Optoelectronic technology application of free-form optical design for achieving arbitrary illumination distribution in LED road lighting Hu Xiaojia, Qian Keyuan (Tsinghua University Shenzhen Graduate School of Semiconductor Lighting, not only can not form a uniform illumination on the road surface, but also a lot of stray light, light effect Low. Therefore, secondary optical design of the LED is required to achieve high efficiency LED illumination.
At present, a number of studies have been carried out to achieve a single LED light source to form a rectangular spot with uniform illumination on the road surface. After secondary optical design of the LED, the photo effect is as shown in (b). This kind of optical design can achieve uniform illumination of the entire road surface by splicing. However, in practical applications, many problems arise. As shown, there is a situation where only the road surface receives light when the two street lighting areas are spliced. If the individual is located there, the driver on the road may not be able to observe his presence, which is very likely to cause traffic accidents. Although the above problems can be solved by overlapping splicing, the uniformity of the brightness of the road surface is poor and cannot meet the driving requirements. Therefore, in the road lighting, the illuminance distribution of the single street lamp in the longitudinal direction of the road surface should be non-uniform, and the overall distribution is strong in the middle and weak in the two ends, and the illuminance distribution in the longitudinal direction of the road surface is uniform by suitable overlapping splicing.
The road lighting evaluation index that is valued is defined as the ratio of the average horizontal illuminance in the strip-shaped area 5m wide outside the roadway to the average horizontal illuminance on the adjacent 5m roadway. As shown, SR is usually required. 0.5. If the street lamp forms a rectangular spot with uniform illumination on the road surface, the width of the illumination area will be wider to meet the environmental ratio requirement, and the light effect is reduced. Considering the environmental ratio and efficiency factors, the illuminance distribution in the width direction of the road should also be changed, and the illuminance of the road on both sides of the roadway is decreasing.
Based on the above considerations, the illuminance distribution of single street lamps in road lighting in the length and width directions of the road should be in a specific distribution. In order to realize this distribution, this paper proposes a new type of LED free optical surface design method to achieve arbitrary illumination distribution. The separation variable is combined with the minimum energy block iterative method. This method forms a mapping between the LED light source and the energy grid of the road surface. For this mapping, the lens surface is constructed according to edge ray theory, Snell's law, and error control method. In the design process, the location and angle of the street lamp are considered comprehensively, and finally the LED optical system with a specific distribution of the lateral and longitudinal illumination of the single street lamp is realized.
1 lens design method assumes that the center S of the LED light source is located at the origin of the orthogonal coordinate system. As shown, the incident light is refracted by the free-form surface with refractive index to become the outgoing light M, and the refractive index of the outer space of the lens is /. The corresponding point of the plane is (:, 3,) and the point is illuminated.
According to Snell's law, at the point P on the free-form surface, the incident ray 3, the outgoing ray 0, and the normal vector N satisfy, and 7, are unit vectors. Combining the energy correspondence and the edge ray theory, the coordinates and normal vectors of the points P(,:y,z) on the free surface can be obtained. 10. The design process of the free-form lens is divided into two steps, which are respectively energy mapping. Establish and structure the lens surface. When establishing the energy mapping relationship, it is assumed that the energy emitted by the light source is equal to the light energy illuminating the receiving surface, and the energy conservation integral equation can be expressed as therein, and the light intensity corresponding to the light source exiting direction i indicates the light source exit angle. The set; EG) represents the illuminance of the point P on the receiving surface, and D represents the illuminated area of â€‹â€‹the receiving surface. M. The method of establishing the energy mapping relationship proposed in this paper is a combination of the separation variable and the minimum energy block iteration. The traditional separation variable method can be used in the design of uniform illumination LED street lamp lens, and good results can be obtained. However, in the design of street lamp illumination with uneven illumination in the horizontal and vertical directions of the road, the result of using the separation variable method alone is not ideal, which has a certain relationship with the limitations of the lens design software. This paper can effectively solve this problem by combining the minimum energy block iteration method.
