Discussion on Several Problems of LED Light Source Applied to Road Lighting

After years of development and accumulation, LED lighting technology has entered a stage of rapid development. As an important part of the lighting industry, road lighting accounts for about 20% to 30% of the lighting power consumption in the whole country, and its status is very important. Therefore, the application of LED in road lighting has also received more and more attention. In order to seize business opportunities, many companies have seized more markets and are rushing to develop various road lamps to meet the needs of users. Even some enterprises that did not produce lighting products have also launched, but for various reasons, the current semiconductors have been created. The lighting products are in a chaotic situation, and some enterprise products have also entered a misunderstanding, which has brought about a great negative impact. The author combines the characteristics of his own enterprise and road lighting to elaborate on some problems encountered in the development and application of LED street lamps.

1, secondary optical design

At present, the high-power LED light effect for road lighting is basically 70-90Lm/W, which has a certain gap compared with the high-pressure sodium lamp (100-120Lm/W), but because the LED has directional light-emitting characteristics, the illumination angle is 120. Within the degree, through the rational design of the optical device, the gap between the two will be significantly reduced.

1.1 Lamp efficiency Generally, the efficiency of traditional high-pressure sodium lamp street lamps is about 75%, and with the directional illumination characteristics of LEDs, the efficiency of lamps can reach 79%;

1.2 Effective use of light-emitting efficiency of lamps The effective utilization rate of luminous flux of traditional street lamps is 50%. Some of the light is overflowing and cannot be used. Especially the light towards the roof can not be accurately controlled on the illuminated surface, causing light pollution, and LED street lights can be accurate. Control the direction of light, effective utilization rate of more than 60%;

It can be seen from the above two points that as long as the secondary optical design of the LED street lamp is reasonable, the effective luminous flux to the target illumination surface is substantially close to that of the conventional street lamp.

1.3 Some inappropriate opinions

There may be some inappropriate views in the current road lighting field, which are summarized as follows:

(1) Emphasize the total lumens of the lamps and the illumination under the lamp, ignoring the application of the lamps and the design of the light distribution;

The result of this is that the street lamp center illumination is too high and the uniformity is not perfect. Both waste energy and have not achieved the desired results;

(2) Mixing of different light distribution requirements

The arrangement of street lamps includes single-sided cloth lamps, cross-border lamps on both sides, opposite cloth lamps on both sides, T-junction cloth lamps, cross-road cloth lamps and corner cloth lamps. Different arrangement modes of street lamps have different corresponding light distribution requirements. If all applications use the same light distribution design, it is not appropriate.

(3) Overemphasizing “environment ratio SR”
The total luminous flux to the streetlights is clearly different due to the different road grades and lighting environments. In addition to illuminating the carriageway, the streetlights should also have some illumination for the auxiliary road and the surrounding environment. This is also the reason why the “environment ratio SR” is proposed in the lighting specification. But the "environment ratio SR" is not desirable for any road. Because the "environment ratio SR" is proposed in consideration of driving safety, there is a considerable level of illumination within a certain range around the lane, so that the driver can determine whether there is a danger of driving around. At present, many urban roads in China use non-isolated or fully enclosed forms (such as viaducts, express lines, etc.), and even special auxiliary lights are provided. If the “environment ratio SR” of a single street lamp is excessively emphasized, the luminous flux of the lamps is increased. Will increase light pollution.

2, LED street light cooling design

2.1 Long-term heat dissipation problem

At present, the heat transfer methods we can use are also basically clear: conduction, convection, radiation and phase change heat transfer (heat pipe is a classic example of phase change heat transfer).
According to the equivalent relationship between luminous flux (lumen) and radiant flux (Watt):
Km = 683 lm/w, which means that 1W of the radiant flux may produce 683 lm of luminous flux in the most ideal case (blackbody radiation).
It can be seen that even if the luminous efficacy of the LED reaches 200 lm/w on one day, only 30% of the energy is converted into light energy output, and the rest are converted into heat energy. Therefore, in any case, LED lamps are required to have heat sinks.

2.2 Ways of cooling design of street lamps

At present, the main heat dissipation methods are: natural convection heat dissipation, fan forced heat dissipation, heat pipe and loop heat pipe heat dissipation. Because the street lamp is used outdoors at night, the heat dissipation surface is mostly located on the side of the lamp body, the shape and volume are limited, the natural heat dissipation condition is better than other lamps, the reasonable structural design and the good heat dissipation material can ensure the LED junction temperature is at normal range.

3, the drive circuit

LED driver circuit is various, the LED circuit used for street lamp should fully consider the power factor and AC/DC conversion efficiency, so that the effect of saving electricity can be truly realized. At present, the nominal power of a considerable number of LED street lamps differs from the actual input power. Very large, causing economic losses to users. The current drive circuit using resonant and synchronous rectification technology has a power factor of 0.9 or higher and a conversion efficiency of over 85%.

4. Engineering application

Due to the irrational selection of lamps, engineering installation and design, the lighting standards are not properly selected, resulting in a large amount of light pollution and waste. This problem has existed for a long time. This phenomenon will be more prominent if the actual application and design conditions of the LED street light do not match. Therefore, companies must correctly guide customers to use LED lamps.

5, intermediate vision

The general road illumination brightness is required to be between 0.1-2.0 cd/m2, which belongs to the category of intermediate vision (0.0034-3 cd/m2). To achieve the highest intermediate visual efficiency, the corresponding illumination can be adjusted according to different light sources and spectral distributions. In order to achieve either yellow or white light, the feeling of brightness should be the same. In the development of road lighting, there is a comparison of the use of yellow and white light. Huang Guang is now the mainstream high-pressure sodium lamp because of its high luminous efficiency and long life. But Huang Guang also has its shortcomings. The wavelength of the high-pressure sodium lamp is close to 589 nm, and the spectral efficiency is high under bright visual conditions. However, when the ambient brightness is reduced to a certain extent, white light having a wavelength of about 500 nm exhibits higher luminous efficiency. LEDs have advantages in this respect due to their wide selection of light colors.