To assure practical economy and driving safety at night, a good road lighting system must provide sufficient and even illumination, a comfortable light color, absence of glare, resistance to water and dust, reliability, long life, low cost, energy efficiency, and easy maintenance.

In addition, the outside design of road lights should reflect the culture of the place where they are installed.

The International Commission on Illumination (CIE) divides roadways into four categories and offers recommendations for the illumination of each category. But regulations and standards vary from place to place because of cultural and environmental differences; in Taiwan, stringent rules on road lighting specifications, installation, inspection, and testing are dictated and enforced by the Economics Ministry`s Bureau of Standards, Metrology and Inspection, the Interior Ministry`s Construction and Planning Agency, and the Ministry of Transportation and Communications.

Taiwan`s Chinese National Standard (CNS) also has provisions for regulating light installation as well as light pole height and interval on motorways, the measurement of illumination and the types of light fixtures, inspection methodology for lamps, inspection standards for high-pressure mercury lamps, and inspection methodology for automatic switch sensors.

Light-emitting diodes (LEDs) seem to be the wave of the future for road lights. LED technology has advanced rapidly over the past few years, and LEDs are now able to produce over 100 lumens per watt. Standards for measuring the impact of ambient temperature on the efficiency and life span of LEDs, however, are still lacking.

Too Much Heat, Uneven Lighting

For high-power LED road lights, service reliability remains a question since the lamps still produce a lot of heat. In addition, the light-distribution of LED lighting fixtures still needs to be improved so that the lights can meet the need for sufficient illumination, even lighting, and lack of glare. Prices need to be brought down, too, as the cost of LED lamps is many times higher than that of conventional fixtures.

Because of these problems, international lighting companies hold a conservative attitude about LED road lamps. Impressed by the potential of LED lamps to save electricity and protect the environment, however, the advanced industrial countries have introduced numerous demonstration projects to highlight the application of LED road lights.

In the world market for LED outdoor lights, streetlights and landscape lights remain the mainstream products. The Chinese are now the major suppliers of roadway LED lights that produce more than 4,000 lumens; and in Taiwan, some manufacturers have introduced 220-watt LED roadway lights that give off 50 lumens per watt and produce a total illumination of 8,800 lumens.

The market for roadway lights is still dominated by high-pressure mercury lamps and high-intensity-discharge (HID) lamps. Mercury lamps are losing ground, however, because of their low efficiency and environmental unfriendliness.

HID lamps, on the other hand, are gaining popularity because of their high efficiency (they produce over 100 lumens per watt) and high light penetration through translucent objects. Lighting manufacturers are now planning to make metal halide lamps (a variety of HID lamps) that give off comfortable white light.

Existing roadway lamps distribute light in a butterfly pattern, with the most intensive illumination in the center and the wings, providing sufficient and even illumination. LED lamps do not have such a good distribution pattern, however, because of limitations of diffusion angle and light intensity. Some LED lamps now have improved angles of diffusion, but center lighting intensity suffers as a result.

Comparison of a 250-watt high-pressure sodium lamp with today`s brightest LED roadway light shows that the sodium lamp is 1.9 times more efficient, and its central illumination is three times higher.



Paying for Efficiency

The best-performing roadway LED lamps claim to be 60% more efficient than high-pressure mercury lamps, meaning that they save a lot of electricity. It would take at least four years, however, for the saving on energy to make up for the added cost of installing an LED lamp. When LED lamps are compared with high-pressure sodium lamps, the period of cost recovery is even longer.

The life-span of an LED lamp depends on its thermal-dissipation design, and dust and bird droppings are big killers of the thermal-dissipation mechanisms of roadway lamps. With life-span verification standards for LED lamps yet to be worked out, users tend to be wary of buying such products.

In Taiwan, LED lighting is a key part of energy-conservation efforts and the encouragement of energy-efficient lighting is national policy. This year, the Bureau of Energy of the Ministry of Economic Affairs will implement a roadway lighting demonstration project that will see LED lighting installed on three or four roads.

In line with the implementation of this project, the government has asked the Energy and Environment Research Laboratories of the government-backed Industrial Technology Research Institute (ITRI) to draft LED roadway lighting standards and set up an inspection lab.

ITRI is helping Taiwan`s LED lamp industry stay ahead of the pack by continuing to develop key LED technologies, including those covering efficient heat-dissipation materials and intelligent control units. (February 2008)

(Note: This article was contributed by Dr. L.L. Lee, deputy director of the Power Electronics Laboratory of ITRI`s Electrotechnology Division, and translated into English by CENS Lighting staff reporter Ken Liu.)
(by Ken Liu)