MULTICOLORED E.L. DISPLAYS USING EXTER- NAL COLORED LIGHT SOURCES

Patented June 3, 1969

A display system for obtaining multicolored images comprising a display surface and an electroluminescent material coupled to an excitation source, the voltage and frequency of which are variable to vary the output color of the display surface over a relatively narrow spectrum of output colors. Means are provided for color biasing the display surface, the means comprising an external source of colored light which is projected onto the display surface where it is additively mixed with the variably controllable colors generated by the electroluminescent material. In a preferred embodiment, the colored biasing light is made substantially complementary in color to a color -within the range of variably controllable colors.

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This invention relates generally to multicolor display systems, and omre particularly, to a system for generating multicolor electroluminescent displays wherein an external light source is provided for color biasing the surface of the electroluminescent display, whereby an increased variety of display colors are obtained. In the past, several methods have been used to provide color displays wherein electroluminescent material was used as the color generating source. However, electroluminescent devices are basically single-colored and in order to obtain multiple colors therefrom it has been required to provide a relatively complex system. Even such complex arrangements which have been developed in the past, provide a rather unsatisfactory multicolor display. One method used in the past has been to combine three separate layers of dissimliar electroluminescent material, each different electroluminescent material selected for the inherent color which it generates. By energizing various points of each layer a mixing of colors occurs, whereby a multicolor display results. However, due to the several layers of material the light intensity of the image output at the surface is reduced due to the filtering effects of the different layers involved. Consequently, the quality of the output colors is substantially reduced. Another prior art method for providing a multicolor display using electroluminescent material involves the principle of subjective color transformation wherein an image having relatively full spectral content is obtained by the interplay of two monochromatic images of the same object to be displayed. The phenomenon of color transformation occurs when the eye perceives two properly selected and superimposed images of the same object, a long wavelength image and a short wavelength image, which are illuminated by two light sources of different colors. The eye responds by assigning a variety of colors to the combined image apart from the two colors actually present. The various colors and hues and saturations thereof are a product of the eye’s response to the interaction of the long and short ‘wavelength images and their intensity patterns. Although only a single layer of electroluminescent material is utilized in such a scheme, it is required to provide, at flicker free rates, alternating patterns of the same image at different wavelengths in

2 order" to induce the subjective colors in the eye of the viewer and obtain the desired multicolor effect. A more recent prior art multicolor display system comprises a surface formed of multiple segments of a translucent substrate material positioned in a manner whereby the substrate segments are viewed edge~on. Each segment has three phosphors thinly deposited thereon, with each such phosphor having a different color generating characteristic. Individual connections are then required for exciting each phosphor of each substrate segment. Although multicolor output is obtainable from such a device certain inherent deficiencies limit the utility thereof. For example, it is extremely diflicult to fabricate a display surface of any practical size due to the large number of segments required, each having multiple connections which must be individually controlled. Secondly, the output radiation from such a display device is highly directional. In other words, the output intensity seen by a viewer is a high order function of the viewing angle, and almost perpendicular viewing of the surface is necessary. The deficiencies heretofore mentioned and others of the prior art are eliminated by the subject invention. Contrasting with such prior art color display systems, the present invention is a very simple device for obtaining multicolored images. In one embodiment of a color display system having a display surface and a variable excitation source, there is provided an illuminating means for providing a colored biasing light which is projected onto the display surface where it is additively mixed with variably controllable colors generated by the display system. The colored biasing light is made substantially complementary in color to a color within the range of variably controllable colors. Multicolored images are produced by an additive color mixing process which occurs at the surface of the display device, not by any subjective reaction process within the eye of the viewer. The output colors obtainable at the display surface are a function of both the frequency and amplitude of the excitation source. A general property of electroluminescent material is the generation of a relatively narrow spectrum of output colors as a function of excitation frequency. Further, the intensity of the output colors varies as a function of the ampltiude of excitation voltage. One embodiment of the present invention employs such electroluminescent material characteristics in cooperation with an external source of light, which source provides color biasing at the surface of the display device. Thereby, a variety of output colors are obtained as the combination of voltage and frequency excitation is varied and the electroluminescent color output mixes with the bias color. The bias color is made substantially complementary to one of the colors within the range of colors generated by the electroluminescent material in order to obtain the widest possible variety of output colors. Accordingly, an object of the present invention is to proyide a very simple multicolor display system, utilizing a biasing light source, the output colors of which are dependent upon both amplitude and frequency of a system excitation source. Another object of this invention is to provide an illuminating means in cooperation with an electroluminescent display device wherein such illuminating means provides a colored source of light at the surface of said -display device for color mixing thereat and resulting in a wider variety of output colors. A further object of this invention is to provide a multicolor electroluminescent display system having a plurality of display areas separately connected to a plurality of variable voltage supply sources wherein the output color from each separate area may be controlled by a variable voltage source and, further, to provide a source of bias light at the surface of the display for color mixing thereat and producing a substantial variety of output colors.

