Description of the Related Art Electroluminescent ("EL") lamps are light sources that contain a special phosphor or combination of phosphors or other electroluminescent material that luminesces in the presence of an electric field. In general, the electric field is generated by electrodes and the phosphor is located between these electrodes. At least one electrode is transparent or translucent so that the light emitted by the phosphor can be transmitted into the environment by the EL lamp. An EL lamp may be composed of more than one cell or channel, which may be selectively powered to emit light independently of the other cells or channels.

FIG. 1 shows an exemplary construction of EL lamp 100. Rear electrode 110 may be any conductive material, such as silver. Dielectric 120 separates rear electrode 110 from phosphor 130. Phosphor 130 may alternately be any electroluminescent material.

Front electrode 140 may be transparent or translucent to allow light generated by the phosphor to transmit from the lamp 100. Front electrode 140 may be indium-tin oxide ("ITO") and may be sputtered on substrate 150. Substrate 150 may be polyethylene terepthalate. The EL lamp 100 may be powered by an AC power supply (not shown).

The electric field strength is a function of the distance between rear electrode 110 and front electrode 140 for a given phosphor 130, dielectric 120, AC power supply, and surface area of EL lamp 100. The electric field strength increases as the distance between the front electrode 140 and the rear electrode 110 decreases. A greater electric field causes phosphor 130 to emit more light and have a shorter lifespan.

The electric field strength is proportional to the applied voltage and inversely proportional to the separation distance between electrodes 110 and 140, for a given phosphor 130 and dielectric 120: E-V/d where E is the electric field strength, V is the applied voltage and d is the distance between front electrode 140 and rear electrode 110.

The applied voltage will increase with decreasing cell size and decrease with increasing cell size: V = I/ (2*fC) where V is applied voltage, f is the frequency of the AC power supply and C-A/d where A is the cell surface area and d is the distance between the front electrode 140 and the rear electrode 110.

As a result, cells with the same electrode separation but a different cell surface area have different electric field strengths and therefore different brightness and aging attributes.

Because power supplies are expensive, it is often desirable to power more than one lamp, or more than one cell on a lamp, with the same power supply. However, as described above, unless the cells have the same surface area, they will present different impedance loads to the AC power supply. The different loads will cause the brightness and lifetimes of the cells to differ.

One potential solution to the above problems is to have a consistent surface area for each of the cells which are using one power supply. However, varying surface areas are desirable to make EL lamps or EL lamp cells of different sizes for aesthetic or practical purposes.

Another potential solution is to add resistors or capacitors to equalize the impedance loads the cells present to the power supply. Such resistors and capacitors may either be added to the cells themselves or be added to the power supply. Similar impedance loads will generate similar electric fields, which will cause the cells to emit similar brightness levels for similar lifetimes.

However, the resistors or capacitors must be hard wired between the cells and the power supply. As a result the addition of resistors or capacitors requires the addition of a hard writing step in the manufacturing process. Therefore, this potential solution significantly increases the EL lamp production time.

SUMMARY OF THE INVENTION The present invention addresses these and other problems within the prior art by providing a multicell EL lamp with a dielectric layer of varying thickness dependent upon cell size.

By adjusting the electrode separation distance as a function of cell area, voltage variations can be adjusted to create a uniform electric field across lamp cells of different areas. By keeping the electric field strength uniform (within +/-10%) across each lamp cell, the brightness and aging of each cell will be uniform regardless of cell size. To accomplish this, the ratio between voltage and distance should also be kept uniform within +/-10%.

Alternately, the same technology can be used to create a gradient effect in one cell of a lamp. By varying the dielectric thickness across one cell the brightness within that one cell can be varied.

An apparatus according to one embodiment of the present invention includes an EL lamp having a plurality of cells, EL material, and dielectric material. The cells have a first electrode and a second electrode that are electrically connected and that are configured to generate an electric field. The EL material is printed between the electrodes and emits light when the electric field is generated. The dielectric material is printed on the EL material between the electrodes. For each EL cell, the thickness of the dielectric material is selected such that the ratio of voltage to separation distance is approximately constant for every cell. Alternately the dielectric thickness is selected to vary across the cell in a desired manner.

According to another embodiment, a method according to the present invention includes the step of printing a first electrode of an EL lamp, which comprises a plurality of cells with varying surface areas on a substrate. An EL material such as phosphor is printed on the first electrode. Then a dielectric material is printed on the EL material such that the dielectric material thickness for each cell is selected to ensure that all cells will have approximately equal electric fields between the first and second electrodes. As an alternative, the thickness of the dielectric material for each cell can be selected to allow a desired non-uniform field strength amongst the cells or within one cell. Finally, the second electrode is printed on the dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood by reference to the appended specification and figures.

FIG. 1 is a cross-sectional view of an EL lamp according to the prior art.

FIG. 2 is a plan view of two cells in an EL lamp according to the invention.

