issue: September 2008 APPLIANCE Magazine
Light Guide Technology
Using Light to Enhance Appliance Safety
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by William S. Lerner, independent inventor
Three patents provide appliance manufacturers with a new level of safety by using light to indicate excessive temperatures.
There are many applications, both industrial and residential, that require a device to warn of extreme temperatures. Historically, these warning devices have been placed at a distance from the “danger zone” for several reasons. The inability to mix electricity with flammable, caustic, liquid, or volatile substances, as well as the limited heat tolerances exhibited by most light sources, have prevented the placement of a warning device directly in the area of greatest need.
Patented technology has made it possible to place warning devices near, or at, the point of use. Utilizing a beam of light, engineers and designers can illuminate a warning symbol to detect excessive temperatures (by design or by malfunction), either hot or cold, at any user set point.
Taken together, three patents (U.S. Patent Nos. 7,173,221; 7,087,865; and 6,806,444) create a system that utilizes a beam of light to provide a warning symbol to indicate excessive temperature. The warning can take the form of any symbol (e.g., line, dot, character, or word). The warning’s brightness or intensity can be coordinated to the temperature. Image transfer and/or image projection may be utilized. An example of image transfer would be the word “HOT” being transferred through the fiber-optic cable, whereas image projection would be the word “HOT” projected onto a surface. This system may also include an aural warning component, whose volume can relate to the excessive temperature level.
The temperature sensor may be of any known type. The sensor may also be a timer that coordinates to an operational state. It can be hard-wired or wireless, and may include nodes and/or motes.
The evolution of the technology began with Patent No. 6,806,444, Fiber Optics for Heat Warning. The second patent (No. 7,087,865) added the use of light guides to the existing technology and describes the detection and warning of excessive temperatures—either hot or cold. The third patent (No. 7,173,221) expands on these concepts, creating a unique portfolio of pioneer technology.
The inventor’s initial research and development efforts focused on thermochromic technologies, such as color-changing inks, dyes, resins, etc. However, these materials had severe limitations. They could not withstand high temperatures, were UV sensitive, and produced or revealed a warning symbol at a slow rate. The materials also degraded over time.
After further research, the technology evolved to use fiber-optic cables, and then solid glass rods to serve as light guides. The cost of using heat-resistant fiber-optic cable was prohibitive for most applications. The use of solid rods presented additional problems: They could not be bent to transmit light properly, they slowed light transmission, and they added unnecessary cost.
Unimpeded, light travels at 186,282,397 miles per second. The instant “on” or binary effect of an LED versus an incandescent bulb enhances the immediate effect of the warning. The goal was to produce a warning symbol or sound alert in the fastest time possible. In most applications, time is critical. A contact burn will occur in only one second at 167°F (75°C); however, most people have the perception of a burn (pain), at 130°F (approx. 54°C).
The inventor has refined the concept, eliminating the need for the fiber-optic cable. In addition to being costly, fiber can slow the transmission of light. Its speed depends on the distance traveled and the material used. The inventor simply removed the fiber, and directed the beam of light through the empty casing, or “jacket,” of the fiber. To clarify, some fiber-optic cable is encased in a metal tube. When the fiber is removed, a simple metal tube remains. The beam of light can travel through that empty space. The tube prevents the light from being visible, eliminating “light bleed,” until it reaches the end point.
To save cost and complexity, the beam of light can travel without a tube, like a standard laser pointer. This would be advantageous in situations where only the end point (or points), needs to be seen. A simple beam of light may be advantageous when components are out of view. If the components were in a dense environment with little room to spare, the beam of light would be preferred.
Simply utilizing a beam of light and a light guide, the patents allow a warning light to be used in locations where this was previously impossible. The light can come from various lighting components such as an incandescent bulb, LED, or laser. If the light’s path is not direct and needs to bend, it can be reflected off of a mirror, or any other reflective surface, and guided to the desired end point.
Gas Cooktop Application
The technology covered by these utility patents is not limited to a particular product or use, but for the purpose of this article, it is helpful to look at how the technology can be applied to a single product.
Currently, there is no system to warn a consumer of potentially dangerous residual heat on a gas cooktop. The burner caps and grates of a gas cooking appliances remain hot long after the flame is extinguished, without any visual clue. Although there are crude warning systems built into electric cooktops, there has never been an attempt to create a similar warning device in a gas-powered cooking appliance. This technology allows a warning device to be placed directly in the center of the gas cap, at the heat source.
