being applied across the conductive layers 17 and 19 and the
 electrochromic medium. As a result, the mirror substantially operates as a
 normal mirror having a high degree of reflectivity. However, display
 driver 50 applies voltages to elements 44 such that an electric field is
 created across particular pairs of aligned display elements 44 of matched
 pairs of displays 40 that are selected by information source 70 and causes
 the electrochromic medium between the selected pairs of display segments
 44 to darken. This electric field is of the same polarity as the electric
 field employed to darken mirror 20 when "night" glare conditions exist.
 The degree of darkening is varied by adjusting the level of the voltages
 applied to the segments which changes the strength of the electric field
 between the segments.
 The contrast that is created with the rest of the electrochromic medium,
 including that between the nonselected display elements that do not have
 an electric field applied across them, takes the shape of the information
 to be communicated by the particular pairs of aligned display elements
 that are selected. The information displayed by these aligned display
 elements thus is visible to the driver via viewing surface 22. The
 conductive lines 46 (FIG. 3) used to apply voltages to particular display
 elements likewise create an electric field that, depending on their size,
 could potentially create a visible contrast. For this reason, the widths
 of the conductive connectors should be small enough such that any such
 contrast would be unnoticeable by the viewer of the display. In FIG. 4,
 for example, the display of the time is 2:18 p.m., although any desired
 information can be displayed by the system of the present invention. An
 electric field is applied across each pair of aligned display elements 44
 of matched seven-segment displays 40 which are selected by the information
 source to display information with, for example, the five pairs of aligned
 elements of the hour digit forming the numeral "2" having an electric
 field applied across them to darken electrochromic medium 15 between the
 elements and, like the remaining portion of the conductive layers 17 and
 19 of the mirror, the remaining two pairs of aligned elements of this
 digit have no electric field applied across them. The mirror shown in FIG.
 1 operates as a normal mirror in which no display information is supplied
 by source 70. This mode is useful during daylight operating conditions
 when it is not desired to display information to the driver.
 A second mode of operation is the night-activated mode. FIG. 5 depicts a
 rearview mirror 7 implementing the electro-optic mirror display system in
 its night-activated mode. In this mode, the photodetectors sense that the
 ambient light is less than a predetermined threshold and that oncoming
 glare exists. This enables the control circuit 30 to apply an electric
 field across the conductive layers 17 and 19 and the electrochromic medium
 to darken the medium 15 to reduce reflected glare to the driver. However,
 the display driver 50 applies a different electric field across particular
 pairs of aligned display elements 44 of display 60 that are selected by
 information source 70. This electric field has a polarity that is opposite
 to the electric field created by circuit 30 and counteracts the electric
 field across the electrochromic medium 15 created by circuit 30 and
 prevents the medium between the selected display elements from darkening
 and thus creates a contrasting display of the information to be displayed.
 As noted above, the degree of contrast can be varied depending on the
 strength of the electric field applied across the aligned display elements
 44 by the display driver 50. The pairs of aligned display elements 44 that
 are not selected by information source 70 to display information have an
 electric field applied across them by display driver 50 which is of the
 same polarity as the electric field applied across the rest of mirror 20
 by circuit 30 such that the electrochromic medium between these elements
 is likewise darkened.
 The information displayed by the selected pairs of display elements is
 viewed by the driver via viewing surface 22 because the higher
 reflectivity of the electrochromic medium in those areas reflects glare
 from surface 26 to provide a contrasting display. As seen in FIG. 5, the
 display is 11:00 p.m. with each of the hour's digits defined by an
 electric field of polarity opposite to that applied across the rest of
 mirror 20. Thus, to form the digit "1", the field applied across two pairs
 of vertically aligned segments 44 of a matched pair of seven-segment
 displays 40 is opposite the field applied to the remaining five pairs of
 aligned elements which have the same electric field applied across them as
 the electric field applied across the rest of mirror 20 by circuit 30.
 The above operational description of the present invention is based on the
 embodiment in which electrochromic material is used for electro-optic
 medium 15. Operation of the embodiment in which P.D.L.C. material is used
 is similar to that described above, except that the mirror is normally in
 a darkened state when no electric field is applied across the
 electro-optic medium. Therefore, the appropriate mode of operation during
 which to apply a particular electric field across the electro-optic medium
 and/or selected pairs of aligned display segments 44 is opposite that
 which is appropriate if electrochromic material is used for electro-optic
 medium 15.
 The above detailed description of the present invention focusses on the
 preferred embodiment in which displays 40 are patterned into both the
 front conductive layer 17 and the rear conductive layer 19. In alternative
 embodiments, only one of the conductive layers is patterned, it being
 preferable in such embodiments to pattern the rear conductive layer 19
 because it reduces the likelihood of double imaging. The operation of such
 an alternative embodiment is essentially the same as when both conductive
 layers are patterned except that display driver 50 applies voltages to
 selected display segments 44 of only one conductive layer, creating an
 electric field between the selected segments and the other unpatterned
 conductive layer. This electric field may be of a less-defined shape than
 the electric field of the preferred embodiment which is focussed between
 aligned pairs of selected segments 44 of matched pairs of displays 40.
 This electric field, however, still provides the contrast needed to view
 the mirror display via mirror viewing surface 22, although display
 resolution may be less than that of the preferred embodiment.
 It will be apparent to those skilled in the art that various modifications
 to the preferred embodiments of the invention described herein can be made
 without departing from the spirit or scope of the invention as defined by
 the appended claims.