Image Formation Optics and Imagers Real world Optics
- Slides: 36
Image Formation: Optics and Imagers Real world Optics Sensor Acknowledgment: some figures by B. Curless, E. Hecht, W. J. Smith, B. K. P. Horn, and A. Theuwisse
Optics • Pinhole camera • Lenses • Focus, aperture, distortion
Pinhole Camera • “Camera obscura” – known since antiquity Image plane Image Object Pinhole camera
Pinhole Camera • “Camera obscura” – known since antiquity Image plane Image Object Pinhole camera • First recording in 1826 onto a pewter plate (by Joseph Nicéphore Niepce)
Pinhole Camera Limitations • Aperture too big: blurry image • Aperture too small: requires long exposure or high intensity • Aperture much too small: diffraction through pinhole blurry image
Lenses • Focus a bundle of rays from a scene point onto a single point on the imager • Result: can make aperture bigger
Ideal Lenses • Thin-lens approximation • Gaussian lens law: 1/do + 1/di = 1/f • Real lenses and systems of lenses may be approximated by thin lenses if only paraxial rays (near the optical axis) are considered
Camera Adjustments • Focus? – Changes di • Zoom? – Changes f • Iris? – Changes aperture
Focus and Depth of Field • For a given di, “perfect” focus at only one do • In practice, focus is OK for some range of depths – Circle of confusion smaller than a pixel • Better depth of field with smaller apertures – Better approximation to pinhole camera
Field of View • Q: What does field of view of camera depend on? – Focal length of lens – Size of imager – Object distance?
Computing Field of View 1/do + 1/di = 1/f tan /2 = ½ xo / do xo xi xo / do = xi / di = 2 tan-1 ½ xi (1/f 1/do) do di Since typically do >> f, 2 tan-1 ½ xi / f
Aperture • Controls amount of light • Affects depth of field • Affects distortion (since thin-lens approximation is better near center of lens)
Aperture • Aperture typically given as “f-number” (also “f-stops” or just “stops”) • What is f /4? – Aperture is ¼ the focal length
Monochromatic Aberrations • Real lenses do not follow thin lens approximation because surfaces are spherical (manufacturing constraints) • Result: thin-lens approximation only valid iff sin
Distortion • Pincushion or barrel radial distortion • Varies with placement of aperture
Distortion • Varies with placement of aperture
Distortion • Varies with placement of aperture
Distortion • Varies with placement of aperture
First-Order Radial Distortion • Goal: mathematical formula for distortion • If distortion is small, can be approximated by “first-order” formula: r’ = r (1 + r 2) r = ideal distance to center of image r’ = distorted distance to center of image • Why (1 + r 2) and not (1 + r)?
Chromatic Aberration • Due to dispersion in glass (focal length varies with the wavelength of light) • Result: color fringes near edges of image • Correct by building lens systems with multiple kinds of glass
Correcting for Aberrations • High-quality compound lenses use multiple lens elements to “cancel out” distortion and aberration • Often 5 -10 elements, more for extreme wide angle
Sensors • Film • Vidicon • CCD • CMOS
Vidicon • Best-known in family of “photoconductive video cameras” • Basically television in reverse ++++ Scanning Electron Beam Electron Gun Lens System Photoconductive Plate
MOS Capacitors • MOS = Metal Oxide Semiconductor Gate (wire) Si. O 2 (insulator) p-type silicon
MOS Capacitors • Voltage applied to gate repels positive “holes” in the semiconductor +10 V ++++++ Depletion region (electron “bucket”)
MOS Capacitors • Photon striking the material creates electron-hole pair +10 V Photon ++++++ +
Charge Transfer • Can move charge from one bucket to another by manipulating voltages
CCD Architectures • Linear arrays • 2 D arrays
Linear CCD • Accumulate photons, then clock them out • To prevent smear: first move charge to opaque region, then clock it out
Full-Frame CCD • Other arrangements to minimize smear
CMOS Imagers • Recently, can manufacture chips that combine photosensitive elements and processing elements • Benefits: – Partial readout – Signal processing – Eliminate some supporting chips low cost
Color • 3 -chip vs. 1 -chip: quality vs. cost
Video • Depending on the scene, pictures updated at 15– 70 Hz. perceived as “continuous” • Most video cameras use a shutter, so they are capturing for only part of a frame – Short shutter: less light, have to open aperture – Long shutter: more light, but motion blur • Television uses interlaced video
Interlacing These rows transmitted first These rows transmitted 1/60 sec later
Television • US: NTSC standard – Fields are 1/60 sec. – 2 fields = 1 frames are 1/30 sec. – Each frame has 525 scanlines, of which approximately 480 are visible – No discrete pixels along scanlines, but if pixels were square, there would be about 640 visible
Television • NTSC standard – Thus, an NTSC frame is about 640 480 – Color at lower resolution than intensity • PAL standard – Lower rate: fields at 50 Hz. (frames at 25 Hz. ) – Higher resolution: about 768 576
- Difference between ray optics and wave optics
- Reflection and refraction venn diagram
- Steps of image processing
- Real vs virtual image
- Virtual and real images
- Convex lens examples in daily life
- Real life problems polynomials
- Real world vs digital world
- What is world of forms
- Formation initiale vs formation continue
- Geometric and photometric image formation
- The most likely cause
- Paraxial
- Geometry of image formation
- Formation of latent image in radiography
- Image formation in mri
- Image formation in camera
- Concave lens image formation
- A model of destination image formation
- Gurney mott theory radiography
- Sar image formation
- Digitize jeol sem
- Formation of image through narrow holes
- The principles
- Image formation outline
- Fundamentals of image formation
- Photometric image formation
- Fundamentals of image formation
- Image formation computer vision
- Formation d'image
- What is lens
- Concave lens ray diagram
- Long-sightedness lens
- Convex lens characteristics
- Retina image formation
- Physics 11-06 image formation by mirrors
- Analog image and digital image