Application of power LED in street lamps

In daily life, most of the light sources people use are close to omnidirectional radiation. Omnidirectional radiation will cause light energy to be emitted to areas that do not need to be irradiated, resulting in energy waste or light pollution. Therefore, in order to make the light energy emitted by the light source be utilized as much as possible without generating glare and light pollution, and to protect the light source from working safely and reliably, it is necessary to use lamps to limit the radiation area of ​​the light source, and to increase the irradiance of the illuminated area as much as possible under the same light power. The use of lamps also involves efficiency issues.

In order to measure the efficiency of lamps in terms of energy utilization, people use the ratio between the light energy output of the lamp and the light energy output of the light source in the lamp to measure its efficiency, which is called lamp efficiency (also called light output coefficient). It is an important indicator for measuring the energy efficiency of lamps. Due to the different uses of lamps, their design forms also vary greatly. The efficiency of different forms of lamps also varies greatly, generally between 50% and 80%. The light output coefficient of high-efficiency lamps can reach more than 80%, and the light energy can be fully utilized. The light output coefficient of low-efficiency lamps is even less than 25%, and most of the light energy is converted into heat energy and wasted. In the design of lighting and lighting projects, choosing lamps with a high light output coefficient can save energy while ensuring safety and preventing glare. If the selected lamps are inefficient or a considerable portion of the light energy is directed into the air, it will cause unnecessary waste.

Analysis of light output coefficient of high brightness point electric light source lamps

High-brightness point electric light sources such as high-pressure sodium lamps and metal halide lamps can achieve a luminous efficiency of 60 to 90 lm/W or more. However, since the light energy they output is too concentrated and omnidirectional, strong glare will be generated if the light source is not shielded during use. Therefore, these light sources must be used with lamps to increase the illumination of the illuminated area, reduce light energy loss, and prevent glare during application. Most of the light energy of these light sources is reflected once or twice in the lamps, and its energy will be attenuated to a certain extent. The lamp efficiency is higher when used for street lamps and floodlights, and lower when used for garden lamps. The actual luminous efficiency of the above point light sources after being installed in lamps is about 30 to 70 lm/W.

Analysis of light output coefficient of energy-saving lamps in some lamps

Energy-saving lamps are widely used now. They are in the shape of tubes, and the typical value of light efficiency is 60 lm/W. When energy-saving lamps are used for general indoor lighting and some outdoor lawn lamps, their light energy can be maximized; but when they are used for garden lamps and downlights, the actual utilization rate of light energy is low.

The milky white bulb garden lamp, which is currently more commonly used, is a lamp that integrates lighting and decoration. Because its light energy is nearly omnidirectional, the light energy actually emitted to the ground is only 50% of the light energy output by the lamp. The total luminous flux of this lamp emitted to the ground can be calculated as follows:
Фg=Фl×δ×50% In the formula: Фg is the total luminous flux emitted by the lamp to the ground; Фl is the total luminous flux of the light source; δ is the transmittance of the milky white bulb lamp, which is generally 0.7; the 50% coefficient is because only half of the total light energy emitted by the lamp is emitted to the ground.

If a 30W energy-saving lamp is used in a milky white bulb, the total luminous flux is 30W×60lm/W = 1800lm; the total luminous flux directed to the ground is:
Фg=1800×0.7×50%=630 lm

It can be seen that the theoretical light utilization rate of the bulb is only 35%. In practical applications, the surface of the bulb is contaminated by dust and the inside is often contaminated by insects, so the δ value is actually lower than 0.7.

Downlights are a common type of indoor lighting fixture. They are installed at a relatively low height. For safety reasons, to prevent glare and to take aesthetic requirements into consideration, light sources are mostly embedded in the tube. When energy-saving lamps are used in downlights, most of the light energy needs to be reflected twice or multiple times before it can be emitted; during this process, the light energy will be greatly attenuated. The light efficiency of vertical downlights is lower than that of horizontal downlights. Therefore, when energy-saving lamps are installed in vertical downlights, the lamp efficiency is sometimes even less than 25%, and the energy-saving effect is not obvious.

