Liquid crystal displays (LCD) in general have become commonplace in a number of applications. These include cockpits in air and ground vehicles, displays for computers, as well as televisions and video monitors. These LCDs operate by applying an alternating voltage potential between opposing electrodes sandwiching a liquid crystal layer. Depending on the voltage potential applied across the liquid crystal, light is either allowed to pass or is blocked.

One common characteristic of liquid crystals used in LCDs is that the performance of the display may be fairly temperature sensitive. When the temperature of the display drops below a particular temperature, the liquid crystal material does not respond quickly, or in a consistent manner, to changes in voltage. Accordingly, many LCDs include a heater which quickly heats the liquid crystal material to a desired minimum temperature, and maintains this temperature, to ensure satisfactory low ambient temperature performance.

One method of heating a liquid crystal display is through use of a transparent conductive coating applied to a transparent glass substrate

proximate to the liquid crystal material. A voltage potential is then placed across the coating, which will generate a significant amount of heat, thus providing a heat source for the display.

BRIEF SUMMARY OF THE INVENTION The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.

The inventor has recognized that one of the drawbacks of using a conductive layer heater is that heat generation may be uneven, creating hot spots on some parts of the display and providing insufficient heating in others.

In a heater configuration where buss bars are positioned along opposite edges of the conductive layer, and a voltage is created between the buss bars, higher temperatures are generated in the center of the display than along the edges. While this non-uniformity may be acceptable in smaller displays, as display size grows these gradients can exceed 50° Celsius and can result in unacceptable display characteristics. The inventor has further recognized that temperature sensors used in heating control are frequently mounted near an edge of the heater, which increases the possibility of LCD damage at the center of the display. As such, the inventor has discovered that through strategic placement of passive conductive elements on the

conductive layer, heating patterns applied to the display are affected in a desired manner.

Described herein is a heating apparatus that can be employed for applying heat over a designated area. Included in the heating apparatus is a conductive layer which is applied to the nonconductive substrate to be heated. Positioned along opposing edges of the conductive layer are electrodes, which are connectable to a power source. Positioned between the electrodes on the conductive layer are conductive elements. These elements have higher conductivity than the conductive layer to which they are applied, and may be located such that when a voltage is applied across the electrodes, they affect the voltage gradients at predetermined locations. This in turn affects the heating pattern generated within the conductive layer.

In one aspect of the invention, the heater apparatus can be incorporated into an LCD. Any material used for the conductive layer, such as indium tin oxide (ITO), must be applied thinly enough so as to be transparent. The electrodes are buss bars which are applied along two opposite edges of the ITO layer. The conductive layer can be intrinsic to the LCD stack, applied to one of the transparent substrate layers which is proximate to the liquid crystal material, or it can be applied to a separate substrate which is then either mechanically attached or adhesively bonded as an additional component to the original LCD stack.

Any conductive elements not directly connected to a power source can include a passive buss bar. Passive buss bars can be positioned anywhere on the conductive layer so as to affect the heating pattern in a desired

manner. When employed as part of the LCD structure, however, the passive buss bars are positioned along the edges of the conductive layer so as not interfere with the viewing area of the display.

In another aspect of the invention, the active and passive buss bars can both be made of the same material, and applied to the conductive layer according to the same process. This application process can include silk screening layers of conductive material onto the conductive layer. The buss bars can be rectangular in shape, where the length of the buss bar is substantially greater than its width. Additionally, the buss bars can be sufficiently thin so as to not create large gaps between the layers of the LCD in which the heating apparatus is located.

Passive buss bars are positioned on the conductive layer so as to affect the voltage gradients across the conductive layer in a predetermined fashion during operation. One characteristic of the passive buss bar is that the voltage drop from one end to the other is minimal. A field of constant voltage can extend from the sides of the passive buss bar, with large voltage gradients being created over relatively small areas at the ends, thus affecting the heating pattern across the conductive layer.

