White LEDs

It's been over 30 years since the introduction of the first LED (Light Emitting Diode) and at long last we now have a white LED that begins to rival incandescent in many architectural and small area illumination applications.

LEDs have enjoyed a tremendous growth over the last several years with new applications ranging from automotive lighting and VMS (Variable Message Signs) to traffic control devices. Much of this is due to the ever-increasing levels of brightness being achieved with new materials and wafer fabrication processes as well as the advances in package and optics design. Several of the most significant areas of expansion however, have resulted from the introduction of the blue LED in the early 1990's.

White Light

This allowed for the manufacture of RGB (Full Color) signage as well as pre-empted the development of white LEDs in the late 1990's. White light is currently achieved by using two different methods. The first is by combining a blue 450nm - 470nm GaN (gallium nitride) LED with YAG (Yttrium Aluminum Garnet) phosphor. The blue wavelength excites the phosphor causing it to glow white. The second method is to combine red, green and blue LEDs in the proper proportion to achieve a white color. The former is presently the most dominant and efficient technique used. Recently, new white LEDs have become available by combining a UV (ultra violet) LED, 380nm, with phosphor. Although this method is relatively new, companies such as TOSHIBA are using this technique to obtain white. In addition, by combining different phosphor types with a UV LED, other colors such as purple, orange, pink etc. can be achieved.

The development of a solid state white light has generated a lot of enthusiasm in the industry. This is primarily due to the significant energy savings possible with LED technology as well as the increased lifetimes over standard incandescent lighting. Although white LEDs are currently not yet the most efficient method for illuminating a large area such as an office or room, the white LEDs has countless applications in architectural lighting, small area lighting, aerospace and automotive interior lighting, exit signs and emergency lighting, flashlights etc. The goal of this article is to present some basic information that will help the user effectively consider the use of white LEDs in their design.

Energy Saver

One of the most common areas of confusion regarding white LEDs is related to efficiency. The standard unit for characterizing the light output of incandescent and fluorescent lamps is the "lumen" which is essentially the total luminous power (luminous flux) emitted from the source. The luminous efficacy is the measure of the light-producing efficiency of the lamp. It is the ratio of thelumens to the total input power of the source. Luminous efficacy is expressed in "lumens/watt." Many tests have been performed to evaluate the efficiency of LEDs compared to incandescent or compact fluorescent. Unfortunately, oftentimes erroneous conclusions are made from the data obtained. For example, LEDs are typically focused or directional devices unlike standard incandescent or fluorescent lamps. If data is obtained only by measuring the light falling on a small area such as a photo sensor, it may yield abnormally high efficiency results for the LED. There is still a significant amount of light being produced by the incandescent or fluorescent that is not being measured. If a reflector were placed behind the incandescent bulb, essentially simulating the LED emitting angle, the efficiency values obtained would be more accurate.

Toshiba TLWA1100W White SMT LEDs

This is an important concept to understand since there is incorrect information circulating in the industry stating LEDs are significantly more efficient than most other forms of lighting. This is not always the case especially in large area applications such as general office or room lighting where existing technology such as compact fluorescent would still be a far more cost effective and practical source of illumination.

In the example shown in "table 1", a comparison was made using a standard 7 watt night light along with white LEDs. If we look at the total lumen output of the incandescent lamp, which in this case is approximately 43 lumens, we would need 43 LEDs at 1 lumen each to obtain the equivalent value. Although the LEDs are more efficient than the incandescent bulb, the cost is significantly higher. In actual use, however, all 43 lumens of the 7W bulb are not effectively utilized since only a small area needs to be illuminated, not the entire room. Therefore, 2-3 LEDs may be more than adequate to light the desired area. In this case, not only would switching to LEDs be significantly more efficient, the cost may actually be less. A similar comparison using a 100W bulb would show that hundreds of LEDs would be required to sufficiently illuminate a room, which obviously is not a very cost effective solution.

White LEDs are slightly more efficient than a 100W incandescent bulb and more than three times more efficient than a 7W night light type bulb as shown in table 2. (Smaller incandescent bulbs are far less efficient than larger types and thus are more suited for replacement by LEDs)

As can be seen, white LEDs are a good choice for use in small area lighting such as in desk lamps, task lighting, exit signs and night lights in addition to backlighting for switch panels, dashboards, palm tops etc.

There are also other issues that may warrant the use of LEDs over incandescent. The typical incandescent bulb emits a considerable amount of energy in the infrared portion of the spectrum, which cannot be seen but is felt as heat. In an environment where many incandescent bulbs are used, such as a room or in a lighted switch panel, the ambient temperature can significantly increase. This may require additional cooling or air conditioning to maintain a specified temperature, which raises energy consumption. By eliminating this radiation with LEDs that virtually run cool, these costs can be reduced.

Lamp Life

An additional factor that must be considered is the useful life of the light source being utilized. Lamp life for incandescent and compact fluorescent is usually defined as the time at which 50% of the test samples have "burned out" or failed to properly operate. LEDs, however, do not generally fail catastrophically but decrease in output over time. Therefore, the life of an LED is commonly considered to be the time at which 50% of the initial brightness remains. It is generally assumed that LEDs have an expected lifetime of 100,000 hours or more if operated within the specified parameters, however, this is not always the case. The lifetime of an LED is dependent on many factors including drive current, type of material used (InGaN, InGaAlP, GaAlAs), environment, packaging and assembly. Since the white LED is relatively new and process improvements are being made at an extremely rapid pace, there is very little long-term data available. Currently, lifetimes of 10,000 hours or greater can be obtained from well-made white LEDs. This is likely to improve greatly over the next several years, closer to the 100,000 hour level typically expected from LEDs. Manufacturers are working on several areas to improve reliability including better heat sinking methods, improved phosphors and encapsulation epoxy. The proper assembly for white LEDs is important; therefore, care should be taken when choosing a supplier of white LEDs. The manufacturer should have experience in the assembly of GaN type product (the basis for white) as well as an understanding of phosphors. In addition, they should be able to provide data as to the estimated lifetime and reliability of the LED even if the data is only preliminary.

The future for solid-state white lighting is very promising. New methods for achieving the required luminosity and efficiency are constantly being introduced. These include increasing the size and shape of the semiconductor device, improving phosphor performance and clustering a number of individual LED devices together. Although LEDs need to produce approximately 100 lum-ens/watt to effectively compete in the general illumination market, at the current 20 lumens/watt (50 lumens/watt possible in the next several years), there are many uses for white LEDs that warrant serious consideration as mentioned previously. In these applications, white LEDs are ultimately the most reliable and efficient choice.

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