Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Encapsulated
Reexamination Certificate
1996-03-26
2003-07-29
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Encapsulated
C257S088000, C257S098000, C257S099000
Reexamination Certificate
active
06600175
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to solid state light emitting devices such as light emitting diodes and more particularly to such devices which produce white light.
BACKGROUND OF THE INVENTION
Solid state light emitting devices, including solid state lamps including LEDs are extremely useful because they potentially offer lower fabrication costs and long term durability benefits over conventional incandescent and fluorescent lamps. Due to their long operation (burn) time and low power consumption, solid state light emitting devices frequently provide a functional cost benefit, even when their initial cost is greater than that of conventional lamps. However, because large scale semiconductor manufacturing techniques can be used, many solid state lamps can be produced at extremely low cost. One such device is the solid state light emitting diode (LED) which has low fabrication costs, long operational lifetimes and low maintenance costs.
Light emitting diodes (LEDs), and similarly constructed super luminescent diodes and semiconductor diode lasers, are commercially available and a wide variety of designs and manufacturing techniques have been developed. In addition to applications such as indicator lights on home and consumer appliances, audio visual equipment, telecommunication devices and automotive instrument markings, such LEDs have found considerable application in indoor and outdoor informational displays. But until recently, LEDs have produced light only in the red, green or amber ranges and have not been generally suitable for replacing, for example, incandescent bulbs, with normally a white luminescence, in a wide variety of display applications. The recent introduction of a bright blue LED, however, allows white light LED systems to be realized and thus has the potential to open the display market to LEDs by providing a practical means to achieve both full color and white light illumination.
The practical advantages of LED displays over those using incandescent bulbs are many. The operational lifetime (in this case, defined as continual illumination) of a LED is on the order of ten years or over 50,000 hours, whereas incandescent bulbs often burn out in the order of 2000 hours, thus leaving an empty pixel in the display message. Such recurrent failures make a display unreadable and, therefore, not useful. These conditions (i.e., broken or missing pixels) require constant repair leading to a significant maintenance problem for providers of display signs based on incandescent illumination devices. With the long operational lifetime of a LED-based sign board, the pixels rarely burn out and the illuminated message remains legible over long operational periods.
Similarly, LED lamps are considerably more robust. When exposed to stress, mechanical shocks, or temperature variations often encountered in an outdoor environment they are less likely to fail than incandescent lamps. This attribute is especially important when the signage is utilized in an environment such as vehicular traffic, e.g., roadway signage to mark highway construction sites, bridges, tunnels, or traffic control markings, in which perishable filaments used in the incandescent lamps frequently break due to constant vibrational motion. Further, incandescent and fluorescent lamps are constructed with fragile glass exterior casings whose breakage makes the lamp useless, and by extension, the message on the sign board illegible. Due to severe environmental conditions on roadways, glass breakage of incandescent and fluorescent lamps is an all too frequent mishap. The solid state LED lamp has no filaments to break and is housed within a durable plastic casing, as the primary device envelope or package (typically being of considerable thickness), thereby exhibiting a high level of imperviousness to extreme outdoor environmental stresses. With respect to outdoor signage applications, displays can contain up to 1 million or more pixels or lamps. Thus the maintenance costs related to replacement of non-operational incandescent lamps or miniature fluorescent (or neon) lamps are high and unfortunately, continual.
Hence, an emerging trend in the manufacturing and marketing of informational displays or signage, especially for outdoor usage, is to utilize solid state LED lamps as replacement for more conventional incandescent bulbs. The major end user benefits are the lower power consumption costs and the longer operational lifetime (hence, reducing maintenance costs). A further benefit is the rapid relaxation times of a solid state device affording an opportunity to display rapidly changing information messages incorporating video or lifelike animation.
Given the desirability of white light displays (e.g., commercial bank “time and temperature” message boards, stadium scoreboards), considerable effort has been expended to produce white light LEDs. Although the recent availability of the blue LED makes a full color, and by extension a white light display realizable, conventionally it has been considered that such a display would require multiple LEDs. The multiple LEDs would be then incorporated into complicated and expensive LED modules to obtain the required broad band illumination necessary to provide white light. Even if a discrete LED lamp were constructed that provides white illumination (as opposed to the utilization of a multitude of single die, single color discrete LED lamps in a module or sub-assembly), the current state of the art requires the utilization of multiple LED dies and typically at least four electrical leads to power these dies. U.S. Pat. No. 4,992,704 issued to Stinson teaches a variable color light emitting diode having a unitary housing of clear molded solid epoxy supporting three LED dies characterized as producing color hues of red, green and blue, respectively. There have been some recent introductions of commercial “full-color” LED lamps, that are essentially discrete lamps which afford a means of producing white light. All currently available examples of such lamps contain a minimum of three LED dies (or chips)—one red, one green and one blue, encapsulated in a single epoxy package. The chips are powered via at least 4 electrical leads. These complicated multiple die, variable color devices provide an expensive and complicated method of offering white light illumination. Furthermore, these multiple die white lamps are rather inefficient in the present state of the art, offering luminosity far below that realized by existing monochromatic light emitting diode lamps, even when a very large quantity of dies are functionally incorporated into the discrete lamp assembly.
The utility of solid state lamps that offer white light illumination is clear. However, at present there is a very limited number of such solid state lamps available. In signage applications where a small pixel of light is frequently required to offer the highest possible resolution of the message or video image, the most practical solid state lamps for display applications are the LED lamps. The LED lamp can have very narrow angles of irradiance and are very small in size when compared with other means of providing a radiant surface. However, the methods of fabricating white LED lamps are limited. A conventional approach is to fabricate a large cluster of red, green and blue LED discrete lamps, housed in multiple lamp (up to 30) subassemblies or modules. By providing multiple power sources to control all of the discrete lamps, these large modules can appear, from a distance, to provide white light by the spatial mixing of blue, green and red sub-pixels of light given off by the individual discrete LED lamps that comprise the module. While the lamps that make up the modules may be individually addressable, and hence, offer the opportunity to, selectively and individually, provide red, green and blue light (or combinations thereof), such modular systems are complex and costly means of providing white light for a solid state display. Further, as these modules are rather large, the ultimate resolution of the display wi
Baretz Bruce
Tischler Michael A.
Advanced Technology & Materials Inc.
Chappuis Margaret
Hultquist Steven J.
Jackson Jerome
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