Known in the art are electroluminescent emitters (conventional electroluminescent panels ELP), in which one electrode is in the form of aluminum foil and the other electrode comprises a transparent conducting film, with an electroluminescent layer between the electrodes. see "Elektroluminestsentnye istochniki sveta" (Electroluminescent Light Sources) Ed. by I. K. Vereshchagin, Moscow, Energoatomizdat Publishing House, 1990, p. 51!. If the film electrode is made on a solid base (e. g., of glass), the ELP has a rigid structure. If the film electrode is made on a polymer film (such as polyamide), a flexible panel is obtained.
An advantage of a conventional ELP design is a planar geometry of the structure, in which the electrodes form a planar capacitor, and the electroluminescent layer that fills up the space between the electrodes is in a uniform electric field of the capacitor. Uniformity of the electric field in the electroluminescent layer allows uniform glow of the ELP to be obtained over the entire area, thus assuring maximum brightness. It is possible if a sub-breakdown working voltage U.sub.br, is chosen so that is limited only by the breakdown voltage Ubr, which does not depend on three-dimensional coordinates, because the field is uniform n the thickness direction of the electroluminescent layer. The design of conventional ELP allows panels of all basic colors to be manufactured in a relatively simple manner, with a low prime cost and high reliability of the products.
However, ELPs of the aforementioned type have a number of disadvantages which result from the problems encountered in forming film electrodes that have to provide high surface areas with uniform properties. These disadvantages limit the possibility of providing long (dozens of meters) planar light sources capable of assuring uniform glow over the entire working area. As the conducting films (e. g., of indium oxide) have high electric resistance (dozens of Ohm per square millimeter), an additional conducting strip (of silver) has to be applied to the edge of the film to assure uniform glow. This complicates the design and manufacture of the ELP. Even under the most favorable conditions such a film electrode absorbs up to 30 to 40% of light radiated by the electroluminescent layer, which is undoubtedly a disadvantage of the ELP from the light efficiency point of view. Another disadvantage of the ELP of the above type is the use of the second electrode of aluminum foil, which is not transparent. This also reduces the total light efficiency, the ELP glow being one-sided (light is emitted through the transparent film electrode only, which in certain applications imposes restrictions upon utilization of such light sources.
Another type of prior art devices is represented by flexible electroluminescent light sources (FLS). In these devices the electrodes are in the form of small-gage conductors (wires), whereby it becomes possible to dispense with the film electrodes and thus to eliminate the disadvantages inherent in the light sources associated with the use of the film electrodes.
Earlier FLS designs with "wire electrodes" are disclosed in U.S. Pat. No. 2,684,450 issued in 1954 to E. L. Mager et al and in U.S. Pat. No. 2,838,715 issued in 1958 to E. C. Payne. The electrodes described in these patents are in the form of a pair of small-gage enameled wires wound on an insulating support and spaced apart at a short distance. The electroluminescent layer fills up the space between the wires.
U.S. Pat. No. 3,052,812 issued in 1962 to F. W. Dow discloses a flexible electroluminescent strand consisting essentially of a first copper wire of a predetermined diameter, a second copper wire of a smaller diameter coated with a high-dielectric material and wound around the first wire, and an electroluminescent phosphor coating applied to the surface of the first wire and engaging the surface of the dielectric coating of the second wire.
However, according to the method of manufacturing of the Dow's FLS described in the aforementioned patent, such an FLS is not suitable for production under industrial conditions. This is because the manufacture of the FLS requires the use of a special frame with hooks for guiding and passing the wires manually one by one.
U.S. Pat. No, 3,571,647 issued in 1971 to B. A. Robinson describes flexible electroluminescent structures made of a deformable electrically conductive material and a second electrode in the form of one or more insulated conductive wires connected to the surface of the deformable electrode. A layer of electroluminescent phosphor covers the conductors defining the second electrode and the exposed portions of the deformable electrode. The deformable electrode is on a substrate which may be rigid or flexible. When an AC voltage is applied across the electrodes, the phosphor layer luminesces with an intensity greatest in the vicinity of the second electrode.
Although this structure is more suitable for manufacturing under industrial conditions, it has the same disadvantages as all conventional flexible electroluminescent light sources, i.e., it does not allow one to control the core separately as a final semiproduct. Furthermore, it is not suitable for production in the form of a "travelling-light" structure and does not provide uniformity of luminescent properties in the longitudinal and transverse direction.
