Opportunities and challenges of LED light source in tunnel lighting

0 Preface

LED (Light Emitting Diode) is a solid-state semiconductor device capable of converting electrical energy into visible light. The principle of LED illumination is to pass current through the compound semiconductor. Through the combination of electrons and holes, the excess energy will be released in the form of light to achieve the effect of luminescence.

Since the birth of the first commercial LED in the 1960s, LED light sources have been rapidly developed and applied. LEDs are mostly used as indicators, display panels, etc. With the advent of white LEDs, their application range has been greatly improved. Now in traffic signals, advertising signs, emergency lighting, LCD backlighting, automotive lighting systems, commercial lighting And a wide range of applications in many lighting fields, such as urban nightscape lighting projects. Therefore, the LED light source is called the fourth-generation illumination source or the green light source, and has become a new generation of light source in the 21st century.

In recent years, with the rapid development of LED lighting technology and the in-depth implementation of China's energy saving and emission reduction policies, LED light sources have also begun to be applied to road lighting and road tunnel lighting. According to statistics, there are already hundreds of road and tunnel lighting companies that develop and produce white LEDs as light sources. Tunnels using LEDs as illumination sources have also gradually increased, such as the Haichangjiang Tunnel and the Beijing Deshengkou Tunnel. Any new thing must have a process of development and improvement. Therefore, in the initial stage of LED lighting application, we should understand and understand the advantages and disadvantages of LED light source in current tunnel lighting as soon as possible, and give full play to the advantages of LED light source. The shortcomings of existence. This provides a reference for the correct use of LED light sources in tunnel lighting, and provides a direction for the future application and development of LED light sources in tunnel lighting.

1 LED light source features

LED light source as a new generation of light source, compared with traditional thermal radiation and gas discharge light sources such as incandescent lamps and high pressure sodium lamps, the current LED light source has the following advantages in lighting:

(1) small size

Compared with the traditional light source, the LED light source is small in size and light in weight, and can be fabricated into various shapes of devices, which is convenient for arrangement and design of various lamps and equipments, has strong adaptability and wide application range.

(2) Good environmental performance

Since the LED light source does not need to add metallic mercury during the production process, the LED will not cause mercury pollution after being discarded, and the waste can be almost completely recycled, which not only saves resources but also protects the environment.

(3) Safe and stable

The LED light source can be driven by low-voltage direct current. The general supply voltage is between 6 ~ 24 V, so the safety performance is better, especially suitable for public places. In addition, under the favorable external environment, the LED light source has less light decay and longer life than the conventional light source, and even if it is frequently switched, it does not affect its service life.

(4) Good earthquake resistance and impact resistance

The basic structure of the LED light source is an electroluminescent semiconductor material placed on a leaded shelf and then sealed with epoxy resin. There is no glass casing in the structure, and it does not need to be inside the lamp like incandescent or fluorescent lamps. Drain or rush into a specific gas. Therefore, the LED light source has good seismic and impact resistance, which brings convenience to its production, transportation and use.

(5) strong directivity

Compared with the traditional light source, the light emitted by the LED light source is oriented, and most of the light emitted from the LED can be directly directed to the illuminated surface, and the utilization rate is much higher than that of the conventional light source.

(6) Fast response time

The incandescent lamp has a response time of milliseconds and the LED light source has a response time of nanoseconds. Therefore, it is widely used in the field of traffic signals and car signal lights.

(7) High luminous efficiency

With the development of LED lighting technology, the luminous efficiency of LED light sources has increased from less than 10 lm/W to more than 100 lm /W, and its luminous efficiency still has a great potential for improvement.

(8) rich in light color

The LED light source can realize light emission and discoloration of various colors in the visible light band by changing current, chemical modification, monochromatic light mixing, etc. Even for white light LEDs, light sources of various color temperatures can be produced. Therefore, it has obvious advantages in interior lighting and landscape lighting.

(9) No flicker, no UV

The LED light source adopts DC power supply, the light is stable and does not flicker, and the spectrum is mainly concentrated in the visible light region, and there is basically no interference of ultraviolet rays or infrared radiation, thereby avoiding the adverse effects caused by the stroboscopic effect and improving the comfort of the human eye.

