Since white LEDs have many significant advantages, there are many advantages to applying them to alternative lighting sources in public urban lighting facilities. However, in order to truly give full play to the advantages of semiconductor light sources, the design of secondary optical systems is crucial. This paper introduces a unique secondary optical system for urban road lighting. It can better meet the relevant standards of urban road lighting, and can be flexibly applied to different road conditions.
Public city lighting has a large share in the lighting market. According to statistics, urban public lighting accounts for 30% of China's lighting power consumption, about 43.9 billion kwh, calculated at an average electricity price of 0.65 yuan/kwh, and annual expenditure is 28.5 billion yuan. . At present, high-pressure sodium lamps are widely used in urban public lighting. Especially on the main road, high-pressure sodium lamps can provide luminous efficiency of more than 100 lm/W. But its own shortcomings are also obvious:
The spectral composition of the light source is yellowish and the color rendering index is extremely low;
The life of the luminaire is short and the replacement work is large;
Inconvenient to adjust the power of the lamp;
With the rapid development of solid state physics in the 1990s and the breakthrough discovery of new semiconductor materials, LED technology has made rapid progress in the past 10 years. The appearance of white LED is driven by its unique low voltage, small size, light weight, good color rendering, good dimming performance, long life (up to 20,000 hours), vibration resistance, non-damage, and environmental protection requirements. So that the semiconductor light source will become an ideal energy-saving light source for urban road lighting. Its remarkable advantages are: high luminous efficiency.
At present, the commercial white LED light efficiency has reached 90-100lm/W, and it is expected to reach 150lm/W or more in two years. This is not the upper limit of LED light efficiency. Experts from all countries have set the goal of light efficiency at 200lm/W. about.
1, long life
The ideal goal is 100,000 hours, and the current commercial white LED life can reach 50,000 hours, 10 to 20 times longer than the traditional light source. When the city road lighting source is made, the light source can be replaced for 10 years, which greatly saves daily maintenance costs. It is convenient to adjust the power of the lamp intelligently; the secondary optical system can be added to make maximum use of the light energy of the LED to meet the specific illumination and light intensity distribution of various applications. In recent years, conventional fossil fuels have become increasingly tense. The prices of conventional energy such as oil and coal have been rising, and the international oil prices have reached historical highs, causing the situation of energy supply to become more severe. Due to these significant advantages of LED and the maturity of white LED technology, it is of great significance to develop an efficient, long-life, green and environmentally friendly semiconductor light source urban road lighting system.
2. Design of secondary optical system
A road lighting system is different from a general lighting fixture in that the illuminated field is a rectangular area. Table 1-1 and Table 1-2 are national street lamp standards. According to the national highway lighting standard, the average illuminance of the main road is 15lx, the uniformity is 30%; the average illuminance of the secondary road is 8lx, and the uniformity is 30%, wherein the uniformity is the minimum illuminance/average illuminance of the illuminated road surface.
Since the high-pressure sodium lamp is a 360-degree full-angle illumination, a reflective device is usually added to the back of the light source to illuminate the emitted light to improve energy utilization. However, due to the large size of the high-pressure sodium lamp source and the lack of effective optical devices, the spot formed by the high-pressure sodium lamp on the road surface is usually a non-uniform spot: the center is very bright and the radial attenuation is fast. As shown in Figure 1.
In order to meet the uniformity requirements of national lighting standards, it is usually necessary to increase the output power so that the illumination of the surrounding dark spots is not less than 5 lx, and the secondary trunk is not less than 2.4 lx. However, a large amount of energy is wasted at the center and the road directly below the street lamp. The outside, which reduces the overall energy utilization.
The existing LED illumination source is a point source with a light intensity distribution approximating the Lambertian source. The individual powers are all about 1 watt, and the road illumination often requires a specific shape of the illuminated field. For example, the illumination area of ​​the main road light is specified by the state. A rectangular area of ​​10x50 square meters. Therefore, to form a road lighting system, it is not possible to simply combine the LEDs together. The circular light field thus formed will be detrimental to the utilization of light energy, and cannot meet the requirements of national standards for the uniformity of illumination of the entire illuminated area. If the light emitted by the LED chip can be reasonably distributed on the road surface and a rectangular area with uniform illumination is formed on the road surface, the energy utilization rate higher than that of the high pressure sodium lamp can be achieved with low luminous efficiency.
Below we estimate the light efficiency of different street lamps from the perspective of system efficiency. As can be seen from Table 2, the system efficiency of LED light sources is much higher than that of traditional sodium lamps.
There are also many manufacturers of various types of road lighting in China. At present, in the field of high-power LED road lighting, some factories have begun to develop, but if it is simple to carry out multi-chip or multi-device combination, it is difficult to reach the country in terms of actual illumination area and illuminance unevenness of effective area. Relevant standards. It is impossible to apply the LED street lamp to the branch road lighting and the secondary road lighting to expand into the main road lighting field. If the power is increased to meet the standard, the high luminous efficiency of the LED will no longer exist.
To design an efficient optical system for semiconductor road lighting, the key problem is to design an effective and reliable optical system so that the light emitted by the light source is evenly distributed on the road surface, that is, to solve the problem of orderly transmission of light energy, which is non-imaging. A major research direction of optics is the design of a given illuminance distribution optical system.