Russian Pat. No, 2,000,678 of 1993 to Ruben Polyan and Sergei Seryogin discloses a flexible electroluminescent light source with wire electrodes, wherein, in order to provide a linear light source, the electrodes (fibers) are positioned along the axis of symmetry, and the electroluminescent material in a dielectric binder fills up the space between the electrodes. Another Russian Pat. No. 2050042 of 1995 to Ruben Polyan and Sergei Seryogin discloses a method for manufacturing the aforementioned flexible electroluminescent light source, wherein a plurality of electrodes are drawn through a plastic mixture of an electroluminescent material with a dielectric binder. The mixture is compacted and fills up the spaces between the electrodes, with subsequent hardening of the dielectric binder and formation of a polymer sheath.
A disadvantage of the construction and manufacturing method described in the aforementioned two Russian Patents consists in that they do not allow a multisectional construction with individually controlled sections for implementation of an idea of "travelling light".
Thus, a disadvantage of all known flexible luminescent sources described above consists in that their electrodes (wires, insulated or non-insulated conductors, or conducting fibers) are either twisted, braided, or laid in parallel and that the electroluminescent material is either placed in the spaces between the electrodes or is applied to the electrode (electrodes). Most often, however, the electroluminescent material is applied to a flexible support to which the electrodes are attached. Since the surfaces of the electrodes are curved and because the layer of the electroluminescent material, which is located between the electrodes, has an intricate and variable-thickness configuration, an alternating electric field that causes the electroluminescent material to glow becomes substantially non-uniform. In the thinnest areas of the electroluminescent material, where the electric field is at its maximum, this material penetrates deeper into the interelectrode space and adjoins the electrodes. Therefore, glow of these zones makes an insignificant contribution to the overall light output of the light source. On the other hand, the working voltage U that has to assure the maximum brightness of glow of the FLS should be chosen to be as high as possible for effective excitation of the thick outer zones of the electroluminescent material in the binder (located close to the emitting surface) in the spaces between the electrodes, thus making the major contribution to the light output and determining the overall brightness of the FLS. This is not, however, possible, because the voltage U necessary for effective excitation of the relatively thick outer zones of the electroluminescent material in the binder will cause break down through the thinner inner zones of the electroluminescent material. For this reason, the working voltage U.sub.br has to be strictly limited on the outer side by the breakdown voltage U.sub.br of the thinner inner zones. Therefore, the thicker outer zones of the electroluminescent material will glow weaker than possible, and the light source will have low brightness. Limitation of U is also necessary because of the possibility of breakdown of insulation on the electrodes at points of their contact or at points were they extend close to each other.
Therefore, substantial non-uniformity of the electric luminescent material that fills in the spaces between the electrodes does not allow the maximum possible brightness of glow of the FLS to be achieved, with increased probability of breakdown through both the electroluminescent layer and insulation of the electrodes, thus lowering overall reliability of the light source and limiting the brightness.
Another disadvantage of all prior art constructions resides in that the electroluminescent layer that fills up the spaces between the electrodes could be formed either by applying a viscous liquid suspension (an electroluminescent material with a dielectric binder) to the electrode (electrodes) or to the substrate which supports the electrode, or by filling the spaces between the electrodes. After application of the suspension and removal of its surplus, the assembly is dried, and the electroluminescent layer is regarded as substantially formed after drying.
When the suspension is applied and its surplus is removed, the relief structure formed by the plurality of the electrodes (characteristic of the FLS structure) interacts with the suspension that behaves like an abrasive material. This interaction results in the electrode insulation being damaged (the emery paper effect), and the preset regular pattern of the electrode arrangement is disrupted. Similar consequences take place as a result of shrinkage of the electroluminescent suspension during drying when internal stresses develop within the body of the interelectrode electroluminescent layer. In addition, the suspension flows under gravity during drawing, whereby uniformity of the electroluminescent layer thicknesswise is disrupted. All these factors result in three-dimensional non-uniformity of the light output of the FLS, and hence in low brightness, brightness nonuniformity over the glow area, and low reliability that is caused by probability of short circuit through the damaged portions of insulation and the thinnest portions of the interelectrode electroluminescent areas. In addition, the use of viscous liquid suspensions results in cracks, bubbles, and voids appearing within the body of the electroluminescent layer after drying. These defects subsequently become the points of concentration of atmospheric moisture that causes accelerated degrading of the electroluminescent layer.
Still another disadvantage of conventional flexible luminescent sources consists in that the need to form an electroluminescent layer in the FLS by applying a viscous liquid suspension to the electrode (electrodes) or to the interelectrode spaces limits concentration of the electroluminescent material in the suspension on the upper side (with maximum not exceeding 2: 1). This, in turn, limits brightness of glow of the FLS that otherwise could be higher with greater concentration of the electroluminescent material.
Manufacture of the aforementioned FLS by a continuous method involves application of a pulling force of the drawing mechanism (in the transport direction during formation of the electroluminescent layer and the sheath) to the plurality of small-gage electrodes (that are normally made of copper due to its low resistance), i. e., lengthwise of the electrodes. The electrodes are thus put under tension, and their insulation may crack. The consequences of this are obvious: putting the electrodes under tension disrupts the regular pattern of their arrangement, thus resulting in non-uniform properties of the produced FLS and in non-uniform brightness over the glow area. Furthermore, eventual cracks in the insulation increase the chance of short-circuiting, thus lowering reliability of the light source.
All prior art constructions of the FLS essentially involve three-dimensional distribution of the electroluminescent material in the binder adjacent to the electrodes, and the thickness of the electroluminescent layer is comparable with the cross-sectional dimensions of the electrodes. As there are no reflecting layers in the construction, radiation originating within the body of the electroluminescent layer (the inner zones adjacent to the electrodes) does not practically reach the surface of the light source, thus lowering efficiency of the light source and brightness of its glow.
In all prior art constructions of the FLS, the glow color (red, yellow, green, blue) is determined by the type of the electroluminescent material used, whereby light sources with a greater variety of the glow colors cannot be obtained.
Moisture resistance that mainly determines life of the FLS depends to a great extent on properties of the thin polymeric sheath, which can prove insufficiently tight in a number of applications.
In all prior art constructions of the FLS the radiating surface (surfaces) glows equally (with one color or a set of different colors) over the entire glow area, thus limiting the possibility of providing light sources with a preset three-dimensional distribution of the glow colors in the glow area.
In all prior art constructions of the elongated FLS (glow filaments, conductors, strips), the breaking strength under tension or bending (except for those described in Russian Pat. No. 2000678 and in aforementioned article of Polyan and Seryogin) depends on the elastic properties and strength of the combination of the electrodes and sheath. According to Russian Pat. No. 2000678 and aforementioned article of Polyan and Seryogin, to assure a required arrangement of the electrodes and to enhance the breaking strength of the FLS, polymer threads are placed into the space between the electrodes, but the cross-sectional areas of the threads are still limited by the thickness of the electrodes. For this reason, the breaking strength of the FLS can prove inadequate, thus limiting the field of application of the light sources.
In all prior art constructions of the elongated FLS with glow filaments, conductors, and strips, high flexibility is assured, owing to flexibility of the system of the electrodes and sheath. For this reason, the FLS cannot be used in applications where the elongated light source has to be plastic (i. e., where it has to retain its shape after deformation) or rigid. This also limits the field of application of the elongated FLS.
In all prior art constructions of the elongated FLS, the power supply wires to which the working voltage U is applied are connected directly to the ends of the wire electrodes and, bearing in mind a small gage of these wires, the soldering points are in the zone most vulnerable to breaking forces. This lowers reliability of FLS.
Another disadvantage of known elongated FLS is that the damage or breakage of the electrodes will make the emitter inoperative over its entire length, thus lowering reliability of the FLS as a whole.
It should be noted that in all prior art constructions of the elongated FLS, it is not possible to turn on independently individual parts (sections) of FLS extending along, or transversally with respect to, the electrodes, since the prior art constructions involve energization of the FLS as an integral unit having a length equal to the length of the electrodes. This does not allow a damaged portion (section) of the FLS to be disconnected with the light source remaining in operation as a whole. This lowers reliability of the light source. In addition, glowing parts (sections) of the FLS extending in series along, or transversally with respect to, the electrodes cannot be switched, thus ruling out the possibility of providing dynamic light effects such as the "traveling light" effect. None of the existing FLS structures possesses moisture-resistant properties which are the main factor in protecting the luminescent layer from deterioration.