In one configuration for the invention, the conductive layer can be substantially rectangular in shape, such that it possesses first and second edges which are parallel to each other, and third and fourth edges which are also parallel to each other. At least one active buss can be positioned on the conductive layer such that it runs parallel to a substantial portion of the first edge. The active buss can also be broken up into multiple electrodes

(multiple actives, or split-actives) which all run along the first edge. To provide for the creation of a voltage potential across the conductive layer, another active buss can also be positioned on the conductive layer such that it runs along a substantial portion of the second edge. This active buss can also be broken up into separate electrodes. When the heater is activated, an electrical potential is created form the first edge of the conductive layer to the second edge. This provides heating for substantially all of the display surface.

As described above, the conductive layer can also include a passive buss bar which can be positioned to run along a substantial portion of the third edge of the conductive layer. In like manner, a passive buss bar can be positioned on the conductive layer such that it runs along a substantial portion of the fourth edge. The closer the passive buss bars are to the active buss bars when positioned, the larger the voltage drop in the vicinity of these items, and heating in this area may be significantly increased.

In another configuration of the invention, the passive buss bars which run along the third and fourth edges of the conductive layer can be broken up into individual segments. For example, the passive buss bar can include a number of segments (multiple passives, or split-passives) which are aligned along the edge in a linear fashion. A number of buss bars can also be positioned along the fourth edge of the conductive layer opposite the third edge. The passive bars along the fourth edge can be aligned in a symmetric fashion with regards to the passive buss bars along the third edge, or,

depending on the heating pattern desired, can be configured in a non- symmetric fashion.

In yet another configuration of the invention, passive buss bars can also be positioned along the first and second edges with the active buss bars.

Depending on the heating pattern desired, the passive buss bar can be placed between two active buss bars or can be positioned at any other desired location along either the first or second edge.

The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.

Figure 1 discloses a cross sectional view of a typical liquid crystal display.

Figures 2a and b disclose views of the heating apparatus with single and split-active busses.

Figures 3a and b disclose diagrams which shows the equipotential lines and current vectors for the configurations shown in Figs. 2a and b.

Figure 4 discloses a view of the conductive layer with split-active buss bars and single passive buss bars along the edges.

Figure 5 discloses a diagram which shows equipotential lines and current vectors for the configuration of the invention shown in Fig. 4.

Figure 6 discloses a view of the conductive layer with split-active and passive buss bars positioned along the edges of the conductive layer.

Figure 7 discloses a diagram which shows the equipotential lines and current vectors for the configuration of the invention disclosed in Fig. 6.

Figure 8 discloses a view of the conductive layer with split-active buss bars along opposite edges and a number of passive buss bars of varying lengths along opposite edges.

Figure 9 discloses a diagram which shows the equipotential lines and current vectors for the configuration of the invention disclosed in Figure 8.

Figure 10 discloses a view of the conductive layer where additional passive buss bars are located between the active buss bars.

Figure 11 discloses a diagram which shows the equipotential lines and current vectors for the configuration of the invention disclosed in Figure 9.

DETAILED DESCRIPTION OF THE INVENTION Disclosed in Figure 1 is a side view of a typical liquid crystal display (LCD) which may employ the heater apparatus described herein. Included in the display stack is a rear transparent substrate 12 which is located adjacent to the lighting source (not shown) of the display. Adjacent to this rear substrate is the conductive layer 14 which is employed for providing the heat source for the display. This layer is typically comprised of indium tin oxide (ITO) and includes external connections to a power source. As an alternate structure, the conductive layer 14 may also be located between the substrate 12 and the lighting source. This heating source will be discussed in greater detail below.

Adjacent to the conductive layer 14 is rear polarizer 16. The rear polarizer is next to the glass substrate 18 upon which the electrodes 20 are disposed. Trapped between the common electrode 24 and the individual electrode 20 is the liquid crystal layer 22. Also included in the display stack are transparent substrate 26, front polarizer 28 and front cover sheet 30.

