Low Power LED Street Light vs. High Power High-pressure Sodium Light

On April 24th, 2008, Mao Yu Hai who is a professor of Tsinghua University, senior member of the Chinese Institute of Electronics and the United States IEEE had mentioned illumination modified coefficient in his blog, “The technology of LED street lamp is really immature?” In the article, he has stated that illumination modified coefficient for LED is 2.35 times, while for high pressure sodium lamp is 0.94 times due to the difference between CRI.

The theoretical basis for this view is the “color modified coefficient K”. This parameter is widely used in the calculation of optical radiation measurement, such as the total luminous flux LED measurement formula, Φt = It / IsΦs? K. For the inference of K value, two researchers from China Institute of Tested Technology have deep discussions in the article “Radiation Intensity Measurement and Color modified Coefficient” published in the 6th issue of “Practical Test Technology” in 2002. In theory, the formula of K is given. Under certain circumstances, K may be the largest source of hidden uncertainty. Also for some typical cases of actual measurement, the color modified coefficient value K was calculated, revealing that the color modified problem may cause a huge systematic deviation in the radiation illuminometer indication. In simple words, the color temperature of high-pressure sodium lamp is less than 3000K, while for the white LED lamp is 3000K – 7000K. Because of this difference, the “visual brightness” of LED lamp is higher than the high-pressure sodium lamp. In the above chart, the value for the illuminance that eye can feel of LED is higher. But the actual measurement of the road will have an error showing the high pressure sodium lamp is higher. This is due to the subtle connection between luminosity and chromaticity, although the academic system still needs improvement. There are also information on the Internet to explain this issue base on the human eye cells and other physiological point of view.

Luminaire efficiency: the ratio of the actual luminous flux emitted by the luminaire and the luminous flux of the luminaire. In the above table, the value for the high pressure sodium lamp is 65% while for the LED lamp is 90%. That is because high-pressure sodium lamp is a point light source which has 360-degree omni-directional light. It is reported that only 40% of them will pass through the transparent parts to the road surface, and the rest will be reflected on the reflective parts within the lamps and then emitted. The light will have a great loss during reflective process, so the high pressure sodium lamp has low efficiency. The luminaire efficiency parameter is directly related to the secondary optical design of the luminaire and the reflective material applied. In contrast, LED is a surface light source; the direction of light is very strong. And there is no reflector required. Through the light distribution design, most of the light will directly reach the ground, and the lighting efficiency can achieve 90%.

With the development of science and technology, the technology of LED lamps has become more and more mature. The luminous efficacy is far higher than the original 90 Lm/W. Suwin company uses XP-G3 Cree chip, and the entire luminous efficacy can reach as high as 145 Lm/W, we can use lower power LED lamp instead of high-pressure sodium lamp.