FIG. 3 is an enlarged cross-sectional view along line 3-3 of FIG. 2.

FIG. 4 is a graph of cell area in square inches versus luminance in fL for an EL lamp according to the invention.

FIG. 5 is a graph of thickness in millimeters versus cell area in square inches for an EL lamp according to the invention.

FIGS. 6A and 6B illustrates an embodiment of the invention where varying dielectric thickness is used to create a lighting effect.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The prior art discloses EL lamps having impedances adjusted by the addition of resistors or capacitors. The present invention avoids the difficulties associated with the use of resistors, capacitors or other added elements by varying the thickness of the dielectric layer in the lamp.

One advantage of providing a dielectric layer of varying thickness in a lamp according to the invention is that no elements need to be added to the lamp to change the impedance load, rather, the lamp structure itself is modifie. Another advantage is that the electric field generated by the lamp cells can be changed, by which cell brightenesses and lifetimes can be adjusted.

In general, the invention includes an EL lamp having one or more EL cells in which the thickness of the dielectric layer varies in order to either equalize the impedance loads of the cells for the purpose of producing approximately uniform electric fields across all lamp cells or in order to vary the impedance loads of the cells for the purpose of producing a gradient effect in one or more cells.

FIG. 2 shows an EL lamp 200 with two light-emitting cells 210a and 210b separated by boundary 205. Each cell may be powered by the same AC power supply.

Cell 210b is smaller than cell 210a. In the prior art, the size difference would ordinarily

cause cell 210b to have a stronger electric field then cell 210a. Thus, cell 210b would be brighter than, and have a shorter lifespan than, cell 210a.

FIG. 3 shows an enlarged cross-section of EL lamp 200 at boundary 205 separating cell 210a from cell 210b. As in FIG. 2, cell 210b is smaller than cell 210a.

The layers are as described above regarding FIG. 1. Cell 210a has its own rear electrode 110a and front electrode 140a. Similarly cell 210b has its own rear electrode 110b and front electrodes 140b, to selectively activate the cells. The cells also have separate phosphors 130a and 130b and dielectrics 120a and 120b.

Exemplary dimensions of the layers of EL lamp 200 are as follows. Rear electrode 110 is about 0.3 millimeters thick. Dielectric 120 and phosphor 130, together, vary from about 1.9 millimeters to 3.75 millimeters thick. Front electrode 140 and PET substrate 150, together, are about 5 millimeters thick. Exemplary cell sizes vary from about 0.5 square inches to about 13 square inches. Those skilled in the art will recognize that other dimensions are possible.

Although FIG. 3 shows discrete layers for dielectric 120 and phosphor 130, these layers may merge together. Phosphor 130 may comprise generally spherical phosphor particles with diameters of about 1 mil to 1.35 millimeters. Dielectric 120 may flow between the phosphor particles and contact front electrode 140.

Although the description has focused on an EL lamp with a sputtered ITO layer, the principles disclosed herein may also be applied to the printable HO lamp as disclosed in U. S. Patent Application No. 08/910,724 entitled"Electroluminescent Lamp Designs", filed August 13,1996, commonly owned by the assignee of the present application, the disclosure of which is incorporated herein by reference.

Dielectric Layer Thickness One feature of the present invention is to define the variables that allow manipulation of the dielectric layer thickness to produce lamps with a given light output level.

The electric field produced by EL lamp 200 is a function of the surface area of EL lamp 200, the distance between electrodes 110 and 140, the AC power supply, and the specific materials used to form EL lamp 200. The brightness of the light output from EL lamp 200 is a function of the electric field.

To solve the brightness and lifespan problems of the prior art, the present invention sets out the following steps to determine the thickness of the dielectric layer.

Those skilled in the art will recognize that these relationships may be rearranged as desired.

A power supply is selected. This choice sets the maximum power supplied to EL lamp 200, setting an upper boundary on the lamp's surface area.

Then a combination of EL materials is chosen. Different combinations of materials in EL lamps according to the invention will emit different levels of light for different lifespans for a given electric field strength.

Third, the number of cells and their individual surface areas are selected. The selection of a particular image to be displayed can be used to determine the cell number and size.

Finally a desired brightness level is selected.

Given the four above-mentioned selections, the thickness of the dielectric layer for each cell may be determined by consulting one of the following tables, which may also be represented in the form of an equation.

Although the description focuses on the"thickness of the dielectric layer,"as stated above, the dielectric material may merge with the EL material. In such a case the "thickness"refers to the distance between the electrodes, which is the total thickness of the EL material and the dielectric material. In FIGS. 4 and 5, the"thickness"reference is the total thickness of the two layers measured after both have been printed.

