A comprehensive discussion on LED street lamp design

With the continuous consumption of earth's energy and the scarcity of resources, the harm of greenhouse effect to human beings, and the serious pollution of the atmosphere to the earth, the international call for energy saving and consumption reduction is getting louder and louder. Today, energy saving ranks first, and road lighting accounts for about 30% of the total lighting electricity consumption, which is closely related to people's production and life. Urban and rural roads will be opened more and more, and they will be opened wider and wider, requiring a large number of various energy-saving lamps to match. With the acceleration of my country's urbanization process, green, efficient, energy-saving, and long-life LED street lights are gradually coming into people's vision. At present, LED lighting technology is becoming more and more mature, and the efficacy of high-power LED light sources has reached 80 lm/W, making it possible to transform urban street lighting into energy-saving.

LED is different from traditional lighting. It has the characteristics of point light source, high brightness , narrow beam output, etc. Therefore, higher requirements will be put forward for designers of new LED lamps. When making LED street lamps, the first thing to consider is to make full use of the limited luminous flux to the effective irradiation range. The street lamp requires the road lighting effect, and the open space beyond the road is not the way of street lamp lighting. Therefore, the distribution range of light should be effectively controlled so that the light emitted by the light-emitting tube becomes a long strip of light spreading along the road direction, and the generation of glare should also be taken into account. At the azimuth angle of the driver's viewing of the lamp, the light intensity of the lamp at 80° and 90° altitude angles shall not exceed 30cd/1000lm and 10cd/1000lm respectively, and regardless of the size of the light flux of the light source, the maximum light intensity at 90° angle shall not exceed 1000cd. Therefore, if the light output angle is not strictly controlled, strong glare will be generated, and some high-brightness LED products will even cause certain light radiation hazards to the human body.

Street lighting is a systematic project. To make good semiconductor street lights, we must first clearly understand the standards and requirements of road lighting and the light distribution principles of conventional lighting fixtures, the differences between the reasonable light distribution of spherical surface light reflectors and point light sources, the complementary advantages and disadvantages, and give full play to the controllable advantages of the beam angle of LED point light sources and the long life of cold light sources .

Semiconductor LED light sources have their own characteristics in structure and optical properties. Therefore, semiconductor street lamps should be designed according to these characteristics. Some manufacturers today use traditional lamp shells to replace semiconductor wicks. Using transition conduction to dissipate heat is not a good way to make a good lamp. The problem is that the external shape of the lamp body must be combined with the heat dissipation of the light-emitting tube, and heat dissipation depends on the area. The surface area of ​​existing conventional lighting fixtures is far from enough. The "Joule heat law" requires a good natural air convection environment. The operating temperature of LED products should basically be controlled below 65°C (the international standard is 80°C. When the LED operating temperature reaches 85°C, the luminous flux will drop by half; the wavelength will become longer, that is, red shift; if it exceeds 90°C, there is a risk of burning). The installation of the light-emitting tube, scientific and reasonable optical light distribution, and the sealing of the lamp body are very important. At the same time, the designed lamp body structure must also be conducive to large-scale industrial production.

The heat dissipation of the light-emitting tube is a key issue to be solved for semiconductor lamps. The light-emitting tube is a cold light source, which does not generate scorching high temperatures like gas discharge lamps. However, the limitations of the light efficiency of today's LED light-emitting tubes also exist in the heat energy and its own temperature resistance is limited. Therefore, the heat generated by the light-emitting tube when working must be effectively dissipated into the air to ensure that the light-emitting tube works at a safe temperature. Only in this way can the semiconductor lamp truly reflect the advantage of long life.

The selection of LED light-emitting tubes for LED street lamps is very important. Some manufacturers today use integrated packages of more than 10W single tubes to make street lamps, which have defects such as difficult optical light distribution, easy glare, and overall heat dissipation difficulties. There are also 3W~5W modules, whose light-emitting tubes have low luminous efficiency per watt and overall heat dissipation problems. Compared with single 1W tubes, the price advantage is not obvious under the same luminous flux. Considering all factors, it is more reasonable to use high-power 1W light-emitting tubes. From the current technical level of high-power light-emitting tubes, 1W has a relatively high luminous efficiency and obvious advantages in energy saving for lighting. It is more reasonable to use 1W as a street light source.

The selection of LED light color is also a factor worthy of attention. In the development of road lighting, there is a comparison between the use of yellow light and white light. Yellow light is the mainstream high-pressure sodium lamp now. Because it is economical and energy-saving, and has excellent fog-penetrating performance, it has become the mainstream choice of street lights in my country. However, yellow light also has its inherent shortcomings: that is, poor color reproduction ability. People have the feeling that under sodium light, the color of any illuminated object is yellowish and distorted.

The goal of white light is natural sunlight. Because it has the best color expression ability, and sunlight is a warm white light. Therefore, if we can achieve white light similar to daylight while taking into account the economic and energy-saving advantages of yellow light, it will be an ideal choice for night lighting. The early stage of white light was metal halide lamps. Although it has a color expression ability several times higher than yellow light, it has not been widely used due to some difficult-to-overcome technical problems such as poor fog penetration and low effective life. The production of LED light sources can make up for this defect. In terms of the color temperature of LED light sources , it is more appropriate to choose around 3500K, which meets the lighting requirements in terms of bright vision and comfort. Moreover, it is feasible in the current development of LED light efficiency and cost-effectiveness, and is suitable for the large-scale promotion of LED street lights.

There are different opinions on using white light above 6000K as street light source, especially for high power above 160W, which needs to be used cautiously on expressways. According to national road lighting standards, expressways and trunk roads must use cut-off or semi-cut-off lamps.

Cut-off type lamps: The angle between the maximum light intensity direction of the lamp and the downward vertical axis of the lamp is between 0° and 65°, and the maximum allowable light intensity at 90° and 80° is 10cd/1000lm and 30cd/1000lm respectively. Regardless of the size of the light flux of the light source, the maximum light intensity at 90° shall not exceed 1000cd.

Half-cut light type lamp: The angle between the maximum light intensity direction of the lamp and the downward vertical axis of the lamp is between 0° and 75°, and the maximum allowable light intensity at 90° and 80° is 50cd/1000lm and 100cd/1000lm respectively. Regardless of the size of the light flux of the light source, the maximum light intensity at 90° shall not exceed 1000cd.

Because the expressway is long, wide, has a large traffic volume, and a high speed, it requires high visual sensitivity. Therefore, it has high requirements for illumination , color temperature, brightness uniformity on the road, glare and other indicators. In order to compare with 400W high-pressure sodium lamps, it blindly requires high power and high brightness. In addition, today's LEDs have difficulties in both light efficiency and heat dissipation costs, and it is also difficult to promote them in terms of cost performance.