The present invention is related in general to flashing warning lights, and, more particularly, to the provision of a high luminous intensity flashing warning light using superluminescent light emitting diodes for improved visibility and lower power consumption than conventional devices.
Flashing (i. e. intermittently or periodically illuminated) lights have long been used to provide visual warnings, and a considerable body of research has been compiled in the fields of physiology, psychology and engineering (and more recently in that hybrid field known as "human factors engineering") concerning human perception of flashing light (i. e. the ability of people to perceive and respond to flashing light). This field of study, which is inter-disciplinary, involves both the illumination art and the study of so-called psycho-visual or psycho-optical sensory phenomena.
From research and study in this field, a large number of factors have been identified and suggested as involved in the human perception of and reaction to flashing light, and although much knowledge on the subject is theoretically based and remains to be confirmed, there have been suggested certain factors which may be applied to the provision of a flashing warning light for improving the visibility of a flashing light, that is, for making a flashing light visible at a greater distance (i. e. "visibility"), and for enhancing the probability that people will not only perceive (i. e. see) the flashing light but will also react consciously thereto (i. e. "attention-getting").
It is suggested for example from the study of human factors that human visual perception of flashing light appears greatest when the light is flashed at a flash rate or frequency in the range of 3 to 10 flashes per second, with a flash duration of at least 0.05 seconds being recommended. Further, for the flashing of a light to be perceived as discrete flashes, the flash rate or frequency must be below the so-called "flicker-fusion" frequency, that is the frequency above which a flashing light appears as a steady light (i. e. due to the phenomenon of "persistence of vision"), this critical frequency being considered to be approximately 24-30 flashes per second.
For simplicity, hereinafter flash rate or frequency will be described in terms of "flashes-per-second" (fps).
Research has revealed other factors to be pertinent concerning perception of light in general, and flashing light in particular. For example, according to Fechner's Law the sensation of light as produced by the eye varies logarithmically with the intensity of the stimulus.
Luminance discrimination has also been experimentally studied, with regard to what psychosensory mechanisms are involved in discerning or seeing light flashes and in discriminating luminance differences between light flashes, in an attempt to establish psychometric curves for these functions. For example, it has been attempted to demonstrate that there are two discrete detection channels, one for long flashes and one for short flashes. Experiments have shown that different slopes are obtained for psychometric curves measured with short and long flashes. The explanation favored is that the visual system is not homogenous; there are at least two detection channels with inherently different slopes, and it is believed that these can be differentially tapped by varying test flash parameters. Results of some experiments tend to confirm this, and suggest that whereas the long flash detection channel is photometrically subtractive or subadditive, the short flash detection channel is photometrically additive and has a much steeper psychometric function slope than the long flash detection channel. It is further suggested that the psychometric function slopes of the different visual sensory channels vary differently as a function of wavelength, and it has been adduced that all three channels of the visual system do not have the same gain but rather differ in spectral sensitivity.
An interesting question concerns the relationship between the light detection and the flicker threshold. When flashes are supplied within a certain interval, they are perceived as being fused and are indistinguishable from continuously supplied light. It has been almost 150 years since it was shown that, under fused circumstances, the mean intensity over time is independent of the actual light-dark ratio. A further question concerns how many extra quanta of light must be added to flashes perceived as fused at the absolute threshold of vision to perceive a flickering light again, or more precisely, in order to see a regular high-frequency flickering light again (since fused light at threshold level is perceived as irregular flickering light). It is has been previously shown that the visual perception system's processing of quantal effects at low luminance levels is essentially nonlinear. Flicker can be detected either by the "on response" or by the "off response" of the visual system to a flash of light. In the case of the on response, extra light quanta have to be supplied so that the threshold set by the adaptational state induced by the previous stimuli is exceeded. A larger interval between flashes leads to a lowering of the adaptational state (because of a decrease of the flux) and thus to lower thresholds. In the case of the off response, the excitation state has to decrease by a certain amount in order to exceed the decrement threshold. If flashes last long enough for a stable adaptation level to be reached, then the threshold no longer depends on the actual flash duration. Experimental results have shown that after 100 msec this stable level can be reached and maintained by a constant intensity in the flashes.