Referring now to Figure 2a and b, disclosed therein are two views of a conventional heating structure as may be employed in an LCD. In Fig 2a, disposed upon the ITO layer 14 are active buss bars 32. A voltage may be applied between these buss bars to induce a current through the ITO layer, thus providing a resistive source of heat. These active busses may be constructed of an electrically conductive material. In one embodiment of the invention, the buss bars may be comprised of a material such as Kapton, on which a conductive material trace layer has been applied. The conductive

layer may include such metals as copper of silver. Buss bar material may also be applied as a paste or frit to the ITO layer by silk screening, or through the use of other similar processes. After application, the busses may be of such minimal thickness so as not to create any appreciable gaps between the conductive layer and any adjoining structure. The creation of gaps may also be avoided through use of the alternative structure described above.

Extending from the active buss bar 32 is connection tab 34. Through this tab, connections may be established with a remote power source, such that when activated, the power source creates a voltage between the buss bars. The resulting current flow through the conductive layer generates the heat used to heat the LCD. Disclosed in Fig. 2b is a heater configuration in which the buss bars 36 are split into two separate elements, referred to as either multiple active, or split-active buss bars. Extending from each buss bar is a connection tab 38 which may be employed to connect to an external power source.

Disclosed in Figs. 3a and b are diagrams which display the current vectors and equipotential lines for the configuration of the heaters shown in Figure 2a and b, respectively, when a voltage is applied across the conductive layer. As seen in Fig. 3a, for the heater configuration where a single active buss bar is used along each of the upper and lower edges of the conductive layer, the equipotential lines are straight across and the current vectors are unidirectional. In Fig. 3b, the equipotential lines are horizontal and the current vectors are unidirectional except near those portions of the conductive layer over which the buss bars do not extend.

One drawback of the heater configurations shown in Figs. 2a and b is that considerable heat is generated in the central area, with less heat generated along the edges and in the corners. Although the diagrams in Figs. 2a and b would lead one to believe that the heating, and therefore the temperatures, would be uniform, thermal properties of the heater and LCD materials, combined with external conditions within which the LCD operates, may cause uneven heating and temperatures. One effect in particular may be aggravated by the structure to which the LCD is connected. This structure will attach to the LCD along the edges and is a more effective heat sink for heat generated at these locations. As such, the center of the display will operate at a higher temperature than the edges. Additionally, because temperature sensors used in heating control are commonly mounted near an edge or corner of the LCD, this difference in temperature between the center of the display and the edges has the possibility to cause damage to the display. This is especially relevant in large displays where the temperature gradient between the center of the display and the edges may exceed 50° Celsius.

In order to address the heating disadvantage of the configuration described above, a heater apparatus which may be employed to affect heating patterns in an LCD is disclosed in Fig. 4. Included on the conductive layer 14 are the split-active busses 36 with the electrical connections 38.

Although this configuration (and the ones to follow) are shown with split-active buss bars, similar configurations may be comprised of single, split, or multiple

active buss bars. Also included are the single passive buss bars 40. The passive buss bars are constructed of an electrically conductive material, which in one embodiment of the invention is the same as the active buss bars. The passive buss bars may be applied to the conductive layer using the same silk screening process as was described above with regards to the active buss bars. This may all be done at the same time during the construction of the heater. The passive buss bars may be sized such that they are significantly longer than they are wide and are positioned near the edges of the conductive layer. This shape is necessitated when used with an LCD. Other shapes for the passive buss bars may be possible for other applications.

