Optical Design of LED-based Automotive Tail Lamps
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Nonimaging Optics and Efficient Illumination Systems IV, edited by Roland Winston, R. John Koshel, Proc. of SPIE Vol. 6670, 66700L, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.732827Fig. 1: From segmented surface to VRSIn this contribution we use the reciprocal process to design an illumination system. A prospective segmented surface is initially modeled as a VRS and then optimized. After that, the VRS is transformed back into a segmented surface. Thiscan be understood as a discretization technique.Figure 2: Functional principle(a) starting system, b) intermediate solution, (c) final solutionFigure 3: Generation of R2 (sectional views of the upper half sections)Besides this, the light redirected by R2 shall hit the FS perpendicularly. This is ensured by the VRS property of the polynomial surface. The result is a real surface with a virtual normal vector function. This is shown in figure 3.4.2 Rotational symmetric internal reflectorNow we introduce1) real light sources (LEDs) with appropriate rotational symmetric COs and2) the internal reflector R1Figure 4: Design of R1 with: (a) point source and b) real sourceThe design of R1 and R2 is finished after several iterations of the two steps (see figure 5). The final solution, which consists of real surfaces and one VRS, is named VRS Design.On the other hand, this system does not create the desired light distribution in the far field yet. It just collimates the light.4.3 Beam forming elements at R2R2 was designed as a VRS. To transform it into a real surface it has to be discretized in a first step. We divide R2 into nine radially spaced segments. In a second step we give them linear shapes which approximate the original function. Each segment’s vertex lies at the smooth polynomial curve of R2 created in 4.1. This basically conserves the light distribution at the FS. The segments still own virtual normal vectors.In order to create the desired non rotational light distribution according to the legal regulation ECE-R 7, we need at least one more degree of freedom. For this reason we divide R2 into four sectors of 90°. This offers the possibility to influence the horizontal and the vertical parts of the light distribution, separately.The slopes of the segments are significant for the shaping of the light distribution in the far field, specified by theluminous intensity values stipulated by regulation.Figure 5: Sectional views of the VRS - and the Polynom Design with principle light rays, rear view of the Polynom Design4.4 Considering asymmetries with free form-VRSThe regulations specify different values in the horizontal and the vertical direction. Therefore, an asymmetric system is required. The Polynom Design partly follows this demand by introducing four sectors with different slopes of the corresponding segments. However, the vertices of the segments still lie on a rotational symmetric surface.In a further design step we use freeform surfaces to better meet the asymmetric demands. Here, the geometrical shape ofthe surface and the virtual normal direction are both defined by independent B-spline functions. This function type isa) shape function b) VNV function (intermediate result) c) VNV function (final result)Fig. 6: Contour graphics of the B-spline functionsThe VRS R2 is described by two B-spline functions: the shape function and the virtual normal vector (VNV) function. The latter one describes a surface whose geometrical normal vectors serve as virtual normal vectors of the shape function. Both functions are symmetric with reference to the vertical and horizontal axis. The surface shape (left picture) is close to rotational symmetry to meet the demand for uniformity in the luminance of the circular FS. The VRS function (right picture) shows elliptical contour lines due to the demand for asymmetric intensity values in the horizontal and vertical directions. The discretization process for a VRS spline surface is equivalent to the former design of a polynomial surface. Each segment is constructed with its vertex and its central slope according to the corresponding values of the two defining spline functions of the VRS. Due to the asymmetry and the stronger local variation of the spline surface, the number of sectors and segments should be increased compared to the Polynom Design.4.5 ResultsIn earlier works we examined different concepts of light coupling into light guide devices and creating homogeneous luminance distributions at the FS [3, 4]. In this contribution we examine the shaping process of the desired light distribution under the condition to double the emitting area of the lamp compared with the former designs in [3] and [4]. We consider the interaction between the shaping process and the luminance at the FS.The impression of the tail lamp from different viewing angles corresponds to the luminance distribution on the output surface.First, we have a look at the homogeneity of the luminance distribution at the FS. Figure 7 shows spot diagrams of the three designs. The spot density can be understood as proportional to the illuminance values or, alternatively, asluminance values integrated over the half sphere.a) VRS Design b) Polynom Design c) Free Form DesignFig. 7: Spot diagrams at the front sideThe spot diagram of the VRS Design imagines the most homogeneous appearance. It is the best one, which is creatableby our principle design. However, we have to remember that it does not work in reality because of its virtual reflecting property.The spot diagrams of the Polynom and the Free Form Design show a good homogeneity, too. Both light guide devicesfed by LEDs offers more uniformity in the luminance distribution than classical designs using incandescent lamps.Now, we check the conformity to the legal regulation ECE-R 7. For this reason, we consider the light intensity values ina stipulated area (between +/- 10° vertical and +/- 20° horizontal) perpendicular to the front side. Tables 1 and 2 showthe simulated values in the demanded range of spatial angles.-20° -15° -10° -5° 0° 5° 10° 15° 20° 10° 25 48 75 84 127 98 70 47 24 5° 34 66 132 170 310 170 140 78 36 0° 43 95 216 384 674 398 205 95 37 -5° 36 71 130 165 288 167 144 70 34 -10° 33 55 67 91 130 93 75 53 30 Table 1: Simulated horizontal and vertical values in candela of the Polynom Design-20° -15° -10° -5° 0° 5° 10° 15° 20° 10° 29 33 63 78 77 86 59 36 32 5° 39 75 112 225 199 225 112 62 37 0° 54 158 147 237 357 245 146 152 64 -5° 38 65 120 223 205 225 124 67 41-10° 31 38 62 88 79 82 58 39 31Fig. 8: Luminous intensity curves vs. azimuth angle: a) Polynom Design b) Free Form DesignThe Free Form Design creates a remarkable and desired difference in the distribution between the horizontal and the vertical values.It is obvious that there is a correlation between the homogeneity of the luminance at the FS and the shape of the light distribution. That depends on the chosen functional principle. The more accurately we approximate the shape of the required minimum light distribution by adjusting the beam forming elements, the lower the homogeneity at the FS results.Another important goal was to develop a thin tail light device. The thicknesses of the devices are the following: a) VRS Design: 16.8 mm, b) Polynom Design: 17.8 mm and c) Free Form Design: 17.1 mm. The aspect ratio is approximately 6:1. Thus, it is permissible to speak of thin automotive lamps.5. CONCLUSION AND OUTLOOKThe application of LEDs in the range of automotive lighting provides the possibility to develop very thin light guidedevices which operate as lamps. Thin lamps provide great benefits to car manufacturers.。
