On the design and key elements of LED streetlights

In recent years, cross-strait development of various types of LED streetlights have been booming, particularly in 2008, when Taiwan and China both initiated several LED streetlight demonstration projects, spurring the growth momentum of LED industry.


 
From LED lamps with 45Lm/W luminous efficiency in 2008, to the 80Lm/W luminous efficiency announced by some manufacturers by the end of 2009, the progress has been phenomenal. Regardless of whether the quality of the 80lm/W LED will pass the CNS15233 criteria announced at the end of 2008, the expansion of each country’s domestic demand in LED lamps, supported by the government, coupled by substantial investments from different industry players, will effectively increase the R&D resources, thereby improving the luminous efficacy of LEDs. In the near future, with its high luminous efficacy, LED streetlight will completely replace existing high-pressure sodium lamps.
 
Thermal dissipation of LED streetlights

All existing designs of thermal dissipation can be broadly categorized into 3 types, namely aluminum alloy heat sink, pin fin heat sink, and heat-pipe. With most of the aluminum alloy type, the heat sink is exposed outside; the advantage of this design is its low cost, especially in module development. However, in the R&D of high-wattage products, weight is an important factor, because according to public regulations, there are certain limitations to the light poles regarding the pressure it is capable of withstanding, thus most of the high wattage products adopt either fin-type heat sink or heat-pipes for heat dissipation.
 
Technical barriers of LED streetlights

In addition to the R&D of heat sink, the other important element in an LED streetlight structure is its power supply. Currently, the most popular LED products are DC-driven, thus in the power supply unit, manufacturers have to customize power supply for each type of LED streetlight, and make adjustments in the circuit to comply with each country’s product safety standards. Based on our experience in the LED streetlight projects, the majority of LED streetlight malfunctions are caused by unstable power supply. Hence, the stability of power supply directly affects the extent of LED streetlight’s luminous decay during the first three years.


 
Optical design – the Achilles’ heel of LED streetlight

The other technical barrier in the development of LED streetlight is the optical design capability. Good optical property in the lighting fixture allows LED streetlights to be uniformly illuminated on the road surface. Taiwan’s NS15233 criteria specifies that, the glare for streetlights should be <30 at a vertical 80-degree angle, and <10 at a 90-degree angle. In addition, at a 65 ~ 95 degree angle, there are specific brightness specifications. As for all other lighting fixtures, it depends on the optical design capability of individual developers, as some projects specify their own luminous requirements on the road surface. A good optical design can effectively improve the lighting quality of LED streetlights that have lower luminous efficacy .
 
So, when both quality and reliability of the products are equal, the comparison of LED streetlights depends on its lighting performance. The majority of current LED streetlights adopt OSRAM’s bidirectional optical design, but each manufacturer still endeavors to enhance the luminous efficacy with its own technological know-how.


 
Establishment of criteria for LED streetlights

Taiwan's national LED streetlight criteria has been officially announced in December 2008, and China's streetlight standards will be announced separately by each province, and product performance tests also vary accordingly. A nation-wide LED streetlight standard will likely be announced in the near term, and the criteria should be in line with both China’s and Taiwan’s CNS standards, to establish a uniform standard for LED streetlight quality across the strait.
As for the U.S. and Europe, LED streetlight standards also specify criteria for the lighting angle, quality, and light decay, etc. With the development of LED streetlight technology, we believe better LED streetlights will be introduced, attracting more LED procurement projects from public sectors, thereby improving the overall market visibility of LED streetlights.

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