The LED light source is divided by a separate variable method. As shown in (a), the energy of the light source is divided into a plurality of energy bars along the 0 direction, and the luminous flux of each energy bar can be obtained by the equation (3): then the energy of the length of the receiving surface is performed by the minimum energy block iteration method. Division. The receiving surface is divided into a plurality of elongated strips in a length direction with a sufficiently small gap mm. Since the illuminance distribution on the receiving surface is known, the luminous flux received by each elongated strip is known, and the luminous flux is sequentially superimposed. When the luminous flux value reaches the luminous flux of the energy bar corresponding to the light source, the superposition is stopped, and the initial and ending strips of the superposition are the boundary of the energy bar corresponding to the light source. After multiple iterations, the receiving surface can be divided into a plurality of energy bars in the length direction, corresponding to the energy bars of the light source. (b) is a schematic diagram of the energy strips divided in the length direction of the receiving surface, and the width of each energy strip is related to the illuminance distribution of the road surface.
The energy bars of the LED light source continue to be divided, as shown, dividing each energy bar into a number of energy blocks along the direction.
The luminous flux of each energy block can be obtained by equation (4): then the energy division of the width direction of the receiving surface is performed by the minimum energy block iteration method. Due to the uneven distribution of road illumination, the division of different energy bars in the width direction may be different, as shown in (b). Finally, a correspondence of energy blocks is formed between the light source and the receiving surface.
2 Design example In the road lighting, the height of the lamp post, the installation angle of the street lamp and the distance between the street lamp and the roadside will have an impact on the lighting result. Therefore, when discussing the road lighting, these factors will be considered comprehensively. As shown, it is a schematic diagram of road lighting, where W is the width of the roadway, S is the width of the area where the illumination on both sides of the roadway is greater than zero, H is the height of the lamp post, and D is the point of the light. The distance between the sides is the installation angle of the lamp, and the distance between the street lamps is A. The inclination angle of the street lamp helps to achieve the overall uniformity of the road illumination. If the value is improperly selected, it may affect the lighting effect of the street lamp, and may also increase the lighting effect. It is difficult to make a streetlight lens. Therefore, it is necessary to select a suitable value of 0. In the width (W) direction, the illumination of the roadway is uniform, and there is a 5m illuminance monotonously decreasing area on both sides of the roadway, that is, the environmental ratio SR=0. 5, As shown in the figure, the lens design is performed using the method proposed in this paper to obtain an asymmetric three-dimensional free-form surface LED lens as shown.
The lens was placed in a road lighting system for simulation, and the illuminance distribution of the single street lamp on the road surface was as shown.
The simulation results are compared with the given road illuminance distribution. The illuminance distribution in the length direction of the road is shown as 0, which is the length of the road and the width of the road. The total illumination uniformity on the roadway is 0.93, and the environmental ratio is 0.55, which meets the design requirements.
3 Conclusion In order to achieve the uniformity of the total illumination and brightness of the road surface in the road lighting, and at the same time to meet the requirements of the environmental ratio, the illumination of the single street lamp in the length and width direction of the road should be given a specific distribution. The free optical surface design method proposed in this paper can effectively realize the arbitrary illumination distribution of the road surface. Based on the law of conservation of energy, the combination of the separation variable and the minimum energy block iteration is used to mesh the light source and the receiving surface, and an energy mapping is formed between the two. For this mapping, the lens surface is constructed according to Snell's law, edge ray theory, error control method, and the like. In addition, this paper analyzes the optimal placement angle of street lamps. The connection between LED and pavement centerline is the most favorable for street lamp design and production. Taking the chord distribution in the longitudinal direction of a given road surface as a cosine distribution and the illuminance in the width direction as a trapezoidal distribution, the street lamp lens is designed by this method, and the position and angle of the street lamp are comprehensively analyzed to obtain an asymmetric discontinuous free-form surface lens. The simulation results show that the illumination in the longitudinal direction of the road is close to the cosine distribution, and the error is less than 6%. The illuminance in the width direction of the road is close to the trapezoidal distribution at different lengths, and the error is less than 10%. The total illumination uniformity of the road surface reaches 0.93, and the environmental ratio reaches 0.55. To meet the requirements of road lighting. The method can effectively realize the design of the LED optical system with arbitrary illumination distribution, and is especially suitable for the street lamp lens design of road illumination.
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