Omni-directional lamps widen the illumination pattern of LEDs
There have been many improvements in the capabilities of LEDs in recent years; however, their limited illumination pattern still poses a problem for many lighting applications. Early LEDs were limited to about a 30-degree viewing angle. Today, it is common for LEDs to provide a 120-degree illumination pattern but the intensity is still reduced to 50% at the extremes of the pattern.
The common incandescent lamp provides an illumination pattern in all directions (omni-directional), and it is this standard that must be met if LEDs are to replace it in many common applications. There are various ways to extend the illumination pattern of LEDs. One of the common methods is diffusion, using for example a "frosted" or diffraction-style lens/canopy. These methods are partially successful but may attenuate the total light output and do not provide an even intensity light pattern in all directions.
Another technique is to mount many LEDs pointing in all directions (similar to a pine cone). This approach is costly and difficult to manufacture, and also provides bright spots where the LEDs are located.
Developing an omni-directional light
Early in 1994, the first replacement LED light bulb for use in construction barricade lights was a bi-directional lamp. It was shaped like a "T" with two LEDs, each pointing in opposite directions, and utilizing a miniature bayonet base. It worked well and provided four times the battery life compared with an incandescent lamp; however, there was one problem. The "T-Bulb"® was intended to be installed pointing at each of two opposing lenses but the lenses could be rotated relative to the LEDs, which greatly reduced the visibility of the light. An omni-directional lamp would solve the problem.
The spring baseball season of 1995 ended in tragedy for two players, when their boat struck an unlit dock after dark. There was a need for a "Dock Warning Light" for nighttime boaters. The Coast Guard was consulted and indicated such a light would be useful and acceptable if it was visible in all directions. A light was constructed utilizing one LED pointing upward at a convex reflector that re-distributed much of the light laterally in all directions as required. This provided the "stepping-stone" for solving the limitation of the "T-Bulb"®.
The simple Omni-Directional technique is based on the reflective law of incidence and reflection. It can incorporate a spherical reflector or any portion of a sphere depending on the application. The size of the reflector, distance from the LED(s) and their viewing angle all determine the resulting illumination pattern. Some applications may require all light to be reflected, while others require only some light to be reflected with the balance allowed to pass-by (or around) the reflector. These variables allow the designer to tailor the light pattern to the application.
Application examples
Consider the light pattern desired for a common table lamp, pathway light or lamppost. Most light is desired downward (toward the table or the ground), and lateral lighting is also important, while upward lighting is least important. The LED or LEDs are positioned to point downward toward a convex reflector. The lamp fixtures typically have a vertical member that supports the lamp assembly and possibly a lampshade. The light that would otherwise be wasted by striking the vertical support instead strikes the convex reflector that is mounted directly above it. The reflected light is directed laterally and upward.
Figure 2 (A) shows a cordless table light, incorporating a single Luxeon LED emitter capable of providing up to 30 lumens of illumination. The prototype operates on 3 "D" alkaline cells and incorporates a dimmer control providing a brightness range of 20% to 100 % (50 to 350 mA). The LED is located near the top of the fixture and directed downward. The heat sink necessary to protect the LED extends out the top into open-air for optimum cooling. The body of the light fixture would obstruct a considerable amount of the conical pattern of light; however, a convex reflector mounted in-line with the LED re-directs that light in all directions providing an even illumination pattern. The body of the lamp can be fabricated from various materials into the desired shape, determined by the application.
Figure 2 (B) shows a second cordless table light utilizing a total of 5 LEDs, 3 white and 2 amber, to provide a "warm" 8-10 lumens of illumination. The prototype is powered by 4 "D" cells (with on/off switch) and incorporates a voltage doubler circuit. The batteries can be expected to last about 225 hours or 45 nights at 5 hours/night. There is a convex reflector in-line with each of the LEDs providing an even mixture and distribution of light. The body of the lamp can be fabricated from various materials into the desirable shape, determined by the application.
A standard medium based light bulb can also be configured with the LED(s) mounted just above the screw base pointing upward toward a convex reflector (figure 3, left). There would be a shadow effect on the back (flat) side of the reflector which can be reduced with diffraction techniques or totally eliminated by adding LEDs to the top (back side) of the reflector (figure 3, right).
Wide illumination pattern
The Omni-Directional technique can be incorporated in any application requiring a wider illumination pattern than provided by the LED itself. Those applications include various warning and emergency lights, marker lights and general illumination applications. For example, aviation warning lights have been constructed for the "big-red" balls mounted on power lines for nighttime visibility.
The extended battery life that is achieved when substituting LEDs for incandescent illuminants is well known. Portable battery powered lamps of all kinds can be produced providing many weeks of operation without solar recharging. These lights can be used for back-up lighting at home or as table lights in restaurants, bars, patios, boats etc.
LEDs used in AC powered lighting, both general and decorative, will save many watts of electricity when compared to incandescent lighting. LEDs are safe and reliable, now brighter, less expensive and can provide the wider illumination pattern required in many applications.