As an example, the following selections have been tested for a lamp similar to EL lamp 200. The power supply and the EL materials used are adapted to give an internal loading of 32 nF. Dielectric Thickness (mm) Cell Area (in2) Luminance (fL) 2.25 0.5 11 2.25lu11 2.25 2. 0 11 2.25 3. 0 10 2.25 4. 0 10 2.25 5. 0 9 2.25 6. 0 9 2.25 7. 0 9 2.25 8. 0 9 2.25 9.0 8 2.25 10. 0 8 2.25-11. 0 2.25 12. 0 7 2.25 13. 0 7

Table 1. Cell Area vs. Luminance For 2.25 mm Thickness FIG. 4 summarizes these values in graphical form, in addition to illustrating dielectric thickness of 2.75,3.25,3.75, and 4.25 millimeters for the same values of cell area. Thus, for a given cell area and desired luminance, a pre-determined dielectric thickness may be selected from the graph.

Similarly, for a given power supply and desired luminance, a dielectric thickness can be selected based on the impedance load of the selected combination of EL materials and the cell area. As an example, the desired luminance is 13 fl and the internal loading is 32nF.

Cell Loading (nF) Cell Area (in2) Thickness (mil) 32 12.0 2.25 32 8.5 2.75 32 6.0 3.25 32 1.9 3.75 Table 2. Cell Area vs. Thickness for 32 nF Loading

FIG. 5 summarizes these values, in addition to the values for EL materials with an internal loading of 22 nF. The values shown in FIG. 5 may be approximated by the following equations: Area22nF =-1.2t2 + 1.44t + 16.395 Area32nF=-1.8t2+ 4. 12t + 11.653 where t is the thickness of the dielectric layer.

Those skilled in the art will appreciate that the concepts described above in terms of creating cells of equal brightness and varying cell sizes can be applied to creating a lamp wherein each cell has internally varying luminance based on a varying dielectric thickness.

Manufacture of EL Lamps According to the Present Invention As described in the above-incorporated Application No. 08/910,724, rear electrode 110, dielectric 120, phosphor 130, and front electrode 140 may be formed by a variety of techniques. Suitable techniques include etching, screen-printing, off-set printing, spray painting, ink jet printing, plotting, rotogravure, doctor blade printing, and squeegee printing. Those skilled in the art will recognize that etching and printing techniques, discussed below, may easily be used to vary the thickness of the dielectric layer formed.

In one embodiment of the invention, the order of printing the layers is as follows.

Substrate 150 may be a continuous section of a translucent or transparent material, such as polyethylene terepthalate. Substrate 150 forms the base layer for printing the other layers to produce lamp 200. Front electrode 140 can be printed, etched, or sputtered onto substrate 150 in a desired cell pattern. Then phosphor 130 may be printed over the front electrode of each cell. Dielectric 120 may then be printed each over the phosphor layer of each cell. The thickness of dielectric 120 may be increased by printing a larger amount of dielectric material or by printing more than one time. This step allows a greater thickness to be printed on smaller cells to allow for uniform electric field strength between all cells.

Finally, rear electrode 110 may be printed or etched over the dielectric layer of each cell.

FIGS. 6A and 6B show a top view of embodiments of the invention wherein the varying dielectric thickness has been used to create a lighting effect. In the embodiments of the invention shown in the figures the dielectric thickness is varied by variation of the number of dielectric prints. The area with the least thick dielectric layer 1, has had dielectric printed only once and is the brightest. The area with the thickest dielectric layer

2, has had dielectric printed seven times and is the least bright area in the lamp. In between 1 and 2 the number of dielectric prints varies with the brightness.

Additional Features It may be desirable that at least one of the electrodes of one cell is not in electrical contact with the electrodes of another cell, to allow the cells to be separately activated. To selectively illuminate the cells, each cell only needs one of its electrodes to be insulated from the electrodes of other cells. Thus, if two cells share the same front electrode 140, the rear electrodes 1 Oa and 1 lOb must be insulated from each other. For example, FIG. 3 shows that each layer in one cell is not in electrical contact with the same layer in another cell (for example, dielectric 120a and dielectric 120b). One advantage of separating all layers at the cell boundaries is that it reduces the amount of materials used. However, if desired, the layers may be printed or etched continuously on EL lamp 200, with the boundaries determined by at least one electrode of each cell, for example, the rear electrode.

Although the description has detailed two cells and two thicknesses for dielectric 120, those skilled in the art will recognize that the principles disclosed may be applied to any number of cells and thicknesses. Furthermore, although the graphs have detailed a relatively small number of cell areas, the principles of the invention may be used for a wide variety of cell areas.

In addition, although the description has focused on the use of varying dielectric thicknesses to create a uniform electric field on EL lamp 200, similar principles may be applied to create a non-uniform electric field, over one or more cells, as desired. For example, a lamp may be printed with continuous front and rear electrodes (that is, it would have one cell) and with multiple dielectric thicknesses between the electrodes. In such a lamp, the electric field would be different for each different thickness. As a result the brightness of the one or more lamps would vary without the necessity of varying cell size. In particular, one cell could within itself include a variation in brightness according to the thickness of the dielectric layer.

It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that structures within the scope of these claims and their equivalents are covered thereby.