16421: Vision Sensors Lecture 3: Optical Elements of Imaging Instructor: S. Narasimhan Wean 5310, T-R 1:30pm – 2:50pm.
Download ReportTranscript 16421: Vision Sensors Lecture 3: Optical Elements of Imaging Instructor: S. Narasimhan Wean 5310, T-R 1:30pm – 2:50pm.
16421: Vision Sensors Lecture 3: Optical Elements of Imaging Instructor: S. Narasimhan Wean 5310, T-R 1:30pm – 2:50pm Lenses Types of Lenses [Wikipedia] Bi-convex Lens [Wikipedia] Bi-concave Lens [Wikipedia] Lensmaker’s Equation f is the focal length of the lens, n is the refractive index of the lens material, R1 is the radius of curvature of the lens surface closest to the light source, R2 is the radius of curvature of the lens surface farthest from the light source, d is the thickness of the lens (the distance along the lens axis between the two surface vertices). Optics of a Two Lens System d object final image f2 i2 o2 i1 f1 o1 image plane lens 2 intermediate virtual image lens 1 • Rule : Image formed by first lens is the object for the second lens. • Main Rays : Ray passing through focus emerges parallel to optical axis. Ray through optical center passes un-deviated. • Magnification: i2 i1 m o2 o1 Exercises: What is the combined focal length of the system? What is the combined focal length if d = 0? Changing Diameter of Collimated Beam Fanning a Laser beam to a sheet Used in Range Scanners Cylindrical Lenses Telecentric Lenses Object-side and Image-side telecentricity: Aperture at the focal point of both lens systems • Sizes of object and image do not change as they are translated. • However, focus does change as in any lens. Eliminating Perspective Distortion Regular Lens Telecentric Lens Illusions with Telecentric Lenses Relay Lenses (1:1 imaging) Wide angle Lenses Circular Fisheye Full Frame Rectangular Fisheye Common lens related Issues Vignetting L3 L2 B L1 A More light passes through lens L3 for scene point A than scene point B Results in spatially non-uniform brightness (in the periphery of the image) Lens Vignetting • Usually brighter at the center and darker at the periphery. Reading: http://www.dpreview.com Vignetting photo by Robert Johnes Chromatic Aberration longitudinal chromatic aberration (axial) transverse chromatic aberration (lateral) Chromatic Aberrations longitudinal chromatic aberration (axial) transverse chromatic aberration (lateral) Chromatic Abberations Reading: http://www.dpreview.com Lens Glare • Stray interreflections of light within the optical lens system. • Happens when very bright sources are present in the scene. Reading: http://www.dpreview.com Geometric Lens Distortions Radial distortion Tangential distortion Photo by Helmut Dersch Both due to lens imperfection Rectify with geometric camera calibration Radial Lens Distortions No Distortion Barrel Distortion • Radial distance from Image Center: ru = rd + k1 rd3 Pincushion Distortion Correcting Radial Lens Distortions Before After http://www.grasshopperonline.com/barrel_distortion_correction_software.html Our Eyes Iris Pupil Sclera Cornea Index of refraction: cornea 1.376, aqueous 1.336, lens 1.406-1.386 Iris is the diaphragm that changes the aperture (pupil) Retina is the sensor where the fovea has the highest resolution Accommodation shorter focal length Changes the focal length of the lens Myopia and Hyperopia (myopia) Astigmatism The cornea is distorted causing images to be un-focused on the retina. Blind Spot in Eye Close your right eye and look directly at the “+” Eyes in Nature Mosquito http://ebiomedia.com/gall/eyes/octopus-insect.html Mosquitos have microscopic vision, but to focus at large distances the eye would need to be 1 m! Curved Mirrors in Scallop Eyes Telescopic Eye (by Mike Land, Sussex) … More in the second part of the course Filters Neutral Density Filters • Spectrally flat from 400-700nm • Homogeneous Glass: Blocks by absorption or by reflection • Light/Exposure Control for Imaging • Transmittance = 10^(-optical density) * 100 Variable Neutral Density Filters Variable Neutral Density Filters Color Filters Filter Book Linear Variable Interference Filters Mirrors Cold Mirrors • Reflect VIS • Transmit IR Hot Mirrors • Reflect IR • Transmit VIS Prisms Image Reflection Right Angle Prism Penta Prism Image Rotation Dove Prism Single Lens Reflex Cameras • • What you see is what you get Single Light Pathway for viewfinder and image 1 - Front-mount Lens (4 element Tessar design) 2 - Reflex Mirror at 45 degree angle 3 - Focal Plane Shutter 4 - 35 mm Film or Sensor 5 - Focusing Screen 6 - Condenser Lens 7 - Optical Glass Pentaprism (or Pentamirror) 8 - Eyepiece (can have diopter correction ability) Canon.com