CA2919096A1 - Visual warning signal for fire safety applications - Google Patents

Abstract

The strobe employs one or more LEDs arranged so as to generate a pattern of light dispersion that adequate illumination is received at all viewing angles appropriate to a wall mounted device. Specifically, the strobe is designed to meet dispersion requirements for light output at all points in a hemisphere below the mounting position.

Description

Visual Warning Signal for Fire Safety Applications Field of the Invention This invention is a visual warning signal apparatus using light-emitting diodes to generate a highly visible and attention-grabbing alarm indication for use as a wall or ceiling-mounted alarm indicator.
The availability of a visible warning signal for fire safety applications has become important due to increasing awareness of the needs of hearing-impaired individuals, especially due to the Canadian with Disabilities Act.
To meet the needs of life safety applications, a visible warning signal must fulfill the following requirements:
= High reliability.
= High visibility.
To make a commercially viable product, the following requirements must also be met:
= Long lifetime.
= High manufacturability.
= Low cost and component count.
The following additional criterion must be met for a battery-powered application:
= Low power consumption.
Current visible warning signals rely on xenon tube technology. A high voltage is applied to a xenon tube. At low voltages, the xenon tube does not conduct electricity, but when a sufficiently high voltage (a few hundred to a few thousand volts, depending on the length of the tube) is reached, the gas is ionized, causing it to become a very good electrical conductor. An electrical charge (often stored in a bank of capacitors) is then able to discharge very rapidly through the xenon tube, resulting in a very bright light.
Creating this high voltage requires a large, complex circuit with a high parts count that can include expensive transformers and capacitors with very high dielectric resistance.
As well, the high voltage requires special safety considerations. Extreme heat build-up also occurs, resulting in poor bulb life. The circuit may also generate radio-frequency transients due to high internal voltage and rapid discharge characteristics.
With the goal of addressing these shortcomings, a new design utilizing high-performance light-emitting diodes (LEDs) is proposed.

Overview of Invention The invention is a flashing light intended to provide a highly visible, attention-getting visual signal.
The invention uses light emitting diodes (LEDs) as the source of illumination to generate a flashing light capable of meeting relevant regulatory standards pertaining to visible signaling devices.
On/off timing is controlled by a logic circuit such as a microprocessor.
Power is provided through a simple two-wire connection. A positive voltage is connected to the anode of the LED, and an electrical return path is connected to the cathode of the LED. If two or more LEDs are used in the circuit, they can be connected in series. If additional photometric intensity is required for the application, a parallel series of LEDs could be added.
Light Source Traditional visual warning signals use a xenon tube strobe as the light source. Xenon tube strobes generate an extremely high-intensity discharge during a very short pulse.
This is desirable in many applications such as stop-motion photography and special visual effects (such as "temporal aliasing" as seen in dance-floor lighting at night clubs) but is of not of particular value to visual warning signals.
This visible warning signal design employs high-power LEDs rather than a xenon tube as the light source.
No currently available power LEDs can equal the peak intensity of a xenon-tube light source. However, because of lower heat buildup and power consumption, LEDs can provide longer pulse duration. In visible warning signal applications, effective intensity, not peak intensity, is the critical parameter. A less intense pulse with longer duration can have greater effective intensity than a more intense pulse with short duration.
The use of power LEDs for visible warning signal applications is intended to overcome shortcomings associated with conventional xenon-tube technology. An LED based design will provide higher reliability, longer life, lower cost, lower parts count, and no need for high-voltage circuitry.
Pulse Shape The LED-based visual warning signal breaks from traditional xenon-tube warning signals by employing a less intense light source for significantly longer pulse duration.

Visible warning devices employing the traditional xenon-tube strobe design use a very intense light for a very short duration to generate the required effective intensity.
Generating this very intense pulse requires complicated electronics, is power-inefficient, and results in short bulb life because of extreme heat generated during discharge.
Although this high intensity, short-duration pulse shape is useful in applications such as stop-motion photography and visual effects, it is of no advantage in warning-light applications. In fact it is a disadvantage, as the intense light is a hazard to someone looking directly at it, and could be disorienting or temporarily reduce vision.
Documentation for xenon-tube strobes warns that looking directly at the strobe could cause visual impairment. High-intensity strobes can be used as crowd-pacifying weapons due to the disorienting effects of intense strobes.
Using a longer pulse-time and lower peak intensity has the following advantages over the xenon tube strobe designs:
= Better power efficiency, resulting in longer run-time for battery-powered applications and allowing the use of lower-cost components.
= Lower peak intensity pulse can be created by simpler, more reliable circuitry.
= Lower peak intensity is less likely to disorient viewers or harm a viewer's eyes.
The quantitative measure of the performance of a strobe is effective intensity. Intensity measures the absolute brightness of light hitting an area. Effective intensity is a more subjective measurement that takes into account how flashing light is perceived by human eyesight. Human eyesight tends to take note of changing light conditions and filter out steady-state light conditions, so flashing lights are more noticeable than a steady light of equal intensity. This effect is quantified as effective intensity.
One of the most common measures of effective intensity is the Blondel-Rey equation.
The Blondel-Rey equation dates back to 1911, but is still considered a good representation of the way human perception reacts to flashing light sources.
UL uses the Blondel-Rey equation when evaluating visual warning signals.
Blondel-Rey equation:
if (t241) >100ms, Effective intensity, le = (integral of light emitted during pulse) / (a + t2 -t1) or if (t2- t1) is less than or equal to 100ms, Ie = (integral of light emitted during pulse) / a where ti is pulse start time, t2 is pulse stop time, and the constant "a"
represents a "visual time constant" during which the eye readjusts its sensitivity. The value of "a" is either 150ms (for near-dark conditions) or 200ms under more typical conditions.
Regulatory bodies typically use a=200ms for purposes of calculating effective intensity.

