The present invention relates to light-emitting diodes (LEDs), particularly optical means for substituting white LEDs for incandescent and fluorescent light bulbs.
Conventional incandescent lamps of less than 100 lumens output can be matched by the latest white LEDs, albeit at a higher price. At this low end of the lumen range, the majority of incandescent applications are battery-powered. It is desirable to have an LED suitable for direct installation in the place of, for example, a burnt-out flashlight bulb.
LED's can offer superior luminous efficacy over the conventional incandescent lamps used in, for example, battery-operated flashlights. Moreover, LEDs are far more tolerant of shock, vibration, and crush-stress. Although they currently cost more to produce than the incandescents, their lifetimes are ten thousand times longer. For the sake of efficacy, flashlight bulbs are run hot so they typically last only a few hours until filament failure. Also, the prices of LEDs continue to fall, along with those of the control-electronics to handle variations in battery voltage.
Indeed, LED flashlights are commercially available already, but their optics have to be adapted to the geometry of light-emitting diodes, which only emit into a hemisphere. Conventional LED lamps are unsuitable for direct installation into conventional flashlights, both electrically and optically. LED lamps are electrically unsuitable because they are current-driven devices, whereas batteries are voltage sources. Typical variations in the voltage of fresh batteries are enough to exceed an LED's tolerable operating-voltage range. This causes such high currents that the Ohmic heating within the die of the LED lamp exceeds the ability of thermal conduction to remove it, causing a runaway temperature-rise that destroys the die. Therefore, a current-control device must accompany the LED lamp.
Conventional LED lamps are optically unsuitable for direct installation into the parabolic reflectors of flashlights. This is because their bullet-lens configuration forms a narrow beam that would completely miss a nearby parabolic reflector typical of flashlights. Using instead a hemispherically emitting non-directional dome, centered on the luminous die, gives the maximum spread commercially available, a Lambertian pattern, with a sin2θ dependence of encircled flux on angle θ from the LED lamp center axis. Since θ for a typical parabolic flashlight reflector extends from 45° to 135°, an LED lamp with a hemispheric pattern is mismatched because it's emission falls to zero at only θ=90°. This results in a beam that is brightest on the outside and completely dark halfway in. Worse yet, even this inferior beam pattern from a hemispheric LED would require that it be held up at the parabola's focal point, several millimeters above the socket wherein a conventional incandescent bulb is installed.
Another type of battery-powered lamp utilizes cylindrical fluorescent lamps. Although LEDs do not improve on their luminous efficacy, fluorescent lamps are relatively fragile and require high voltages.
There is thus a need in the art for an effective, low voltage and optically suitable LED lamp with which current incandescent bulb flashlights can be retrofitted by direct installation of the LED lamp into the parabolic reflectors of current flashlights.