具部分背向照明之LED套件設計;LED Lens Design with Partial Backward Lighting

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题名:	具部分背向照明之LED套件設計;LED Lens Design with Partial Backward Lighting
作者:	李良泰;Lee, Ted Liang-Tai
贡献者:	光電科學與工程學系
关键词:	發光二極體;套件;透鏡;模擬;背向照明;LED;Add-on;Lens;Simulation;Backward;Illumination;Lighting
日期:	2018-01-30
上传时间:	2018-04-13 11:01:00 (UTC+8)
出版者:	國立中央大學
摘要:	在日常生活中會遇到許多交通標誌，在時間與節能減碳的推演下，LED交通標誌的使用量越來越多，但是在高速公路上的使用並無統一規範。 LED雖然有壽命長、節省能源與穩定度高等優點，但同時具有高亮度及高光源指向性等特性，若應用不當則容易產生眩光。我們先前的研究發現，高速公路上的 LED 標誌白色字體與綠色背景的對比度最好落在 9:1 至 10:1 之間，方可有最佳的舒適度與判別度。可是現今採用點矩陣設計的高速公路交通標誌，普遍都使用砲彈型 LED，此型式的 LED 雖然有便宜、普遍等優點，但是當使用在高速公路標誌上時會產生無限大的對比度，容易產生眩光問題。因此在本研究中我們提出一個有部分背向照明的 LED 套件。此套件目的是當直接加裝在既有的砲彈型 LED 上方時，會讓部分的光背向反射，以達到我們所期望的 10:1 對比度。我們先利用改進的光源模型建立量測與模擬流程及中場理論，來建立出砲彈型白光 LED 的光源模型。有了光源模型之後我們就可以開始進行套件的設計，Lens 1 到 Lens 3 主要是參考現今已有的設計加以應用，Lens 4 之後的都是結合或改進前面的套件而衍生出來的版本。設計完的套件會藉由 ASAP 光學追跡軟體進行向前的配光曲線與背向照明的光分佈分析，藉由此分析篩選出發光角在 60° 與符合我們 10:1 對比度目標的套件。模擬結果得到 Lens 2 與 Lens 11 是效果最卓越的兩個套件。 Lens 2 是一個薄塑膠套管，上面鍍上 90% 高反射率的鍍膜。這個套件的背向照明雖然不是很均勻，但是能量的值與範圍都還在規範內。突出的是他的前向正規化之後的配光曲線幾乎與原本的光源模型相同。 Lens 11 為一個粗的塑膠圓柱，其上緣外側有 45° 的切割，此切割面上鍍有一半反射、一半穿透的鍍膜。這個套件的前向正規化之後的配光曲線雖然與原本的 LED 光源模型有差異，但是也都還在發光角 60° 以上的規範內。以背向照明的分佈來說，此套件明顯的比 Lens 2 均勻，能量上也接近往前比上往後 10:1 的比例。以本研究設計出來的 11 個套件中，Lens 2 與 Lens 11 的結果都是可以接受的，接下來所需考慮的是實際上生產的成本與實際的表現。先忽略製造成本的話，建設使用本研究提出的 Lens 2 與 Lens 11 在高速公路的點陣式 LED 標誌上，可以有效地產生適量的背後照明來達到最佳觀看舒適度與判別度。;In this thesis, a lens that induces backward illumination when fitted on bullet-shaped LEDs is designed and simulated. Traffic signs play an essential role in our daily driving life. With costs plummeting and growing environmental awareness, traffic signs are utilizing more and more LEDs, but there are still no regulations for the usage on the freeway in Taiwan. Our previous studies have found that the contrast ratio of LED signs on the freeway should be around $ 10:1 $ between white font and green background to have the best comfort and legibility. Nowadays freeway traffic signs are adopting the dot-matrix design and typically use bullet-shaped LEDs. Although this type of LED has the advantages of being cheap and easy to obtain, it tends to cause infinite contrast and ghostly effects when used on a freeway sign. So in this thesis, we propose a LED lens that induces partial backward lighting. The goal of this lens is to allow part of the light to be directed backward when retrofitted directly over an existing bullet-shaped LED to achieve the desired $ 10:1 $ contrast. We first use an improved LED source model technique and midfield theorem to establish the LED source model for the sample bullet-shaped white LEDs. Using the LED source model, we then designed a total of 11 lenses. Lens 1 to Lens 3 is designed similar to existing designs commonly found in practice. Lens 4 and later designs are either a combination or improvement of earlier designs proposed. The lenses are simulated with the bullet-shaped LED source model to obtain front-facing normalized angular intensity distribution curves and backward illumination distributions with ASAP ray-tracing software to see if the designs meet our $ 10:1 $ contrast goal and fall within the luminous angle of 60° FWHM specifications. Lens 2 and Lens 11 are the two lenses that were the most effective. Lens 2 is based on a thin plastic sleeve coated with a 90% highly reflective coating. Although the backward illumination pattern of this lens is not very smooth, the total backward energy is still within the specification. What is striking is that the front-facing normalized angular intensity distribution curve for this lens is almost the same as that of the LED source model. Lens 11 is based on a thick plastic cylinder with a cut of 45° on the outside of its upper edge, which is coated with 50% reflective and 50% transmissive coating. Although the front-facing normalized angular intensity distribution curve is significantly different from that of the LED source model, it is still within the 60° FWHM luminous angle specifications. Where it stands out is the backward illumination distributions, this lens has a significantly more continuous backward illumination distribution than Lens 2 and also has a power ratio of front-facing power to backward lighting power close to $ 10:1 $. Out of the 11 lenses designed in this study, both the results from Lens 2 and Lens 11 are acceptable, so it boils down to the cost of production and the priority of the two properties --- front-facing normalized angular intensity distribution or backward illumination distribution --- in the application. Therefore, ignoring the manufacturing costs, Lens 2 and Lens 11 proposed in this study both meet our criteria and can be used to retrofit freeway LED signs using bullet-shaped LEDs. This modification will efficiently produce the appropriate amount of backward lighting needed to achieve the best viewing comfort and legibility.
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