The theory behind the Blondel-Rey equation for flashing light is that the human eye is considered to behave like an ideal integrator for periods up to 100ms, and after 100ms the eye begins to adjust to the increased brightness and becomes less sensitive.
The light begins to stop being perceived as a "flash" and begins to become part of what the eye perceives as ambient light and therefore is not perceived to be as intense.
As a result of this equation, using a pulse duration of 100ms maximizes effective intensity of the flash. For a pulse of constant intensity, such as the output of the LEDs, the integral portion of the equation is equal to (pulse amplitude) * (pulse duration).
100ms represents the longest pulse time that can be emitted without the (t2 -t1) term entering the denominator and decreasing the effective intensity.
The choice of a 1-second flash rate is motivated by the goal of reducing power-consumption and heat buildup. UL specifies that visible warning signals have flash rates between one and two flashes per second. Using a once per second flash rate gives the longest allowable off-time and therefore also the lowest possible average power dissipation. This gives the best power consumption and thermal buildup characteristics that can be achieved without compromising the effective intensity of light output.
Fault Detection The ability to detect a malfunctioning strobe is desirable in safety applications. Some of the existing patents for LED strobe devices employ a complicated means of detection employing an optical sensor to ascertain that the strobe is operational. The optical sensor method requires an expensive photodetector component and adds to the cost and component count of the circuit. A simpler and less expensive means of detecting malfunction is desirable.
The constant-curent power source could be used as a means of identifying a malfunction.
A constant current source provides a set current, and adjusts its voltage output to whatever level (within limits) is required to drive that amount of current through the connected load. The voltage across an LED is quite stable (around from 3 to 4 volts for typical white LEDs) regardless of the amount current passing through it, so the voltage required to drive a constant current would be fairly predictable. The constant current source could detect voltages outside the expected range. For example, a series of two white LEDs would require from six to eight volts. If the constant current source finds itself generating 4 volts to drive the specified current, it is likely that one of the LEDs has short circuited. If the constant current source finds itself producing the requested current at near zero voltage, the whole strobe assembly may be short circuited. If the constant current source is at its voltage limit and unable to drive the specified current through the strobe, a malfunction has occurred: either the strobe has been disconnected or an LED
has burned out.

The predictable characteristics of LEDs combined with a constant current power supply provide the invention with a simple and effective means of fault detection.
Dispersion Pattern An indoor safety warning application requires the strobe to be visible from every conceivable viewing angle in its intended mounting location. For a wall mounted unit, light must be visible forward and to 90 degrees left and right of the mounting location, and to a lesser extent up and down as well.
For a ceiling mount application, a safety warning light must produce a hemispherical radiation pattern, to be visible from any direction below it.
While other inventions in the field have employed a multitude or plethora of LEDs mounted in such a way as to generate a wide viewing angles and light outputs that could be viewed from long distances, this invention is intended for indoor applications, and as such need only generate sufficient light output to capture the attention of occupants of the room it is installed in. The level of light output required for this purpose is quantified in fire safety codes and other regulatory documents pertaining to visible signaling devices.
Existing LED strobe implementations employ a large number of LEDs mounted with the main axis of light generated on a plane that is parallel to the axis of mounting. This arrangement is appropriate for applications where the objective is to be visible in a horizontal plane, such as on road vehicles or as a warning between airplanes traveling at the same altitude. However, it is not suitable for interior safety applications, where the peak intensity is required on an axis perpendicular to the plane of mounting.
Other related patents are likewise intended for outdoor applications such as emergency vehicles or hazard markers. The basic intent of such applications is different from what is required for an indoor visible warning device.
A vehicle flasher or hazard marker is intended to alert approaching observers of the presence of the hazard, or alert observers to the approaching vehicle. They are intended to provide high visibility (and in some cases a distinct identifying signal) to observers on a horizontal plane with the marked object.
For an indoor visible warning device, the objective is not to announce the presence of an object, but to capture the attention of the occupants of the room.
While other inventions in the field have employed a multitude or plethora of LEDs mounted in such a way as to generate a wide viewing angles and light outputs that could be viewed from long distances, this invention is intended for indoor applications, and as such need only generate sufficient light output to capture the attention of occupants of the room it is installed in. The level of light output required for this purpose is quantified in fire safety codes and other regulatory documents pertaining to visible signaling devices.