Other objects and advantages of the present invention will become apparent and the invention will best be understood from the following description with reference to the accompanying drawings in which:

FIGURE 1 is a schematic representation of an embodiment of the invention; 10

•FIGURE 2 depicts the standard chromaticity diagram and indicates the range of variable colors which are generatable by the device of this invention;

FIGURE 3 is a schematic representation of one embodiment of the present invention wherein the display jg panel comprises a plurality of separately energized areas;

FIGURE 4 is an isometric view of a portion of an electroluminescent display panel for use in a preferred embodiment of the present invention;

FIGURE 5 is an exploded view of a few of the ele- 20 ments which make up the display panel of FIGURE 4;

FIGURE 6 is an isometric view of an embodiment of the present invention showing means for providing colered bias light "by use of edge lighting panels;

FIGURE 7 is a schematic end view showing the em- 20 bodiment represented in FIGURE 6; and

FIGURE 8 is an isometric rear view showing an alternate embodiment of the present invention.

Referring now to FIGURE 1, there is provided an electroluminescent display panel 10 comprising an opaque 30 conductor 12 formed by vacuum deposition of aluminum at the backside of the panel, a transparent conductor 14 of tin oxide, also formed by deposition process at the front surface of the display panel, and a single layer of electroluminescent material 16 situated between the 35 two conductors 12 and 14.

There is also provided a voltage source 18, which is variable in both amplitude and frequency, connected to the conductors 12 and 14 by lines 20 and 22. There is further provided an illumination source 24, external from 40 the display surface, which is shown providing a source of radiation for color biasing the surface 14 of the display device. The color of the biasing light from source 24 may be any color other than those generated toy the display panel 10, but for practical reasons, in order to 45 obtain the greatest differentiation of output Coiots the biasing light should be made substantially complementary to the natural output color of the electroluminescent material 16 used in the panel 10. The inherent color generated by commonly used electroluminescent materials 50 falls within the range of green to blue-green. In such case, the biasing color should be red or some color relatively close to red, and therefore substantially complementary to the natural output color of the electroluminescent panel. 55

With the source 24 turned on, and in the absence of excitation from the voltage source 18, the display panel will appear the color of such bias source 24, that is, red. Upon excitation of the electroluminescent material, color mixing will occur at the display surface 14 and, depend- 60 ing upon the excitation intensity, that is, the amplitude of the voltage of source 18, the display color will change from that of red to white, and as the intensity of excitation is increased, the display output color will become essentially blue-green. The output color of the electro- Gd luminescent display panel 10 is further variable as a function of the frequency of the excitation source 18. As the frequency is decreased the natural color output of the electroluminescent material 16 shifts to the green ^0 end of the spectrum, whereas as increase in frequency will push the output color to the blue side of the spectrum.

As a means of further understanding the various color combinations obtainable as a result of combining the bias 75

color and the variable colored output of the electroluminescent device itself, a description of the standard chromaticity diagram shown in FIGURE 2 follows.

Referring now to FIGURE 2, the chromaticity diagram represents the relation between light wavelength combinations and perceived color. Along the vertical axis is indicated the percent of the color green; along the horizontal axis is indicated the percent of the color red; and the percent of the color blue is the remainder after subtracting the sum of the two indicated percentages from one hundred percent. Point iR on the diagram represents the red bias color. White is represented by point W, blue is represented by point B, blue-green by point BG, green by point G, yellow-green by point YG, yellow by point Y, orange by point O, and magenta by point M.

In the operation of the electroluminescent panel 16 of FIGURE 1, the output color seen at the panel surface, in the absence of excitation source 18, is represented by the point R, the bias color emanating from illuminating source 24. As voltage excitation is increased, and beginning at a low frequency level, the output color of the panel 16 would vary along a line passing from point R to point G. In other words, the color output would shift through orange, yellow, and yellowish green, until at high excitation amplitude the color green would be generated. Further color variation is now possible by increasing the output frequency of source 18 while maintaining high voltage excitation. As the frequency is increased, the color output of the surface of the panel 16 will shift along the line shown passing from point G, through point BG to point B, which represents the color blue. At such high frequency excitation, a decrease in the voltage amplitude of source 18 would shift the output color back towards point R passing through point M, which represents the color magenta. The color W is obtained by the combination of equal intensity complementary colors, and in the example under consideration blue-green BG represents the complement of the bias color red R. An intermediate excitation frequency of intermediate amplitude will generate the proper intensity of blue-green, which when mixed with the bias color red will produce a white output color.

It may now be appreciated that any color shown on the chromaticity diagram and located within the substantially triangular area defined by points R, G, and B may be generated at the surface of the display panel 10 merely by altering the amplitude and frequency of the output of excitation source 18, in conjunction with applying a red bias light at such display surface. In the absence of such colored bias light, output colors would only vary in intensity while ranging between points B and G. A display panel designed to simultaneously provide a multitude of output colors is shown in FIGURE 3.

Referring now to FIGURE 3, there is depicted a simple schematic representation of the present invention, wherein the display panel comprises a plurality of separately energized areas of electroluminescent material. There is provided a transparent conductor 25 at the viewing side of the panel, which covers the total display surface, and a single layer of electroluminescent material 26. There is further provided a plurality of separate, opaque conductors, 30, 32, 34, 36 and 38 covering the electroluminescent material 26 on the rearward side of the panel. As shown, opaque conductors, 30 and 32, are connected to a voltage source 40 and conductors 36 and 38 are connected to a voltage source 42. Both sources 40 and 42 are also connected to transparent conductor 25. Conductor 34 is shown connected to no energizing source. However, by closing switch 47 conductor 34 would be connected to source 42 through a resistor 45. A pair of resistors 44 and 46 are inserted respectively between source 40 and conductor 32 and between source 42 and conductor 36. An illumination source 48 provides, when energized, a red bias light which impinges upon the surface of the display panel.