During operation of the heating apparatus, a voltage is created between the active buss bars. Current flow through the inherent resistance of the conductive layer provides a source of heat. Disclosed in Figure 5 is a diagram of the voltage equipotential lines and the current vectors for the configuration disclosed in Figure 4. As can be seen in the diagram, the placement of the passive buss bars has the effect of creating an area of substantially constant voltage from one end of the buss bar to the other. This area of constant voltage extends a significant distance from the buss bar toward the center of the display. As a consequence, the placement of the passive buss bars creates large voltage gradients between the ends of the passive buss bars and the ends of the active buss bars. The increased current flow through these areas provides for increased heating in the corners and reduced heating in the center. The net effect of placing passive buss

bars along the side, therefore, is that a considerable amount of the heating which would typically occur near the center of the screen is shifted towards the edges, where it more effectively compensates for the heat transferred to the LCD mounting structure.

Another configuration for the placement of passive buss bars is disclosed in Figure 6. In this configuration the single passive buss bars have each been split into two segments, 50 and 52, referred to as split-passives.

These are positioned along opposite edges of the conductive layer such that substantial gaps are left between each passive buss bar segment, in addition to the gaps between the passive and active buss bars. Disclosed in Figure 7 is a diagram of the equipotential lines and current vectors for the configuration of the invention disclosed in Figure 6, when the heating apparatus is operational. As in the previous configuration, there is a minimal drop in voltage from one end of each passive buss bar to the other. In the configuration shown in Figure 6, however, not only are there significant voltage gradients between the ends of the passive buss bars and the active buss bars, but between the ends of passive buss bars 50 and 52 as well.

This particular configuration has the additional effect of creating a significant voltage gradient along the center of the side edges of the conductive layer, which in turn provides additional heating in this area.

Disclosed in Figure 8 is yet another possible configuration for the placement of passive buss bars on the conductive layer. In this configuration, more than two passive buss bars (multiple passives) are positioned along opposing edges of the conductive layer, between the active buss bars. The

lengths of the individual passive buss bars is determined by the desired affect to the heating pattern.

Disclosed in Figure 9 is a diagram which shows in particular the equipotential lines and current vectors for the configuration shown in Fig. 8, when the heater apparatus is activated. In particular, it is once again seen that a relatively constant voltage is created in the vicinity of each passive buss bar and that significant voltage gradients are created at each end of the passive buss bars. In this configuration the passive buss bars 60,62 and 64 may be of any desired length in order to affect the heating pattern for the system in a desired manner. For example, if more heat is desired at either an upper or lower corner of the display, the passive buss bars 60 and 64 may be either lengthened or shortened in a relative manner, with the length of buss bar 62 either changing, or remaining unchanged, as necessary. The net affect of this configuration is that the multiple side passives, with the gaps biased towards the corners, create multiple high gradient areas along the sides.

In order to further customize the heating patterns applied to a particular display, the passive buss bars may be positioned on the conductive layer in a non-symmetrical fashion. If it were shown, for example, that due to the characteristics of the structure within which a display or other apparatus was operating, that more heat was absorbed on one side versus the other, the passive buss bars may be positioned such that a greater or lesser amount of heat is directed to a particular side of the display.

Yet another configuration for the placement of passive buss bars is disclosed in Figure 10. There may be situations which require changing the heating patterns along the edges of the conductive layer in which the active buss bars are positioned. In this situation, a passive buss bar may be positioned along the same edge as the active buss, such that areas of relatively constant voltage may be created, at a voltage different from that of the active buss bars. As can be seen in Fig. 10, passive buss bars 70 are located between the active buss bars 36. In this configuration, multiple passive buss bars are also included along the adjacent edges of the conductive layer.

Disclosed in Figure 11 is a diagram which shows the equipotential lines and current vectors for the configuration disclosed in Figure 10. As can be seen, these passive buss bars"pull"the voltages along these edges closer to the supply voltage,"flattening"the voltage gradient relative to those shown in Figure 9. This increases the current and the temperature along the upper and lower edges. One skilled in the art would realize that, although only a single passive buss bar is shown positioned between the active buss bars in Figure10, depending on the heating pattern desired, more than one passive buss bar could be positioned between the active buss bars.

The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other

variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.