This design generates a hemispherical pattern rather than a flat circular pattern. This is achieved using Lambert lens LEDs mounted so that the main axis of light output is perpendicular to the surface on which the strobe is to be mounted. Lambert lens LEDs have a dome-like lens over the emitter, producing a radiation pattern that can be viewed from over 90 degrees past the main axis. This pattern makes Lambert lens LEDs suitable for high visibility from wall and ceiling mounted positions without the need for specially shaped enclosures or mounting mechanism.
Lambert lens LEDs mounted on a flat surface can produce adequate levels of illumination at all angles required for wall or ceiling mount applications. Mounting the LEDs on a slightly curved surface, such as a specially shaped heat-sink, could further enhance the light dispersion for specific applications.
Intended Use and Other Potential Uses It is anticipated that the invention will be of exceptional value in replacing conventional xenon tube strobes for interior visible warning devices in wall and ceiling-mounted applications.
The invention is intended to comply with relevant standards governing visible warnings for fire safety applications. However, it is also anticipated that the invention could be applied to other warning applications, such as a visible warning for Carbon Monoxide detectors, or as a replacement for audible indicators in any other device where the hearing impaired would want a highly noticeable indicator. The invention could be applied as a ring indicator for a telephone, for instance.
In addition to interior safety warning applications, it is also anticipated that the invention could be fitted with a water-resistant housing to be used in exterior wall-mount applications where the hemispherical pattern of illumination would be advantageous.
It is also anticipated that the device could be fitted with protective covering for use on interior floors as an exit route marker.
It is also anticipated that the invention could be fitted with a rugged and weatherproof protective covering for use on road surfaces as a lane marker. The low power consumption, long bulb life, resistance to vibration, and hemispherical range of visibility would be suitable for use on roads or airport runways.
Description of Related Art US Patent 6,483,254 (Nam H. Vo and Nicolo F. Machi) discusses the use of light emitting diodes in a strobe application. However, Vo and Machi discuss a design for a ring shaped configuration that is intended for vehicular use and does not generate a light pattern appropriate for use in wall or ceiling mounted applications. Wall and ceiling mounted applications require a roughly hemispheric pattern, while Vo and Machi's design is designed to generate a flat circular dispersion along the plane of mounting.
US Patent Application 20070035255 (Schuster, James, et al) discusses a strobe light employing an LED light source. However, the Schuster and James application claims a light source with an intensity of 15 Candela or higher, while the method of this application proposes using a less intense light source in combination with the effective intensity effect to create an effect equivalent to a much more powerful light source without consuming nearly as much energy. As well, the Schuster and James patent fails to address the issues of supervision and fault detection, and of light dispersion pattern and viewing angle.

Claims

Claims What is claimed is:
(1) A strobe light comprising one or more LED for use as a wall mounted visible warning indicator.
The strobe employs a pulse and amplitude optimized as per the Blondel-Reye equation to meet relevant standards for fire-safety applications with the lowest peak intensity requirements possible, and the lowest possible energy consumption. The strobe will employ a square pulse of light with constant intensity, with an on-time and an off-time optimized to generate an effective intensity that the human eye perceives to be five times the actual intensity, while also reducing power consumption by 90% compared to the steady state for the same light source.
The strobe employs a constant-current power source, as a means of ensuring adequate light output.
The strobe employs a fault detection system based on measuring the voltage across the LED array at the set current for the power source, thereby taking advantage of the predictable forward-voltage characteristics of Light Emitting Diodes.
The strobe employs one or more LEDs arranged so as to generate a pattern of light dispersion that adequate illumination is received at all viewing angles appropriate to a wall mounted device. Specifically, the strobe is designed to meet horizontal dispersion requirements for light output in a 180 degree arc forward of the mounting position, and meet vertical requirements from straight ahead to directly below the mounting position.
(2) A strobe light comprising one or more LED for use as a ceiling mounted visible warning indicator.
The strobe employs a pulse and amplitude optimized as per the Blondel-Reye equation to meet relevant standards for fire-safety applications with the lowest peak intensity requirements possible, and the lowest possible energy consumption.
Specifically, the strobe will employ a square pulse of light with constant intensity, an on-time of 100milliseconds, and an off-time of 900 milliseconds. This pattern generates an effective intensity that the human eye perceives to be five times the actual intensity, while also reducing power consumption by 90% compared to the steady state for the same light source.
The strobe employs a constant-current power source, as a means of ensuring adequate light output.
The strobe employs a fault detection system based on measuring the voltage across the LED array at the set current for the power source, thereby taking advantage of the predictable forward-voltage characteris tics of Light Emitting Diodes.