LED is a new light source widely used in automotive lighting in recent years. Due to its high light energy utilization, fast response, good seismic performance, long life and small size, LED has gradually become a signal light, tail light, instrument Z lamp, The main replacement light source for vehicle lighting such as ambient lights. In terms of headlamps, with the improvement of LED power and the improvement of stability, more and more models have adopted LED as the main light source for headlight function lighting. However, the current models that use LED light source headlamps are mainly small cars, and there are few reports on large passenger cars using LED headlamps.
Compared with small cars, the application of LED lights on large passenger cars has advantages, as follows:
(1) Compared with the small car, the passenger car has a large installation space, and the front part is usually in a vertical state, which is more effective than the front streamlined car, which is convenient for optical and structural design;
(2) Most of the cars are front-engine models, and the headlamp light source is close to the engine, which makes the heat dissipation design difficult. The engine of the modern large-sized passenger car is at the rear end of the car, which can reduce the difficulty of heat dissipation design and the heat resistance of the
LED chip . Can also be reduced to reduce costs;
(3) Most of the cars are private cars. Large buses are usually buses. Large passenger cars tend to be higher in frequency of use. The long life and shock resistance of LEDs are more conducive to the bus.
(4) The price of general large passenger cars is much higher than that of ordinary cars (non-luxury cars). The proportion of car lights in the whole car price is relatively low. After replacing the LED headlights, the impact on the car price is not great, which is more conducive to Promotion of LED headlights.
In summary, the application of LED headlamps on large passenger cars has very significant technical and cost advantages. In this paper, for the prototype of an existing passenger car headlight, according to the technical requirements of GB25991-2010 "LED headlights for automobiles", the high beam of the headlights is carried out by using the LED light source under the condition that the size of the external frame is unchanged. The optical system design of the low beam and the simulation results were verified.
Headlight design standard
1.1 Light source The standard light source is the central module of the headlamp system. The entire optical system is designed according to its light field distribution. As a headlight source,
high-power LEDs should have high brightness and moderate color rendering index to alleviate driving fatigue. According to the provisions of GB4785—2007 “Installation Regulations for Exterior Lighting and Light Signal Devices of Automobiles and Trailers”, the light source color should be white, the total luminous flux should not be less than 1000 lm, and the chromaticity coordinates should be within the boundary range shown in Figure 1.
1.2 Test screen light standard car headlights are divided into low beam and high beam, the lighting effect of the two needs to be different, respectively, there are corresponding design standards. When low-beam lighting, it should not make the other driver dazzle, while ensuring that the road has sufficient illumination to achieve normal lighting, and the main purpose of high-beam illumination is to illuminate the road farther ahead. The different functions of the two determine its design. The indicators and methods are also very different. GB25991-2010 "LED headlights for automobiles" specifies the indicators of the headlights, wherein the detection of low beam and high beam illumination is done on the light distribution screen 25 meters before the reference center of the headlights. The specific requirements of the low beam and high beam types on the light distribution screen are shown in Figure 2.
It can be seen from Fig. 2(a) that the illuminance value of each point on the test screen should be strictly controlled, and two distinct cut-off lines must be formed, which are asymmetric light distribution. In order to avoid the glare of the driver, the illuminance above the cut-off line should be as small as possible. The lower part of the cut-off line corresponds to the near and the lane lighting, and should have a large illumination to ensure a good lighting effect. The high beam illumination standard is relatively simple, ensuring sufficient illumination farther in front of the lane. Figure 2(b) shows the illuminance requirements of the measuring point on the test screen in the high beam standard. The maximum illumination area is near the HV point. Side illumination is decreasing.
Optical system design
2.1 Light source selection <br> According to the GB4785-2007 standard, the OSRAM special lamp chip series products are selected. The chromaticity indexes are all in line with the requirements, and the light intensity distribution is approximately Lambertian. According to the above requirements of the headlight flux (total luminous flux ≥1000lm) and the full utilization of energy, the 1×3 high-power LED chip LE-UWD1W3-01 is selected as the light source, and the maximum luminous flux of a single can reach 630lm, 2 The total luminous flux requirement can be achieved. Color coordinates: Cx=0.3349, Cy=0.3405, which is the point A in the chromaticity characteristic boundary of Fig. 1 on the chromaticity diagram, which satisfies the system design index. To simplify the design, the LED light source was set to a 1 mm x 3 mm size illuminator in subsequent simulations.
