In road lighting, with the development of white LED technology, the luminous flux of a single LED keeps increasing. It will become a trend for LEDs to replace traditional light sources for LED street lights. The light intensity distribution of the light source in all directions of a space is called light distribution. Excellent light distribution of LED street lights will make full use of the luminous efficiency. The light coverage of multiple street lights will make the illumination include all areas. The light distribution of LED street lights is often better than that of traditional street lights, which is one of the important advantages of LED street lights. Research on the light distribution of LED street lamps can greatly increase the utilization rate of light and reduce the waste of energy, which is of great significance to the road lighting system.
The characteristics and requirements of LED street light distribution
Light distribution shape characteristics of LED street lamp
If there is no light distribution for the LED light source, the light will be very scattered. When used on the road, the area of the road surface illuminated by the LED street light will be a very large spot. In this way, part of the light shines on the area outside the road surface, causing a certain amount of waste. It is difficult for this kind of LED street light to illuminate the entire road surface. As shown in Figure 1(a).
After the distribution of LED street light, the shape of the light spot is rectangular, as Figure 1(b). The light will shine on the road relatively evenly, which ensures that there are no blind spots on the road. After the distribution of the LED street light, the width of the light irradiation is close to the width of the road. This makes full use of the light and can fully illuminate the road.
Standard Street lighting distribution requirements
Roads mainly include expressways, trunk roads, secondary trunk roads and branch roads. These roads have different classifications, different road conditions, and different requirements for road lighting. The street lamps on these roads have different densities and luminous fluxes, and the cost of street lamps used is also different.
Express roads and main roads generally need street lamps with high power and large luminous flux. And secondary trunk roads and branch roads need street lamps with low power and low luminous flux. At present, there are not enough LED street lights used on expressways and main roads. Although LED street lights have many advantages over traditional street lights, the technology of LED street lights is not particularly mature. Besides, the cost of LED street lights produced by many manufacturers is relatively high. LED technology still needs further development.
Light distribution curve of LED street lamps
The spatial distribution of luminous intensity is often referred to as a light distribution curve. Below the street lamp, the light intensity should be the minimum, and as the elevation angle θ increases, the light intensity I increases. The functional relationship is:
That is, the light distribution curve expression. The schematic diagram of the ideal light distribution curve of a single lamp for road lighting is in Figure 2.
Due to the complexity of the optical design, it is difficult for the shape of the light distribution to completely conform to the functional relationship. The projection range of the θ angle can decrease, and the distance between the lamps can also decrease to obtain uniform illuminance. Generally speaking, it is hoped that after light distribution, a wide-angle “batwing” light distribution can be achieved.
LED street light distribution scheme and analysis
The light distribution scheme of LED street lamps must meet the requirements of road lighting. This requires that the aperture size of LED street lamps to be close to the width of the road. If it is too small, there will be no lights in some areas. If it is too large, it will cause light pollution and energy waste. For street lamps, illumination evenness and aperture size are two important parameters of street lamp light distribution. The design of the light distribution of LED street lamps is also around these two parameters.
Primary light distribution of LED street lights
The first light distribution of LED street lamps is mainly the packaging process in the manufacturing process of LED street lamps. Different packaging of LED street lamps has a great influence on the light efficiency. Excellent light transmission technology can reduce light loss and increase light efficiency. The lens packaging process of LED street lamps plays an important role in the first light distribution of street lamps.
If LED street lamps adopt double-headed lens packaging technology, the lens will refract LED lamp head, and the light will emit in a “bat wing” shape. After subsequent processing methods, LED street lights can realize “bat wing” light distribution as a whole. The primary light distribution structure and light distribution curve of the double-headed lens are shown in Figure 3(a) and Figure 3(b). The design of this lens becomes the core of its primary light distribution.
Secondary light distribution of LED street lights
LED street lights use lenses or reflectors to modify the direction of light transmission. This process is called secondary light distribution of LED street lights.
The secondary light distribution of LED street lightss mainly has the following conditions.
Total reflection lens LED secondary light distribution
light is emitted from a relatively optically dense medium to a relatively optically sparse medium. When the incident angle is greater than the critical angle, total reflection will occur. A full-axis symmetric total reflection lens is in LED street lights to constrain the light within a certain range. This method is called the LED secondary light distribution of the total reflection lens. This light distribution method increases the utilization rate of light. For example, controlling the beam angle within the range of ±30°is beneficial to further light distribution design. The total reflection lens is shown in Figure 4.
Free-form surface lens LED secondary light distribution
Different from the full-axis symmetrical total reflection lens, the free-form surface lens has different lengths of light distribution on the X-axis and Y-axis, and adopts a rectangular free-form surface optical element with light distribution. This design aims to meet the road lighting requirements better. For example, if longer illumination length on the X-axis direction is needed, the X-axis light distribution angle can be increased. If the illumination length doesn’t need to be that long on the Y axis, the light distribution angle can decrease. In order to achieve the light distribution effect, the design of the curved surface contains high technical quality. In order to achieve the best effect, the manufacturer often conducts multiple tests.
