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研究生： 杜昇翰
Shin-Han Tu
論文名稱： 離焦對影像品質的影響
Effects of defocusing on image quality
指導教授： 梁忠義
Thomas C.Y. Leung
口試委員:
學位類別： 碩士
Master
系所名稱： 理學院 - 光電科學與工程學系
Department of Optics and Photonics
畢業學年度： 91
語文別： 英文
論文頁數： 62
中文關鍵詞： 標準差 、對比 、自動聚焦
外文關鍵詞： standard deviation, contrast, Autofocusing
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    • 摘要
    在數位影像擷取元件日益發達的今日社會，透過各式各樣的影像擷取裝置，如CCD及CMOS去擷取一個影像已經是十分平常的問題。 在整個成像元件所組成的系統中，如何鑑別一個影像是否為最佳聚焦影像是一個相當重要的研究方向。在主動式的自動聚焦方法中，經常透過雷射光及超音波等的主動訊號發射器，再配合訊號接收器。如此一來便可知道光學系統與物體之間的關係。但是這樣一來對系統的整個體積、重量以及製造成本方面，都是比較大的負擔。
    在本篇論文中介紹了靜態的影像鑑別方法，透過系統離焦對影像對比的改變來對系統的狀態做定量的工作。由空間頻率開始，計算空間頻率隨著離焦程度的衰減。再將衰減的量反應至邊緣的損耗最後透過統計的方法求得一定值。除了自行設計的物空間圖案外更對真實空間的人物及風景做一評鑑，在距焦附近的效果是顯著的。在文章的最後除了直接對灰階值做運算處理之外，更對系統的穩定度及運算時間做一比較及說明。本文所做的一切都希望將來能應用為離線操作。

    Abstract
    In modern society the digital imaging devices are popular productions. Generally, CCD and CMOS image sensor are used to be an important element in common digital device. The most importance of an imaging system is to evaluate the image quality and the entire system situation and then modulate itself to the perfect situation. In active auto focusing method the laser and ultrasonic emitter source and signal detector are necessary. Active methods are accurate than passive methods, but the cost of manufacture and volume and weight of the entire system are higher than passive methods.
    In this thesis, we introduce a static image evaluation. The decrease of contrast will be measured to evaluate the situation of the entire system. We calculate the decay of spatial frequency spectrum versus defocus level first. The information related with edges will be calculated to evaluate the situation of the system by statistics methods. Besides the designed pattern object, we use the real image to be a test of our method. The effect is obvious near the focal point. Finally, the stability of the system is measured. The methods mentioned in this thesis will be used in off line situation, in the future.

    Content Chapter I Introduction……………………………………………………1 Chapter II theory…………………………………………………………3 II-1 Image…………………………………………………………...3 II-2 Digital image……………………………………………………7 II-3 Simulation of defocus…………………………………………12 Chapter III Experiment setup……………………………………...……17 III-1 Object generator and image sensor…………………………...19 III-2 Lens system…………………………………………………..30 Chapter IV Experiment……………………………………………….…36 IV-1 Experiment patterns…………………………………………37 IV-2 Spatial frequency analysis…………………………………..38 IV-3 Spatial differential…………………………………………...41 IV-4 Single line gray level method……………….……………….49 IV-5 Center area method…………………………………………..52 IV-6 Real image…………………………………..……………….54 Chapter V Discussion…………………………………………………..58 References………………………………………………………………61 Figure caption Fig. 2.1 Schematic diagram of an image system…………………………3 Fig. 2.2 A 1-dimensional Dirac-delta function and its spatial frequency spectrum………………………………………………………….5 Fig. 2.3 Coordinate system of the image………………………………....8 Fig. 2.4 Images of different pixel sizes…………………………………..9 Fig. 2.5 The spatial distribution of the gray level of the image………...12 Fig. 2.6 The spatial frequency spectrum of the gray level of the image..13 Fig. 2.7 (a) Gaussian PSF with σ=3 (b) Spatial frequency spectrum of the PSF in (a)………………………………………………...…14 Fig. 2.8 The spatial frequency spectrum of image and object……..……15 Fig. 2.9 Comparison of gray level distribution of (a) object and (b) image produced with a Gaussian PSF with σ=3…………….……….16 Fig. 3.1 Schematic diagram of the experiment setup…..……………….17 Fig. 3.2 The schematic block diagram of OV7620……………………..22 Fig. 3.3 Gray level of an edge along the x-direction with (a) and without (b) edge enhancement…………………………………………..23 Fig. 3.4 Gray level versus exposure time………………………...……..24 Fig. 3.5 Simple model of image system…………………………….…..26 Fig. 3.6 Linear region of gray level vs exposure time relationship……..28 Fig. 3.7 Lens structure…………………………………………………..31 Fig. 3.8 MTF of the lens when the image is formed on the focal plane…………………………………………………………….32 Fig. 3.9 MTF of the lens when the image is formed 0.0625 mm beyond the focal plane………………………………………………..…32 Fig. 3.10 Spot size of a point object versus with lens position…………34 Fig. 4.1 Experimental object patterns……………………………..…….37 Fig. 4. 2 The images and the spatial frequency spectrum of focused image and an image when the lens is displaced by –0.0625mm…..…..38 Fig. 4.3 The amplitude of main spatial frequency versus lens displacement of PIXEL10……………………………...……….39 Fig. 4.4 The amplitude of main spatial frequency vs lens displacement of PIXEL2…………………………………………………………40 Fig. 4.5 Gray level distribution of the image of focus PIXEL10……….41 Fig. 4.6 Differential of the gray level distribution of the image of PIXEL10……………………………………………………..…42 Fig. 4.7 Gray level distribution of the image of PIXEL10 formed on a plane –0.125mm from the best focusing plane………...……….43 Fig. 4.8 Distribution of the spatial differential of the gray level distribution of the image of PIXEL10 formed on a plane –0.125mm from the best focusing plane………………....43 Fig. 4.9 STD of ?gray level/?pixel of the image of the PIXEL10 for various image positions……………………………………..….44 Fig. 4.10 STD of ?gray level/?pixel of the image of the PIXEL5 for various image positions…………………………..…………….45 Fig. 4.11 STD of ?gray level/?pixel of the image of the PIXEL2 for various image positions……………………………………….46 Fig. 4.12 Distribution of the spatial differential of the gray level distribution of the image of PIXEL5 formed on a plane –0.125mm from the best focusing plane……………….47 Fig. 4.13 STD of Single line gray level along an x direction scan line to various image positions…………………………………….....50 Fig. 4.14 STD of Single line gray level along a y direction scan line to various image positions………………………………………50 Fig. 4.15 STD of gray level combing two scan lines each along the x and y direction, versus image position…………………………….51 Fig. 4.16 STD of gray level of the image in the center area of PIXEL10……………………………………………………...52 Fig. 4.17 STD of gray level of the image in the center area of PIXEL5……………………………………………….………53 Fig. 4.18 Three typical images……………………………………….....54 Fig. 4.19 The spatial frequencies along AA’ of Fig. 4.18(a)……………54 Fig. 4.20 The spatial frequencies along BB’ of Fig. 4.18(b)………...….54 Fig. 4.21 The spatial frequencies along CC’ of Fig. 4.18(c)……………55 Fig. 4.22 The spatial frequencies along CC’ of Fig. 4.18(c)…………....55 Fig. 4.23 Standard deviation of spatial differential along AA’ of the printed document image versus lens position………………...56 Fig. 4.24 Standard deviation of spatial differential along BB’ of the face image versus lens position…………………………...……….56 Fig. 4.25 Standard deviation of spatial differential along CC’ of the scenery image versus lens position……………………….…..57 Fig. 5.1 Standard deviation of gray level of the image of PIXEL10 vs image positions plotted with error bar…………………….….58

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