With regard to critical fusion frequency as a function of mean intensity at low luminance levels, it is has been suggested that the critical fusion frequency increases from 6 to 25 Hz with increasing stimulus size. It has also been found that, at higher luminance levels, brief flashes need a longer interval to elicit flicker perception than do long-lasting flashes.
In summary, it may be concluded that simple flashes of light elicit a whole range of complex responses from the visual system relating to retinal potentials, subcortical potentials, primary-visual-cortex and associated area potentials, and generalized non-specific responses of the cortex.
Various different types of flashing lights have been known to be used for providing visual alert or warning lights, and have employed incandescent lamps, rare gas discharge lamps and, more recently, light emitting diodes as an illumination means, with some associated control circuitry. However, each of these previous types of illumination means has its disadvantages. Further, the design and operation of such previous types of flashing lights did not take into account the various factors such as flash rates and durations for optimizing the psychosensory perception of flashing light. Still further, the previous flashing light devices could not provide effective light output with low power consumption (i. e. efficiency) at desirable high flash rates, or could not do so without severely sacrificing device power consumption and reliability of the light source, and thus could not provide reliable low power operation and were thus not suitable for use in portable lightweight battery powered equipment.
For example, while incandescent light sources have commonly been used in flashing warning lights, there is the problem that, typically, incandescent light sources are not able to come to full brightness and to then cool off to extinction (i. e. turn on and off) within the higher optimum flash rate frequencies for attracting attention; the flashing character of tungsten-filament lamps is typically degraded significantly above flash rates of 9 fps. Furthermore, because of the inherent thermal inertia of incandescent light sources (once turned sufficiently on to emit light, there is a significant delay in extinction to the off state), such light sources cannot provide flashes of relatively short duration, nor can such light sources provide adequate on-off contrast when operated at higher flash rates. As a consequence, incandescent light sources are not suitable for use as warning lights at those flash rates and flash duration periods to which human visual perception is most sensitive but are constrained to use at lower frequencies and longer flash periods.
Still further, incandescent lamps are inefficient due to their emission of considerable energy at wavelengths outside the visual spectrum, and suffer inherent increased power loss, thermal inertia and filament degradation when operated at higher intensity and/or flash rates. An incandescent flashing light with adequate intensity for outdoor use usually requires larger size batteries to compensate for the excessive power loss in the form of heat, thus rendering it impractical for applications requiring reasonably small size and light weight necessary for portability. Durability of incandescent flashing lights is also degraded due to the thermal stress on the filament and mechanical shocks received by the filament.
Rare gas discharge lamps (e. g. Xenon, Argon flash tube lamps and strobes), while capable of operation at higher flash rates are, however, limited to extremely short flash durations which cannot be lengthened. Thus, such light sources are incapable of longer flash duty cycle operation. Furthermore, rare gas discharge lamps are relatively expensive and must necessarily be energized with high voltages and currents, and thus flashing warning lights of this type require complex charging and discharging circuits and consume considerable power. Furthermore, a large amount of energy is required to produce the flashing action of a rare-gas lamp; it tends to deplete ordinary batteries quickly if flashed at an optimal frequency of 3 to 12 Hz continuously such as that required by a warning light. Therefore, rare-gas discharge lights for extended flashing time are only feasible where a large power source is available, such as the utility power, or a power generator, but not in a portable application. Furthermore, being glass-encased, gas discharge flash tubes are susceptible to mechanical shock damage and to gas leakage rendering them inoperative.
Ordinary light emitting diodes (LEDS) are relatively durable mechanically and electrically (as long as their current supply is properly limited) and most readily lend themselves to low voltage-low current operation and electronic control for both flash rate frequency and duration. However such ordinary LEDs as have previously been used as light sources in flashing warning lights were of insufficiently low light intensity output. Hence the use of such low luminosity light emitting sources in visual warning devices has been of limited effectiveness, being restricted to subdued light environments such as for indoor activities, or where the ambient or background light level is quite low so that sufficient contrast can be obtained with the relatively dim illumination intensity of ordinary LEDs to render them visible against a background. Thus, ordinary LED flashers have only found wide application in toys, jewelry and other devices where visibility requirements are not critical. Examples of such prior devices are found disclosed in U.S. Pat. Nos. 3,786,246 and 3,812,614 (flying disc type toys); U.S. Pat. No. 4,308,572 (clothing ornament); U.S. Pat. No. 4,170,036 (jewelry); and U.S. Pat. No. 4,383,244 (skate wheel).