(10) Brightness is adjustable

According to the principle of illumination of the LED light source, the luminance or output luminous flux of the LED changes substantially with the current. The working current can be large or small within the rated range, and has good adjustability, which lays a foundation for the LED light source to achieve on-demand illumination and brightness stepless control.

It is precisely because LED light source has the advantage that the above traditional light source can't match, LED light source has become the focus and direction of the development of lighting industry in various countries.

2 Characteristics of tunnel lighting and requirements for light source

Tunnel lighting is a functional lighting designed to ensure smooth traffic in the tunnel. The purpose of the lighting is to provide the driver with a safe and comfortable visual environment during the tunnel driving, to ensure traffic operation and improve transportation efficiency. Since the tunnel is a semi-enclosed space, the tunnel is much more complicated in terms of driving visual characteristics than other lighting. It requires not only 24 hours of uninterrupted lighting, but also daytime lighting is more complicated than night lighting. Therefore, tunnel lighting has the following characteristics:

(1) Segmented lighting

Segmented lighting is the most prominent feature of tunnel lighting. This is because the light environment inside and outside the tunnel has changed a lot during the daytime, and the vision needs a certain adaptation time to see the road. Therefore, in order to allow the driver to have a good visual transition at the entrance and exit of the tunnel, the tunnel illumination is divided into several different lighting segments: the entrance section, the transition section, the intermediate section and the exit section, and each illumination section The road surface brightness level requirements are different, as shown in Figure 1. The road surface brightness level of the entrance section and the transition section needs to be determined according to the brightness outside the tunnel hole.

(2) Strengthening lighting

Since the tunnel has different lighting requirements during the day and night, the lighting for the entrance section, the transition section and the exit section is basically composed of basic lighting and reinforced lighting. The former is considered by the middle section lighting, and the latter can be reinforced by the lamps with higher power. illumination. At the same time, there are corresponding lighting requirements for the lighting at the emergency parking zone and the connecting channel.

(3) Avoid / smoke effect

Due to the limitation of ventilation conditions inside the tunnel, the exhaust gas from the car is not easily diluted quickly, and it is easy to accumulate and form smoke, resulting in a smoke effect. Therefore, the impact of tunnel lighting design and light source selection should be considered.

It is precisely because tunnel lighting has the characteristics as described above that China's current industry standard 5 road tunnel ventilation lighting design specification 6 puts the following requirements on the light source and lamps of tunnel lighting:

(1) Under normal circumstances, it is advisable to select a light source with high efficiency and good fog permeability;
(2) Light tunnels with high color rendering index can be selected for tunnels near cities and towns with short tunnels and diesel vehicles, emergency parking belts, pedestrian crossings, and crosswalks for vehicles;
(3) The service life of the light source should be not less than 10 000 h;
(4) The degree of protection of the luminaire shall not be lower than IP65;
(5) The luminaire shall have an anti-glare device suitable for the characteristics of road tunnels;
(6) The structure of the luminaire should be easy to replace the bulb and accessories;
(7) Lamp parts should have good anti-corrosion performance;
(8) Installation of lighting fittings should be easy to operate and adjust the installation angle;
(9) Luminaires shall not intrude into the boundaries of tunnel construction;
(10) The luminaire arrangement in the middle section shall satisfy the flicker frequency below 2. 5 Hz or above 15 Hz.

3 LED light source opportunities and challenges in tunnel lighting

With the rapid development and popularization of LED lighting technology, the application of LED light source in tunnel lighting has gradually increased. LED light source has advantages in tunnels due to its small size, good environmental performance, safety and stability, high luminous efficiency and good adjustability. There are good prospects and opportunities in lighting.

(1) has good optical properties

In terms of luminous efficiency, although the current LED tunnel light is not the light source with the highest luminous efficiency, it still has certain advantages compared with fluorescent lamps and metal halide lamps. In addition, because the LED light source has strong directivity and small volume, in LED packaging. The light distribution design can be carried out. Through the scientific and rational secondary optical design, the LED tunnel light can achieve higher luminous efficiency than the high pressure sodium tunnel lamp.

