Emerging Solid State

The electric lighting era started during the latter part of the 19th century with two types of light source technologies, incandescent and electric discharge. Both technologies have coexisted over a century and have provided many different light sources for a variety of lighting applications. In general, the light sources based on incandescent technology have lower source efficacy and a shorter operating life compared to light sources based on electric discharge technology. However, incandescent light sources have lower initial costs.

Light emitting diodes (LEDs) are the first signs of an emerging third type of light source technology, known as solid-state lighting. The potential for significant energy savings and long life has attracted the community interested in general lighting, and during the past few years many articles have appeared around the globe predicting a bright future for the technology.

LEDs are semiconductors and were first developed three decades ago. Applying an appropriate voltage across the semi-conducting element enables the radiation process. The emitted radiation is in a very narrow wavelength band.

The chemical composition of the semi-conducting element determines the wavelength band of the radiation. Most of the early LED devices produced light in the red to yellow region. After the development of the high-brightness blue LED, and subsequently the white LED in the mid-1990s, the technology became a potential candidate for the general lighting industry. Presently, LEDs produce light in a variety of colours covering the visible spectrum. White light emission from LEDs is generated by combining the blue LED with a suitable phosphor or by grouping multiple coloured LEDs, such as the red, green and blue, in the correct proportions.

Proponents of solid-state lighting technology naturally tend to highlight its positive aspects while pointing out the negative aspects of traditional technologies. Due to its low efficacy and short lamp life the incandescent lamp is always the first target for displacement by any new light source. In the past we have seen other technologies including compact fluorescent lamps attempting to displace incandescent lamps without much success, especially in residential applications. Some of the marketing brochures and technical publications that have appeared during the past couple of years have created an impression that LED light sources are also more energy efficient and have a much longer lamp life, in the order of 100,000 hours, compared to incandescent lamps.

Does this mean we are ready to screw out our incandescent lamps and screw in the LED light sources? Not so fast.

Not so fast

It is true that in certain applications such as exit signs and traffic signals LEDs have been shown to save over 80 per cent energy compared to their incandescent counterparts. Such large energy savings are realized because with the incandescent lamp the coloured signs and signals are created using optical filters that absorb, and thereby waste, significant portions of the radiant energy generated by the light source. However, when it comes to general lighting with white light it is a different story.

Consider a 100-watt incandescent lamp and a phosphor based 5-mm white LED. The table at left summarizes some of the expected performance characteristics for the two light sources.

According to the table, it appears that 1,700 white LEDs are needed to create the same amount of flux generated by the 100-watt incandescent lamp. This is not a cost effective solution for most general lighting applications.

The 100,000-hour lamp life for LEDs has been used as a justification for significant cost savings since these sources will be replaced far less frequently than traditional light sources. It is worth pointing out that unlike the lamp life definition for traditional light sources, which is the time at which 50 per cent of the lamps have burned out here, the life for LEDs is based on the projected time for the light level to reach 50 per cent of its original value. The white LEDs test at the Lighting Research Center exhibited life values less than 10,000 hours, at 20 mA drive current, which is the recommended operating current for these devices.

The incandescent lamp and the white LED have some similar characteristics. For example, changing the applied voltage to an incandescent lamp changes the light output, efficacy and life. Likewise, changing the drive current to the LED changes the light output, efficacy and life. These quantities are interrelated, and it is impossible to change one without affecting the other. The table above shows, for example, how the lamp life and luminous efficacy of a 100-watt incandescent lamp varies when the lamp operating voltage is reduced. If the incandescent lamp is operated at 70 per cent of its rated voltage the lamp life increases to 100,000 hours but reduces the luminous efficacy to 9 lumens per watt. Is this bad?

The major LED manufacturers understand these issues well and are developing high-flux white LED packages to meet the requirements of general illumination. The new high-flux LEDs that would enter the marketplace during the next few years would have luminous efficacies in the range of 20 to 30 lumens per watt, and light output in the range of 50 to 100 lumens per device. Furthermore, better thermal management within these new devices would improve their reliability compared to the presently available 5-mm devices. These LEDs, however, will arrive in the marketplace at premium prices. It seems that the Edison lamp and other discharge light sources are safe for at least another decade.

Dr. Nadarajah Narendran, Ph.D. is director of research/research associate professor at the Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY.