Master Thesis - Fachbereich Informatik

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2.2. ILLUMINATION 17 tubes alternate at about 25kHz, what is far beyond what can be captured by a video camera. Fluorescent lights exist at different sizes, shapes, and setups. Beside the common light tube, there are also fluorescent ring lights or rectangle area lights. Low costs and a long life-time make fluorescent lights even more attractive. Light Emitting Diodes A LED is a semiconductor device that emits incoherent, monochromatic light with the wavelength depending on the chemical composition of the semiconductor. Today, different wavelengths of the visible spectrum for humans ranging from about 400 to 780nm, as well as ultraviolet or infrared wavelengths, can be covered by LEDs. The emitted visible light appears for example red, green, blue or yellow. Furthermore, it is possible to produce LEDs that appear “white” by combining a blue LED with a yellowish phosphor coating. LEDs have many advantages compared to other light sources. Due to the small size, they can be used for a variety of lighting geometries [18]. This includes ring lights, dome lights, area or line lights, spot lights, dark-field lights and backlights. Theoretically, each single LED in a cluster can be controlled independently. Thus, it is possible to generate different illumination conditions (for example different lighting angles or intensities) with a single setup by enabling and disabling certain LEDs, e.g. automated or software controlled. It is also possible to use LEDs in strobe light mode. Another advantage of LEDs is their energy efficiency and long lifetime with only a little loss in intensity over time. Thus, LEDs have low maintenance costs. Operated at DC power, LEDs do not produce any flickering visible as intensity changes in the video image. Halogen lights Halogen lamps are an extension of light bulbs and filled with a halogen gas (e.g. bromine or iodine). With respect to machine vision applications, halogen lamps are often used in combination with fiber optic light guides [18]. The emitted light of a light source is transferred through this fiber optic light guides, allowing for very flexible illumination setups and geometries. This includes ring lights, dome lights, area or line lights, spot lights, dark-field lights and backlights as for LEDs. Furthermore, there are a range of fiber optic bundles at different sizes to route and position the light for user-defined lighting. One disadvantage of halogen lamps is a large heat development. Thus, usually active cooling is required. Nevertheless, due to the bright “white” light emitted by halogen lights (color temperature of about 6000K), they are also called cold light sources in the literature. If heat development of the light source can be harmful to heat sensitive objects, fiber optics can be useful to keep the light source away from the point of inspection. Like LEDs, halogen lamps do not produce flickering effects, if the light source is DC-regulated. Thus, halogen lamps qualify for high accuracy inspection tasks. Xenon lights, often used for strobe light mode, are quite similar to halogen lamps. These lights allow for very short and bright light pulses, which are used to reduce the effect of motion blur. Besidethedifferentwaysoflightgeneration,therearemultiplepossiblesetupsofhow light sources are arranged. Especially LED lights and fiber optics are very flexible as introduced before. They can be adapted to a wide range of machine vision tasks at almost any size and geometry.
18 CHAPTER 2. TECHNICAL BACKGROUND (a) (b) (c) (d) Figure 2.5: Incident lighting setups. (a) Indirect diffuse illumination over a hemisphere. (b) Diffuse ring- or area light setup. (c) Darkfield illumination. (d) Coaxial illumination. 2.2.2. Incident Lighting Incident lighting or front lighting is characterized by one or more light sources illuminating the object of interest from the cameras viewing direction. This includes diffuse front lighting, directional front lighting, polarized light, axial/in-line illumination and structured lighting [18]. Figure 2.5 gives an overview on different incident lighting setups. Diffuse Lighting Diffuse lighting reduces specular surface reflections and can be seen as a uniform, undirected illumination. It is usually generated by one or more light sources that are placed behind a diffuser at a certain distance. This yields the effect of one uniform area of light. A diffuser can be a pane of white translucent (acrylic) glass, mylar or other synthetic material. Instead of using a diffuser in front of a light source, indirect lighting can also result in diffuse illumination. A simple, but effective method reported in the literature [7] converts a Chinese wok into a hemisphere for diffuse lighting. The inner side of the wok is painted white. The camera can be placed at a hole on the top of the hemisphere (the bottom of the wok). The light sources are arranged in a way they can not directly illuminate the object, but the emitted light is reflected at the white screen inside the hemisphere (see Figure 2.5(a)). A diffuse illumination can also be achieved using a ring or area light source as in Figure 2.5(b) Directional Lighting Directional lighting is achieved by one or more directed light sources at a very low angle of incidence. The main characteristic of such type of illumination is the
Page 1 and 2: Fachbereich Informatik Department o
Page 4: Acknowledgments First of all, I wou
Page 8 and 9: Contents Acknowledgments iii Abstra
Page 10 and 11: Contents ix 5.3.7. TubeLength .....
Page 12 and 13: List of Tables 1.1. Rangeoftubetype
Page 14 and 15: List of Figures 2.1. AccuracyandPre
Page 16 and 17: 1. Introduction Heat shrinkable tub
Page 18 and 19: 1.2. PROBLEM STATEMENT 3 hazards, r
Page 20 and 21: 1.4. RELATED WORK 5 Length [mm] Tol
Page 22 and 23: 1.5. THESIS OUTLINE 7 The vision pa
Page 24 and 25: 2. Technical Background 2.1. Visual
Page 26 and 27: 2.1. VISUAL MEASUREMENTS 11 (a) Fig
Page 28 and 29: 2.1. VISUAL MEASUREMENTS 13 Figure
Page 30 and 31: 2.1. VISUAL MEASUREMENTS 15 � xd
Page 34 and 35: 2.2. ILLUMINATION 19 effect of comp
Page 36 and 37: 2.3. EDGE DETECTION 21 i 2 i 1 0 (a
Page 38 and 39: 2.3. EDGE DETECTION 23 N × 1 kerne
Page 40 and 41: 2.3. EDGE DETECTION 25 These operat
Page 42 and 43: 2.3. EDGE DETECTION 27 (a) 0 ◦ (b
Page 44 and 45: 2.4. TEMPLATE MATCHING 29 accuracy
Page 46 and 47: 3. Hardware Configuration This chap
Page 48 and 49: 3.2. CAMERA SETUP 33 3.2. Camera se
Page 50 and 51: 3.2. CAMERA SETUP 35 eachcolorchann
Page 52 and 53: 3.2. CAMERA SETUP 37 Figure 3.4: Co
Page 54 and 55: 3.2. CAMERA SETUP 39 adjusting scre
Page 56 and 57: 3.2. CAMERA SETUP 41 further distan
Page 58 and 59: 3.3. ILLUMINATION 43 f V 12mm 129mm
Page 60 and 61: 3.3. ILLUMINATION 45 (a) (b) (c) Fi
Page 62 and 63: 3.3. ILLUMINATION 47 (a) (b) Figure
Page 64 and 65: 3.4. BLOW OUT MECHANISM 49 A light
Page 66 and 67: 4. Length Measurement Approach Whil
Page 68 and 69: 4.2. MODEL KNOWLEDGE AND ASSUMPTION
Page 74 and 75: 4.3. CAMERA CALIBRATION 59 4.2.7. B
Page 76 and 77: 4.3. CAMERA CALIBRATION 61 Figure 4
Page 78 and 79: 4.3. CAMERA CALIBRATION 63 is compu
Page 80 and 81: 4.4. TUBE LOCALIZATION 65 (a) (b) F
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4.4. TUBE LOCALIZATION 67 � 1 Psm
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4.4. TUBE LOCALIZATION 69 Global Th
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4.4. TUBE LOCALIZATION 71 but also
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4.4. TUBE LOCALIZATION 73 Global Re
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4.5. MEASURING POINT DETECTION 75 f
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4.5. MEASURING POINT DETECTION 77 T
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4.5. MEASURING POINT DETECTION 79 -
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4.5. MEASURING POINT DETECTION 81 0
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4.5. MEASURING POINT DETECTION 83 o
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4.5. MEASURING POINT DETECTION 85 (
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4.5. MEASURING POINT DETECTION 87 w
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4.6. MEASURING 89 side of the tube,
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4.6. MEASURING 91 fcor(x) =−(c1x
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4.7. TEACH-IN 93 inthemeasuringplan
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4.7. TEACH-IN 95 The pair of a pixe
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5. Results and Evaluation 5.1. Expe
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5.1. EXPERIMENTAL DESIGN 99 between
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5.1. EXPERIMENTAL DESIGN 101 where
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5.1. EXPERIMENTAL DESIGN 103 Ground
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5.2. TEST SCENARIOS 105 enough, the
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5.3. EXPERIMENTAL RESULTS 107 The t
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5.3. EXPERIMENTAL RESULTS 109 Detec
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5.3. EXPERIMENTAL RESULTS 111 Lengt
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5.3. EXPERIMENTAL RESULTS 113 GTD [
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5.3. EXPERIMENTAL RESULTS 115 gray
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5.3. EXPERIMENTAL RESULTS 119 (a) (
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5.3. EXPERIMENTAL RESULTS 121 (a) (
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5.3. EXPERIMENTAL RESULTS 125 Total
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5.3. EXPERIMENTAL RESULTS 127 Color
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5.3. EXPERIMENTAL RESULTS 129 Task
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5.4. DISCUSSION AND FUTURE WORK 131
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6. Conclusion In this thesis a func
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Appendix 135
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138 APPENDIX A. PROFILE ANALYSIS IM
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140 APPENDIX A. PROFILE ANALYSIS IM
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142 APPENDIX B. HARDWARE COMPONENTS
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144 APPENDIX B. HARDWARE COMPONENTS
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146 Bibliography [18] C. Demant, B.
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148 Bibliography [51] K.K. Pingle.
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Master Thesis - Fachbereich Informatik

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