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.