In order to be both effective and practical, a portable warning light should satisfy at least the following five requirements:
1. Adequate visibility, and attention-getting quotient, which involve considerations of various factors such as the luminous intensity as well as the on-off contrast ratio of the light source, flash rate/frequency, and flash duration/period;
2. Controllability, which involves the relative ease of controlling the light source for effective flash rate frequency and flash duration;
3. Extended operating battery life, which is a critical factor and requires balancing the interdependent factors of the power available, the light output intensity, and the permissible weight of the device;
3. Durability, which concerns the reliability of the device;
4. Light weight and small size, which requirements constrain the use of large and/or heavy batteries and thus affects the available power, limiting permissible power consumption in order to achieve adequate operating life; and
5. Cost, which is often of paramount concern since complex devices not only adversely affect economy in manufacture, but also the applicability of such devices to use by consumers.
Unfortunately, although numerous prior flashing light devices are known, these prior devices have failed to meet or satisfy all of the above-noted requirements for use in a portable flashing warning light.
Various prior safety flasher light devices have been proposed. Exemplary of such prior devices are those safety lights disclosed in U.S. Pat. Nos. 4,423,473 and 4,451,871. In these devices, a penlight battery power supply is coupled to an ordinary LED mounted within a lensed reflector housing by means of a position sensitive mercury switch, so that, when the device is worn or carried, the position sensitive switch will on account of sensing the wearer's movements connect power to the LED to cause intermittent bursts of light to be emitted thereby. It is described that because power is supplied only intermittently to the LED, the light source LED may be operated from a battery source which provides current to the light source LED in excess of its maximum current rating to provide light of greater intensity than is normally producible from such light source (i. e. an ordinary low luminosity LED). Such a device however does not produce flashes at any particular effective flash rate (apart from being responsive to the rhythm of the wearer's motions sensed thereby), nor of any particular flash duration and thus is not optimal for attracting attention, nor reliable since it regularly will subject the LED to an overcurrent condition which while perhaps brief risks damaging the LED junction nevertheless should the mercury switch connection remain on for too long. Another motion-switched intermittently flashed safety light device is disclosed from U.S. Pat. No. 4,535,392.
In U.S. Pat. No. 4,523,258 there is disclosed a safety belt with flashing LEDs for joggers in which an array of LEDs arranged along a reflective belt are connected to a battery-powered oscillator circuit including two separate oscillators, one oscillating at &lt;1 to 5 Hz, and another oscillating at 3 Hz. Sets or subsets of the LEDs in the array are alternately driven by connecting these oscillators to opposite nodes of the array, such that one oscillator forwardly biases the LEDs and the other oscillator reversely biases the LEDS, such that LEDs of different sets are driven only when forwardly biased and thus flash alternately. However, such a device is only suitable for low ambient light conditions.
In U.S. Pat. No. 4,819,135 there is shown a bicycle lighting device in which strings of LEDS are mounted along the frame tubes of a bicycle and flashed in sequence to provide a broadside flashing triangular slow moving vehicle signal to motorists. U.S. Pat. No. 4,763,230 shows a string of LEDs adapted to be secured to the spokes of a bicycle wheel.
Other portable safety flasher lights are known from U.S. Pat. Nos. 3,153,745; 3,840,853; and 4,323,879. Flashing or blinking signal light devices for bicycles and other vehicles are also known from U.S. Pat. Nos. 2,661,406; 3,764,976; 3,916,377; 3,974,369; 3,987,409; 4,019,171; 4,388,559; 4,550,305; 4,598,339; and 4,692,736.
The use of light emitting diodes instead of incandescent lamps in flasher lights offers advantages in longevity and lower power requirements. Devices using flashing LEDs are also known from U.S. Pat. Nos. 3,737,722; 4,228,484,; and 4,228,485. U.S. Pat. No. 4,271,408 discloses an array of LEDs mounted on a reflectorized substrate to form a colored light source, for use in signs. U.S. Pat. No. 4,654,629 discloses a vehicle marker light for end-of-train equipment use having arrays of LEDs which are driven to be flashed at prescribed or different flash repetition rates at night or during other low visibility conditions.