2.2 Overall structure <br> Different from the traditional halogen or xenon lamp source, the high-power LED is a surface light source, and the light field is Lambertian distribution. Therefore, the automotive headlamp optical system using LED as the light source cannot follow the light distribution system of the traditional light bulb. . Currently, there are three main types of automotive headlamps: reflective, projected, and free-form. The projection headlights use an ellipse to reflect and converge the light at the baffle of the modulated light type. Finally, the light distribution lens is used to meet the lighting requirements, and the baffle blocks part of the light, resulting in waste of energy. The free-form surface type headlights divide the light distribution system into small modules. Each module completes a part of the light type requirements, which is difficult to describe with mathematical expressions, and the required processing precision is high and difficult. Parabolic reflective headlamps, as a traditional car headlight structure, are large in size, but are not affected by large passenger cars with a vertical front and large installation space, and their energy utilization is high. The technology is mature and simple, and is the preferred type for this design. The working principle is as follows: the light source emits light, and the parallel light is reflected by the parabolic reflector, and the obtained parallel light forms a desired light shape through the light distribution lens, and the basic structure thereof is shown in FIG.
It is available from the vehicle headlamp low beam light distribution standard, in which the low beam test screen has a horizontal cut-off line and a diagonal upward 15° cut-off line, and a single parabolic system is difficult to form two cut-off lines at the same time, so in this design Two parabolic systems are used to form two cut-off lines to meet the light distribution requirements. Figure 4 shows a schematic diagram of the combined light distribution scheme.
2.3 Parabolic Reflector The source of illumination is incident on the parabolic reflector to form parallel light for subsequent lens light distribution. The design of the parabolic reflector includes the determination of the focal length, length and height. It should satisfy: 1 should not be too small, because most of the light emitted by the chip is concentrated at ±60°, too small will cause some light to leak; 2 can not be too large, must meet the present There is a reserved space requirement for the car. The paraboloid of rotation used in this paper is shown in Fig. 5(a). Its focal length is 15mm, its length and height are both 60mm, and its normal direction is Z-axis positive and focal length is f. Its mathematical expression is:
When the point source is placed at the focus of the paraboloid of rotation, the light is reflected by the reflector parallel to the Z axis. The LED used in this design is 1 mm × 3 mm, and the outgoing light of the non-focus portion of the light source does not exit parallel to the Z axis. Now, the exit light with a certain offset from the focus is analyzed. As shown in Fig. 5(a), the offset in the Z-axis direction is assumed to be d. According to the law of reflection, the angle between the emitted light and the horizontal direction is β. Satisfy:
Among them, β is positive for the top and negative for the downward direction. The light intensity distribution of the LED is approximately Lambertian, and is distributed from -90° to 90° in the YZ plane, and α is clockwise. Taking the offset d from -0.2f to 0.2f, the change of β with the light exit angle α is shown in Fig. 5(b). It can be obtained from FIG. 5 that when the light source is not offset at the parabolic focus, it is emitted as parallel light; when the light source offset is positive, the reflection angle β is also positive, that is, upward illumination, and vice versa. Therefore, in a low beam design, the light source should be placed on the same side of the focus. Since the LED emits an approximate Lambertian distribution, the light energy is mainly concentrated at ±10°. Therefore, when one end of the light source is placed at the focus and the other end is far away from the focus, most of the light energy is emitted horizontally, and other light rays are scattered upward. Then the illuminance on the light distribution screen gradually becomes smaller from the top to the bottom, and a clear cut-off line can be formed by the subsequent lens astigmatism. The high beam has no cut-off requirements, so the light source can be placed at the focus to obtain a uniform light pattern that meets the light distribution requirements. 2.4 The light from the reflector from the reflector is mostly parallel to the normal direction of the paraboloid, so it is necessary to use a lens to spread the parallel light to meet the light distribution standard. The concave lens, the convex lens and the cylindrical lens all have astigmatism, but the angles of the outgoing light of the first two lenses are large, which is not conducive to the formation of a clear cut-off line. In this design, a combination of a plurality of cylindrical lenses is used to form a diffusing plate for light distribution. Considering that there is a sharp edge between the cylindrical lenses in Fig. 6(a), there is a problem of processing difficulty, so it is converted into the structure in Fig. 6(b), and according to the light distribution standard, the illumination on the light distribution screen is the middle. The large sides are small, and the illuminance of the middle portion can be enhanced by adjusting the size of the cylindrical lens on the basis of the spread as shown in Fig. 6(c), and the design derivation is as shown in Fig. 6.