In terms of service life, although in practical applications, due to the influence of packaging technology, LED light source is difficult to achieve theoretical life of more than 100 000 h, but at this stage there is still an actual service life of about 20 000 h, still meet tunnel lighting The requirements for the light source have a high service life.

In the light source color, the white LED light source can reach the color rendering index of 80 or more. The tunnel lighting can make the driver have better definition and visibility, greatly improve the recognition ability and reaction speed, and avoid traffic accidents. At the same time, the color temperature of the white LED light source can also be customized according to the requirements, so it is beneficial to study the visual effects of different color temperature light sources in the tunnel illumination, and select a light source more suitable for the tunnel illumination color temperature.

(2) Conducive to improving the lighting quality of the tunnel

For most conventional light sources, light sources with high power tend to have high luminous efficiency and large luminous flux. Therefore, in the case of road lighting, under the premise of ensuring uniformity of road illumination, a light source with a relatively high power is generally selected, which can improve efficiency, increase the spacing between lamps, reduce the number of light poles and lamps, and reduce costs. For tunnel lighting, although the installation of the luminaire does not require the addition of a light pole, since the conventional low-power light source has relatively low light efficiency and needs to increase the number of lamps, it is also possible to use a light source with a high power.

For the LED light source, although the high-power LED light source is applied to the tunnel illumination, the power of the single light source is generally between 1W and 3W, and the tunnel light is composed of a plurality of high-power LED light sources through string and parallel combination. Therefore, LED tunnel lights can be combined as needed to reduce the power of a single tunnel light and shorten the distance between the lamps, thus bringing the following benefits:

1) Avoid glare

Since the power of a single tunnel light is reduced, its brightness is reduced, thereby avoiding glare interference caused by excessive light source during driving, especially in backlighting conditions.

2) Improve brightness uniformity

Due to the short distance between the lamps, it is not easy to appear between the lamps/dark area 0, and with good secondary optical design, the brightness uniformity and target visibility of the tunnel surface can be greatly improved.

3) Avoid stroboscopic effects

The flicker effect is a phenomenon that occurs when lighting is periodically changed by lighting. The flicker frequency (ie the frequency of the stroboscopic effect) f = v / s (where v is the vehicle speed and s is the distance between the lamps) should meet the requirements of less than 2.5 Hz or higher than 15 Hz. Therefore, the distance between the shorter lamps is easy to make the flicker frequency higher than 15 H z, and the light source with uniform brightness distribution is also beneficial to reduce the discomfort caused by the stroboscopic effect.

4) Enhance visual guidance

Because the distance between the lamps is short, the order is arranged, and the LED tunnel light has a certain brightness, it is easy to form a brighter light band, reminding the driver to pay attention to the change of the road direction, so that the tunnel has good visual guidance, as shown in the figure. 2 is shown.

As can be seen from the above, the LED tunnel light can greatly improve the lighting quality of the tunnel through scientific design and arrangement, especially LED strip light source or strip light fixture.

(3) Intelligent stepless control for tunnel lighting

In the tunnel lighting standard, it is stipulated that the illumination brightness of the inlet section, the transition section and the exit section should be adjusted according to the brightness and traffic volume outside the tunnel. Traditional tunnel light sources (high-pressure sodium lamps and metal halide lamps) mainly rely on turning off some of the light sources to achieve dimming requirements. This method will increase the number of lamps and lighting circuits on the one hand, and reduce the tunnel lighting quality on the other hand, which is easy to produce / Zebra effect 0.

Since the LED light source has good brightness adjustability, it can realize on-demand illumination and brightness stepless control. The stepless dimming performance of the LED tunnel light is one of its biggest advantages and highlights. If the intelligent lighting control system is combined with the intelligent stepless control of the tunnel lighting, the inner lighting can be strengthened in the day according to the actual brightness change outside the tunnel hole. Real-time adjustment control is carried out. In the middle of the night, the basic illumination brightness of the tunnel is adjusted synchronously according to the change of traffic volume, and no zebra effect is generated. 0, effectively avoiding excessive illumination, and maximally saving lighting energy consumption while satisfying safety and comfort.