Analysis of light output coefficient of LED lighting fixtures

For power LEDs, the radiation direction of light energy is unidirectional. When LEDs are packaged, if lenses with different refraction angles are installed at the light outlet, the emitted light beams will have different radiation angles. Designers can reasonably select LEDs with different radiation angles according to the characteristics of various types of lighting.

当LED用于路灯时，可选择一组发射角为1400的大角度LED。安装时可将LED的光轴线向外倾斜300～450，这可使被照区域的照度变得更为均匀。LED发出的光无须使用任何灯罩进行反射，也无须使用任何光罩加以装饰。它的大部分光能均直接由光源射向地面。由于路灯灯具高度在3.5m以上，且单位面积内的光功率比白炽灯还低，故不足以产生眩光，无须使用任何光罩加以遮挡。因此，LED路灯灯具的效率可高达95%。如上面提到的乳白色球泡灯，用30W节能灯时射向地面的总光通量为630 lm，而采用45 lm/W的LED时只需14W，无须外罩，其节能效果是相当显著的。

When LED is used in downlight, a group of small-angle LED with a radiation angle of 600 can be selected, and the optical axis of the LED is perpendicular to the ground when installed. This design allows most of the light energy to be directly directed to the illuminated area, and only a small amount of light energy is emitted after one reflection. Therefore, the lamp efficiency of LED used in downlight can reach more than 90%.

In addition to the above two types of lamps, when LEDs are used in various lighting fields, as long as designers have a full understanding of the performance of power LEDs, they can design various types of lamps with very high light output coefficients.

Advantages of LED in lighting

LED has some special advantages in the field of lighting. First, the luminous efficiency of LED has reached 47 lm/W, which is close to the typical value of energy-saving lamps. This year, there will be 95 lm/W power LEDs on the market; its advent will make LED one of the light sources with the highest luminous efficiency. Therefore, there is no longer a major technical barrier to replacing light sources such as incandescent lamps and fluorescent lamps with LEDs. It is just a matter of time. Secondly, power LEDs have unidirectional radiation characteristics. Most of the light energy it emits can reach the illuminated object directly without reflection, so that the light energy can be emitted to the maximum extent, greatly improving the efficiency of lamps. Third, LEDs have a very long life and are green and environmentally friendly. The average trouble-free working time of power LEDs that can be purchased is mostly above 40,000 hours; if it works 11 hours a day, it can be used for ten years. Almost all of its waste can be recycled. This not only saves energy, but also saves resources and protects the environment. Other light sources will pollute the environment after being discarded, especially when fluorescent lamps are discarded, the mercury in the lamp will cause serious pollution to the environment and water sources. Fourth, LED light sources can be flicker-free, UV-free, and have extremely low heat radiation, which is harmless to human eyes. Using LED for indoor lighting or for lighting in places where people need to work and study for a long time can really protect the eyes. Fifth, there is the aesthetic demand. With its appearance characteristics, LED will give designers a lot of imagination in lamp design. It can integrate lighting and colorful night scene decoration, making the city more beautiful and the city night more brilliant.

With the above-mentioned excellent characteristics, the country's determination to build a conservation-oriented society, and the fact that the "National Medium- and Long-Term Science and Technology Development Plan" (2006-2020) has listed energy-efficient, long-life semiconductor lighting products as a priority topic, it is believed that within a few years, LED will be widely used in the lighting field.

Comparison of investment returns using LED lamps

In the street lighting projects of residential areas and urban secondary roads, the cost of cables, transformers and lighting cabinets accounts for a considerable part of the total project investment. If LED lamps are used in the engineering design stage, it can not only save a lot of cable costs, but also save the capacity of power transformers and save lighting distribution cabinets as appropriate. Therefore, in terms of the total project investment, the total investment does not increase significantly when LED street lamps are used. Although the investment cost of LED street lamps has increased when replacing the original lighting lamps in residential areas and roads, its operating cost is quite low compared with other light sources because of its low energy consumption, life of more than 40,000 hours, and basically maintenance-free. Taking the milky white bulb lamp introduced above as an example, a 30W energy-saving lamp or a 100W incandescent lamp is installed in the bulb. The luminous flux they project to the ground is equivalent to that of a 14W LED lamp.