Traditionally, there were three main impediments to creating this type of warning device. From a safety perspective, it was not desirable to pass electrical wiring through an area with flammable gases. Even if it were possible to wire a light source in this area, such wiring would adversely affect the flow of gas through the burner cap, negatively impacting the gas distribution and making it difficult to create a consistent heat source. Finally, light bulbs and LEDs cannot withstand the direct and reflected heat produced by the burner, which can exceed 1200°F (approx. 649°C), especially in commercial settings.
Using the method described in these patents, a warning light can be placed in the center of the gas distributor cap. A red LED, positioned beneath the cooktop, will direct a beam of light through a clear glass ceramic disc that is flush mounted in the bottom of the gas distributor unit. The LED is held in place by a temperature-resistant adhesive. The beam of light passes through the slug and gas (which is clear, and residue free), and illuminates a second flush-mounted glass ceramic disc in the center of the cap. The surface of the top slug is “sanded” to catch the light. It also makes the product scratch resistant. Subsequent minor scratching only assists in light capture.
Placement of the technology in the gas cap is the most dramatic for illustration purposes, but it is not limited to that position. The patents offer the end-user unlimited possibilities for placement, shape, size, brightness, color, symbol, and design. The beam of light can be projected from the area directly below the cap or it can be offset, coming from the side of the cap as well. With the use of a light guide, the beam can be sent in any direction. While any glass ceramic material can be used, the material cited in the above example is Robax by Schott Home Tech North America. This material is available in various stock sizes such as 3, 4, and 5 mm. It is clear and can be easily machined. In some forms, bends, curves, and angles are possible.
The optical properties for a stock 3-mm-thick round slug with a 10-mm diameter are represented in Figures 1 and 2. The human eye cannot discern the difference between light traveling through air and light traveling through Robax. Keep in mind that typical uncoated glass reflects approximately >8% of the light back.
Figure 1. Optical properties for a stock 3-mm-thick Robax material.
Graphs reproduced with permission of Schott Home Tech North America.
Figure 2. Optical properties, UV range, for a stock 3-mm-thick Robax
Graphs reproduced with permission of Schott Home Tech North America.
The temperature tolerance range for this clear glass ceramic is –400° to 1400°F (–238°C to 760°C). However, the light guide can be any clear material, from simple plastic to the glass ceramic. The choice of material will depend on its location and the temperatures involved. If the temperatures exceed the disk’s capabilities, the warning symbol can be projected on the surface of an object. The light guide can be placed at a safe distance from dangerous temperatures.
Expanding on the basic theme of using light to project a warning presents several additional possibilities. In addition to using the beam of light to directly illuminate a symbol (red dot, stop sign, etc.), the light can be projected through a danger zone with the projected symbol serving as the warning. The surface temperature is not relevant if the symbol is projected directly onto it.
As mentioned earlier, the path the light takes can be controlled with the use of light guides—simple hollow tubes used to direct the light to its intended target. The light guide can also take the form of a flexible rope. A flexible cable can be rolled and stored so that it is accessible for emergency situations. The warning can be projected from the end, or the side of the cable. The rolled and stored cable can be paired with a self-contained light source to become a self-contained, easily transportable unit. Up to 80% of the safety system’s cost can be due to the “hard wiring” of the system—a portable system as described mitigates much of that cost.
Three patents offer engineers and product designers a blank slate to design excessive temperature warning systems. The warning can be audible, visual, or both. The system can indicate status, malfunction, deviation, or any information concerning temperature or environmental change. The system can be hard wired or wireless. The warning can end at a single point, or multiple points on a surface, using one light source. The warning symbol can be projected from a light guide to any surface when traditional warning lights would fail due to extreme temperature.
Safety is paramount. Bulbs fail, and wires melt at extreme temperatures. These patents allow an engineer or designer to incorporate a safety system in any environment.
Special thanks to Ted Wegert, director of applications engineering, Schott Home Tech North America.
About the Author
William S. Lerner is an independent inventor who holds 14 patents in five different fields. He is a graduate of George Washington University in Washington DC. If you would like to contact Lerner, please e-mail firstname.lastname@example.org