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The freeform TIR lens is designed using the method described in the section 1.2, and the lighting simulation (ray tracing) is performed. In order to improve the illumination performance of the freeform TIR lens, we reduce the area of the target region where the illuminance value is relatively large, and enlarge the area of the target region where the illuminance value is relatively small using the inverse feedback optimization method. The overall illuminance uniformity is improved, and the cross-section diagram is shown in Fig.6(a), and the illuminance distribution is shown in Fig.7(a). Then the freeform surface
$ {S}_{2} $ is Fresnelized. The main parameters of the Fresnel lens are summarized in Tab.1. CREE XLamp XR-E series is selected as the light source. The luminous flux is 100 lm, the divergence angle is 170°, and the surface light source size is 2 mm×2 mm; the target plane is 10 m away from the light source, and half-diameter of the target plane is 2.5 m. In order to further improve the optical illumination effect, the freeform Fresnel TIR lens with different number of segments is optically simulated. The simulation results are shown in Tab.2. When the number of segments is larger, the illumination effect is better, but the manufacturability gets worse. As a tradeoff, the number of segments N is defined as 22 in our work. The 2D cross-section diagram is shown in Fig.6(b), and the illuminance distribution is shown in Fig.7(b). The height of the freeform TIR lens without the Fresnel surface is 8.60 mm, and the height of the freeform TIR lens with the Fresnel surface is 1.08 mm, which decreases by 7.52 mm.Parameter Specification Light source size/mm 2×2 Half-diameter of the lens/mm 24 Half-diameter of the target plane/m 2.5 Distance between target plane/m and the light source/m 10 Lens material PMMA Number of segments N 22 Table 1. Main parameters of Fresnel lens
Number of segments N 16 18 20 22 Illumination uniformity 72.3% 75.4% 78.1% 82.0% luminous efficiency 89.2% 92.3% 93.3% 96.6% Table 2. Illumination effect of freeform Fresnel TIR lenses with different numbers of segments
According to the illuminance distribution diagram, the illuminance uniformity of the two lenses is obtained according to the uniformity calculation shown in Eq. (20):
The luminous efficiency values given by the software are listed in Tab.3. The corresponding weight, volume and superficial area of the two lenses are listed in Tab.3. The 3-D structure diagrams of the freeform TIR lens is shown in Fig.8(a) and the freeform Fresnel TIR lens is shown in Fig.8(b).
Volume/mm3 Weight/g Superficial area/mm2 Illumination uniformity Luminous efficiency Freeform TIR lens 27437.27 27.44 5977.53 82.0% 98.2% Freeform TIR Fresnel lens 21940.06 21.94 5899.18 82.0% 96.6% Table 3. Comparison of lens size and illumination performance
It can be seen from Fig.7 and Tab.3 that the far field illumination uniformity of both two TIR lenses is 82.0%, indicating that the original illumination performance is maintained. The luminous efficiency of the freeform TIR lens with Fresnel surface slightly decreased from 98.2% to 96.6%, but it still maintained a high level. The TIR lens with Fresnel surface is reduced by about 20% in volume and weight compared with the TIR lens without the Fresnel surface. This shows that the Fresnelization of freeform refractive surface of the freeform TIR lens can significantly reduce the volume and weight of the lens and shorten the optical path length, thus effectively improving the heat dissipation efficiency and service life of the lens, while maintaining the original illumination uniformity.
Optical design of freeform Fresnel TIR lens for LED uniform illumination
doi: 10.3788/IRLA20200183
- Received Date: 2020-05-19
- Rev Recd Date: 2020-07-30
- Available Online: 2021-02-07
- Publish Date: 2021-02-07
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Key words:
- optical design /
- Fresnel TIR lens /
- Snell's law /
- heat dissipation
Abstract: A new design of total internal reflection (TIR) lens was presented which had a freeform Fresnel surface in the central part of the front to improve the heat dissipation capability. Snell's law and the reflection law were applied to construct the freeform refractive surface and the freeform reflective surface for the TIR lens. The freeform refractive surface was transformed into the freeform Fresnel surface with universal design method of Fresnel lens. The simulation result for the freeform Fresnel TIR lens obtained by Monte Carlo ray tracing shows that the far field illumination uniformity of 82.0% and the luminous efficiency of 96.6% are achieved for the light source size of 2 mm×2 mm, in the meanwhile the lens weight is only 21.94 g. The freeform Fresnel TIR lens has nearly 20% reduction in lens weight and volume, only a 2% reduction in luminous efficiency, and no reduction in illumination uniformity compared to the TIR lens without the Fresnel surface. The result indicates that the Fresnelization for freeform surface of TIR lens can significantly reduce the volume and weight of TIR lens and shorten the optical path length, thus effectively improve its heat dissipation efficiency and service life while maintaining a high performance.