However, none of the known devices satisfactorily meet the myriad requirements for an effective portable safety warning flasher of high attention-getting visibility at low power consumption and light weight with low cost and high reliability, and thus there has remained a need for a device which can satisfy these requirements.
In copending prior U.S. patent application No. 07/420,068 there was disclosed a battery-powered flashing superluminescent light emitting diode warning flasher light directed to resolving the above stated requirements by taking advantage of the unique characteristics of recently commercially available so-called "Superluminescent" or "Superbright" lightemitting diodes (otherwise known simply as "superluminescent diodes" and abbreviated hereinafter for convenience as "SLDs"). SLDs are known for example from U.S. Pat. No. 4,634,928 and have semiconductor device constructions which provide relatively broad spectral width light output when operating at relatively high powers. Commercially available SLDs emit light at powers as high as 4-6 milliwatts or more in steady state, and are relatively inexpensive, not being very much more costly than ordinary low-luminosity LEDS. For example, the "Radio Shack" catalog lists as available a 5,000 mcd SLD (No. 276-086), and a 21000 mcd SLD rated at 20 milliamp at 1.85VDC (No. 276-087) as well as a blinking high intensity GaAlAs device with 1,200 mcd output and an 8 .degree. viewing angle and rated at approximately 40 milliamps at 1.7VDC (No. 276-020).
For purposes of the present description, a superluminescent light emitting diode (SLD) may be defined as one which has a light output (luminosity) of 1,000 millicandels or more per 20 milliAmperes supply current input when driven by a power signal current at 3 to 12 Hz with a variable duty ratio of 10 to 40%.
Furthermore, an SLD consumes only 20 milliamperes to generate its rated nominal light output (above 1,000 milliCandels). Since light emitted from an SLD is generated by the quantum release of light energy by excited electrons at the P-N junction of the diode (rather than by thermal radiation), its efficiency while less than that of other electroluminescent devices is still phenomenal especially when compared with conventional light sources since the light output spectrum of an SLD can be selected to fall completely within the visible range and the spectral bandwidth of the light output can be varied to increase with increasing optical output power. Higher operating power can be achieved in a pulse mode, and pulsed radiation output power of 50 mW has been obtained from SLDS. As a specific example, for one GaAlAs/GaAs red SLD device commercially available from Sharp Corporation of Japan under the model designation no. GL5R43, the manufacturer specifies an absolute maximum peak forward current rating of 300mA when the SLD is driven in a pulse mode with a pulse width of 1msec or less and a pulse duty cycle of 1/16 (approximately 6%), however under these rated conditions the actual pulse/flash frequency is approximately 60Hz which is well beyond the resolving response frequency limits of the human eye, so that such pulsing of the SLD is not discernible as discrete flashes.
The battery-powered flashing SLD safety warning light disclosed in copending prior U.S. application No. 07/420,068 uses a unique double oscillation circuit to effect the flashing of one or more SLDS. This type of drive arrangement takes advantage of the short rise and fall time of an SLD, typically in a fraction of a microsecond. The copending prior application also discloses a novel method of pulsing an SLD between the full on and full off states in the attention getting frequencies of 3 to 12 Hz. In accordance with the disclosed method of the copending application, the duty cycle of an SLD can be easily varied, preferably between 10 to 40%, to maintain a high on-off contrast ratio which further enhances the perceptibility of the light. In general, the double oscillator circuit allows a low frequency pulse drive signal (3 Hz to 12 Hz) to "gate" a high frequency (1 kHz to 5 kHz) drive pulse train (FIG. 2). By doing so, the emitted light appears to the human eye as 3 to 12 Hz flashes, while the SLD is actually being pulsed at the most efficient high frequency range (above 1 kHz) during the on-time of the low frequency period. Since the human eye cannot resolve pulsing light above 30 Hz, the high frequency pulsing above 1 kHz occurring during the "on time" period of the low frequency duration will appear only as a continuous illumination within each low frequency pulse period.