External lens and reflector LED secondary light distribution
This light distribution method is relatively simple. Using parabolic reflectors and optical lenses, the light emitted by this light distribution method is much worse than the above two, but this light distribution method is simple and cheap.
Tertiary light distribution of LED street light
There are multiple LED unit modules arranged and combined on the LED street lamp. In order to meet the requirements of road lighting, it is necessary to perform a tertiary light distribution on the LED street lamp. The first and the second light distribution have made relatively good adjustments to the light of the LED unit modules, and the tertiary light distribution is to arrange the LED unit modules reasonably. There are several methods for tertiary light distribution.
(1) Flat light distribution
This type of LED street lamps use free-form surface optical element (lens or reflector cup) with asymmetrical rectangular light distribution in the XY direction. Since the rectangular light distribution has been completed on a single LED optical element, it only needs to arrange the LED modules on the panel.
(2) Curved light distribution
Multiple LEDs are arranged to form an LED module. The LEDs on the LED module use an axisymmetric total reflection lens or reflective cup for light distribution. The radiation angle width of the light distribution through the lens or reflective cup is sufficient to cover the road. By arranging the LED modules on the curved surface, and adjusting it, the lamp can generate a nearly rectangular light pattern in the direction of the road.
(3) Multi-fold light distribution
LED light has good directionality. In order to obtain better road light distribution, the LED projection directions of each group are designed to illuminate their respective areas. The relatively simple way is to adopt the V-shaped surface method. In the multi-fold light distribution design, the high-power LEDs of each group in the street lamp are on different planes. By adjusting the relative angles of each group, the lamp can obtain the light output characteristics and approximate rectangular lighting effect, which meets the road lighting standards. Fig. 7(c) is a schematic diagram of a multi-fold LED street lamp.
(4) Reflective cup light distribution
The design of LED reflector realizes the output characteristics of street light. An asymmetric reflector in the XY axis direction is separately designed for a single LED. This solution is similar to a flat light distribution. But different from using a free-form surface reflector to obtain light output close to the “batwing” shape, multiple reflectors are designed and arranged to get the same road lighting effect.
Analysis of the light distribution scheme of LED street lightss
No matter adapting what technical means or scheme, as long as its light output characteristics meet the requirements of night road lighting standards, this technology or scheme has good use value. Each of the above light distributions has advantages and disadvantages, as shown in the following Table. A light distribution scheme often requires multiple combinations and repeated designs to meet the standards. For example, the combination of LED primary light distribution (lens packaging process) and secondary light distribution of lamps, the combination of LED secondary light distribution unit and tertiary light distribution of lamps, the secondary light distribution of reflectors of LED street lamps, etc.
Comparison of light distribution schemes
Comparison of light distribution schemes | |||
| Light distribution scheme | advantage | disadvantage | |
| primary light distribution | adopt double lens packaging process | good bat-wing light distribution curve | Packaged lenses have poor versatility |
| Total Reflection Lens | Component design and processing are relatively simple | Multi-grain combination, average uniformity | |
| secondary light distribution | free form lens | Good uniformity and batwing light distribution curve | Component design and processing are more complex |
| external lens and reflector | Component design and processing are relatively simple | Light output loss is relatively high | |
| tertiary light distribution | flat light distribution | The mechanical structure, heat dissipation and power control are relatively simple, and the overall uniformity is better | Difficult to use when the roads are quite different
|
| curved light distribution | Adjust arc curvature to adjust light distribution | The design of heat dissipation and lamp holder is more complicated | |
| Multi-fold light distribution | The combination of the heat dissipation plane and the installation arc surface is good, which reduces the difficulty of the process | Difficult to adjust the angle | |
| Reflective cup light distribution | Make full use of the light leaked from the surrounding area, and the light output rate is high, similar to flat light distribution | The design of free-form reflective cups are more complicated | |
At present, power-type white light LEDs are developing in the direction of single-chip and high-power ones. Due to the bottleneck of chip heat dissipation, it is relatively difficult for single-chip and high-power LEDs with multi-chip packages to dissipate heat. And the luminous efficiency will also be relatively low.
What to choose
Therefore, considering factors such as packaging, heat dissipation, luminous efficiency, and color rendering, street lights usually select a single LED power of 1 to several watts and a luminous effect of 90-100 lm/W. The overall power is achieved through a multi-element array hybrid method. In terms of light distribution of LED street lamps, first, LEDs with better primary light distribution are reasonably arranged on the same plane. Then, free-form surface optical elements (lenses or reflector cups) are applied to the LEDs on the plane as a whole to perform secondary light distribution. This solution reduces the loss of LED output light and improves the efficiency of lamps. What’s more, it simplifies the relative design and processing difficulty of lamps and ensures the uniformity, illuminance and light pattern of the road surface. Therefore, this program has good research prospects.
Summary
The light distribution design of LED street lamps is one of the keys to the application of LED road lighting. Even in the future design of smart city lighting, it will be a key point. This article mainly analyzes the light distribution scheme of LED street lights and provides an ideal solution. Unreasonable distribution of LED street lamps can be a key problem of LED street lighting. But with the continuous development of LED technology, the prospect of LED street lamps used in road lighting will be brighter.