(4) Easy installation and maintenance

The LED tunnel light is assembled by modules assembled from multiple LED light sources. The installation is modular, eliminating the need for redundant wiring and easy installation.

LED tunnel lights are also superior to traditional light source tunnel lights in terms of maintenance. When the tunnel light of the traditional light source reaches the life limit, the entire luminaire must be replaced. The LED tunnel light is modular, and once the individual light source is extinguished, only one of the modules is replaced, and the whole luminaire is not caused. Unusable, from this point of view, LED tunnel lights also have the advantage of saving energy. At the same time, the long life of the LED light source means that there is no need to change frequently, which reduces the potential for traffic disruption and reduces the maintenance costs paid for this.

Although LED light sources have some advantages over conventional light sources in tunnel lighting and are being promoted in tunnel lighting, LED light sources still face the following challenges in the popularization and application of tunnel lighting.

(1) High price

The high price is an unavoidable problem in the promotion and application of LEDs, especially the high-power LED light source. A 100 W LED tunnel light is similar to a 150 W high-pressure sodium lamp, but the price of a high-quality 150W high-pressure sodium lamp is about 800 yuan, and the price of a set of high-quality 100W LED lamps is about 4,000 yuan. The price difference is 5 times. Therefore, the one-time investment and product cost performance are important reasons for the current popularity of LED tunnel lighting.

(2) Heat dissipation problem

Conventional light source arc tubes such as high-pressure sodium lamps and metal halide lamps require external bulbs to maintain a high wall temperature, and closed lighting fixtures are advantageous for such light source operation. However, this is very harmful to the LED light source. This is because when the LED is working, 70% or more of the electrical energy is converted into heat that cannot be released by the radiation, and the ventilation inside the tunnel is not good, and the temperature is easily increased. LED semiconductor light-emitting devices will have obvious changes in their characteristics as their temperature changes. In particular, the increase in PN junction temperature will result in changes and attenuation of various aspects of the device. This change is mainly reflected in three aspects:

1) reduce the external quantum efficiency of the LED;
2) shorten the life of the LED;
3) The main wavelength of the LED is caused to shift, causing the color of the light source to shift.

Therefore, heat dissipation is an important challenge faced by LED light sources in tunnel illumination. Otherwise, it will greatly affect the normal operation of LEDs, reduce light efficiency, accelerate light decay, and shorten life.

In addition to the above two bottlenecks restricting the development of LED tunnel lighting fixtures, the related issues of LED light source efficacy, packaging process and materials, drive circuits, secondary optical design, products and test standards are still not mature and perfect. It will be the challenge for LED light source in future tunnel lighting and other lighting applications.

4 Conclusions and prospects

As a new generation of illumination source, LED light source has been strongly supported and promoted by national and local governments for its high efficiency, energy saving and environmental protection. Its application fields have gradually expanded from low-power lighting such as signal indication and emergency lighting to high-power lighting such as road lighting and road tunnel lighting. In tunnel lighting, in addition to good optical performance, LED light source also has the advantages of improving tunnel lighting quality, adopting intelligent stepless control and convenient post-maintenance, which brings good application of LED light source in tunnel lighting. opportunity. At the same time, however, LED light sources still face significant challenges in terms of high price and heat dissipation in current tunnel lighting, as well as the immature challenges of LED lighting technology.

Therefore, in the current tunnel lighting, excessively exaggerating the effect of LED light source, quick success and blind use, and the application of LED light source in tunnel lighting should be further studied and tested, and standard LED lighting design and management should be established. The operation system realizes the standardization of LED lighting products and truly takes advantage of the LED light source in tunnel lighting. At the same time, we should also see the development momentum and broad prospects of LED lighting technology. After solving the current problems, LED light source is undoubtedly the direction of tunnel lighting and even other lighting fields.

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