The headlamp system requires a total of four diffusers, all of which are composed of a combination of cylindrical lenses. The adjacent cylindrical lenses are arranged in tangent, and the cylindrical lenses of different diffusing plates have different sizes, and the smaller the radius of curvature, the larger the astigmatism. Moreover, the number of cylindrical lenses should not be too small, which will result in a thick cylindrical lens; too much astigmatism will be caused. After many design and debugging, the number of cylindrical lenses is determined to be 10. The low-beam 0° astigmatism plate and the high-beam upper scatter light plate cylindrical lens are vertical arrays, the purpose is to spread the light level. The specific parameters and structure are shown in Figures 7(a) and 7(c). . The low-beam 15° diffuser is an oblique 15° cylindrical lens array, forming a diagonal upward 15° cut-off line. All cylindrical lens radii are 12 mm, as shown in Figure 7(b). The function of the diffuser under the high beam is to enhance the illumination of the middle area on the basis of the scattered light. Therefore, the middle part still uses the cylindrical lens astigmatism, and the two sides are directly transmitted to the middle area of the light distribution screen to achieve the target light distribution effect. The specific structure and parameters are shown in Figure 7(d).
Light distribution system simulation and structural design
3.1 Low-beam module <br> As described above, a combination of two parabolic reflectors is used to form a complete paraboloid of rotation for light distribution, forming a horizontal cut-off line and a diagonal upward 15° cut-off line. The structure is as shown in Fig. 8(a). Before and after the LED chips are respectively placed in focus, the reflector emits light mostly for parallel scattering, and the rest is incident downward toward the cylindrical lens. The LED chip model and the light source file are provided by OSRAM, the reflector is set to total reflection, and the lens material is PMMA. The specific model is shown in Fig. 8(b). The light distribution screen is set to a square of 10 meters, 25 meters away from the light source, and finally the light distribution effect on the test screen is as shown in Fig. 9. The illuminance values of the respective measurement points are shown in Table 1.
From the illuminance map, two obvious cut-off lines are formed on the test screen, and the illuminance shows the effect of weakening both sides. By comparing the illumination and illumination standards of each measurement point, it fully meets the requirements of low beam illumination in GB25991-2010. In the simulation, the luminous flux of a single LED light source is set to 600 lm, which is lower than the luminous flux (630 lm) emitted by the actual LED chip at the rated power, which can reduce the heat generation of the LED chip and facilitate the heat dissipation of the lamp. The light source utilization rate of the scheme reaches 66.5%, which is much higher than the 40% efficiency of the general halogen low beam lamp. At the same time, considering the above-mentioned design of the low beam scheme, the position of the light source is very ideal, and installation errors will inevitably occur in practical engineering applications. A certain offset is set on the position of the light source to investigate its influence on the light distribution. The results show that the position error of the light source can still meet the requirements when it is controlled within 0.6mm, 1mm and 1.5mm respectively in the x, y and z directions.
3.2 High-beam module <br> The high-light module's light distribution requirements are relatively simple, so that the test area only needs to meet the illumination requirements, and the parallel light can be uniformly illuminated by the cylindrical lens. The low beam design concept can be applied during design. The reflector above can be used to spread the light for the entire illumination area, and the lower reflector is used to enhance the illumination in the middle area. The design entity model is shown in Figure 10, and Figure 11 shows the effect of the illuminance map on the test screen.
From the high beam test screen illumination effect diagram, the illuminance distribution is very symmetrical and uniform, showing that the middle strong side is weak, and the illuminance gradient changes relatively flat, that is, the field of view changes little, which is very suitable for high beam illumination. The high beam simulation value and the national standard comparison table are shown in Table 2, and all the indicators are in line with the standard. Finally, the structure and heat dissipation system design of the car headlights are adopted, and the heat pipe and fins are adopted. The entire optical system is assembled in the existing large passenger car headlight housing. Figure 12 shows the assembled LED lamp assembly drawing. The headlamps designed with this design can be well installed inside and meet the design requirements.
in conclusion
This paper introduces the scheme of replacing the LED light source with a traditional parabolic reflector optical system for a large passenger car headlight. The design idea and implementation method of each part of the optical system structure are given, and the design model is verified and analyzed. The indicators of far and near light illumination meet the requirements of the national standard. The program has the typical features of simple design, easy processing and installation, and can give full play to the advantages of LED vehicle lighting, which is very beneficial for popularization and application on large passenger car headlights.
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