車用魚眼鏡頭之設計

Abstract

魚眼鏡頭是一種具有大於180°超大視場角的鏡頭，運用非相似成像原理引入大量負畸變(Distortion)以達到大視角在有限尺寸像面之成像。本研究是以反望遠(Reverse Telephoto)結構為基礎，考量引入大量負畸變伴隨的像差問題，並利用光學軟體zemax設計出一款搭配30萬畫素成像感測器、適於車用環景影像的魚眼鏡頭。 經魚眼鏡頭結構分析及像差校正的結果，本研究採用了1枚球面玻璃及3枚非球面塑膠，設計光學總長為11.92mm之複合材料透鏡組，相較於總長16.5mm之5枚透鏡組，大幅減少鏡片組長度；此外本研究採用超短焦距概念，使得有效焦距為0.8mm，在視場角150°時相對畸變僅為-50%。相較於目前魚眼鏡頭，其有效焦距為1.34mm而相對畸變-70%，因此本研究提出之魚眼鏡頭設計具有較小的畸變更利於鏡頭邊緣之成像。當搭配1/4”COMS成像感測器（系統截止頻率90lp/mm），此魚眼鏡頭具有水平視角192°，以函蓋較廣之環景取像範圍；當視場角為最大時，此鏡頭之調制轉換函數 (Modulation Transfer Function)值為0.347(大於0.2)，代表此鏡頭具有較佳之環景邊緣影像。

Fisheye lens are the ultra wide-angle lens with the view angle larger than 180 degrees. By applying non-similar imaging principle, fisheye lens could introduce negative distortion to have ultra wide view angle imaging onto a limited image plane. To design proper fisheye lens with 0.3M pixel sensor for the vehicle camera, the current study utilizes reverse telephoto lens as the foundation, combining with optical design software to deal with the aberration comes with the negative distortion. After structural analysis and aberration convergence simulation, the proposed lens were composed by a four-piece compound material lens, including one piece of glass and triplet aspheric plastic lens. The total length of the proposed lens is 11.9mm, which is significantly reduced from the current five-piece lens with 16.5mm in length. In addition, the lesser relative distortion could be achieved by adopting the ultra-short focus design concept. The 0.8mm effective focal length makes a relative distortion of only -50% at 150 degree view angle, while the current lens with effective focal length of 1.34mm would cause a relative distortion of -70%. When the proposed fisheye lens are functioned with 1/4" CMOS image sensor, the 192 degree of horizontal field view angle could be achieved to ensure the coverage of panorama images. The Modulation Transfer Function at the maximum field reaches 0.347 when the cutoff frequency of system is 90lp/mm, which ensures the image quality at the edges of the panorama image.