Battery-powered flashing superluminescent light emitting diode warning flasher lights have wide usefulness in applications in the field and on unpowered, horse-drawn and human-powered vehicles and where no other power source is available, for example as bicycle-mounted safety lights, bicyclist and jogger safety lights, road hazard flasher lights, breakdown warning lights, and as slow-moving vehicle warning lights. Other applications include electrically lighted ornaments, apparel ornaments, "electronic safety flares", helmet-mounted safety lights, emergency flasher lights, roadside construction site warning flasher lights, shoe lights, roller skate lights, hunter visibility safety lights, traffic control and warning lights, driveway lights, runway and helipad marker and landing "strobe" lights and traffic wands. The use of a small battery power source which is rechargeable by a solar energy converter (i. e. solar cell) make such applications even more advantageous by making battery replacement unnecessary or at least a less frequent necessity, an important consideration in remote locations.
The present invention can be used with multiple SLDS of a single emission color or with multiple SLDs of different emission colors. When groups of SLDs of different emission colors are used, these groups of differently colored SLDs can be turned on and off simultaneously to produce a color different from that of any of the SLDs' singly. Furthermore, groups of multiple color SLDs can be operated in such a way that each color group of SLDs are turned on and off simultaneously and sequentially in a manner that produces leading and trailing effects in combination with the chosen colors. In this way, even greater spectral width of the light output can be obtained, compensating for the spectral shift in the eye's response to light as may occur at twilight and dawn, and providing an enhanced visually perceptible effect for attracting the attention of observers.
Furthermore, because of the combination of high luminosity afforded by the utilization of SLDS, the novel drive technique implemented herein therefor and the tailoring of the flash rate and duration made possible by the present invention, the flashing light output signal from the present invention may be easily optimized for attracting attention in a manner offering a significant optical or visual effect enhancement over conventional warning lights, in accordance with psychosensory precepts, while providing a compact, simple low cost flashing safety warning light device having substantially improved reliability and long operating life with inexpensive low voltage battery power.
The present invention is amenable to various implementations for flashing safety warning lights, both portable and otherwise. As noted above, one possible implementation is as an "electronic flare", e. g. for disabled vehicles, while another possible implementation is as a substitute for the incandescent "blinker" lights as are commonly mounted on saw-horse stands for marking hazards and construction areas along roadways, with the benefit of extended operating life and reliability. The applicability of the present invention is not limited to warning indicators, however, as the brilliant flashing light produced by the present invention will also be useful anywhere a portable flashing light for getting attention might be desired, such as on advertising displays, wearing apparel, toys, campaign buttons, etc. And as noted above, portable safety flashers are also usable by pedestrians and joggers for alerting motorists to their presence on and alongside roadways. In general therefore, the applicability of a low cost battery-powered portable flashing SLD light is very wide, and the exemplary uses noted should be understood to be merely illustrative and not limitative, since the present invention is of wide applicability.
As noted above, the SLD drive circuit arrangement disclosed in copending prior application No. 07/420,068, in order to efficiently and effectively flash a SLD, utilizes dual oscillators for driving the flasher SLD, a high frequency oscillator (HFO) and a low frequency oscillator (LFO) which can be used to "gate,, the output of the HFO, and a power or output driver stage for supplying drive current to the SLD(s). While such a drive arrangement is both effective and efficient, the circuitry, requiring two oscillators and a power stage, remains relatively complex to implement. This complexity arises in part from the fact that while suitable integrated circuits are available for implementing the circuit functions, the cost and power consumption of suitable ICs for use in consumer devices limits their usefulness in such a circuit arrangement where battery life and manufacturing cost are important considerations, and thus makes the use of discrete component circuitry more practical.
Furthermore, the use of a dual oscillator-power driver circuit arrangement as in copending application No. 07/420,068 is, while most effective for flashing a SLD, relatively costly to manufacture and necessarily will consume more battery power in comparison to the use of a single oscillator circuit in the oscillator stage.
Thus, it remains desirable to provide a battery-powered flashing SLD safety warning light which is simple and economical to manufacture and which is able to deliver effective illumination levels with high on-off contrast for high visibility and attention-getting performance while still providing long battery life.