iam11_lecture04

E2 21 22 23 24 25 26 27 28 image sensorics 1 12 image

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Unformatted text preview: quantities E1, E2 one has the correspondence: ratio E1 E2 ←→ E1 logarithmic measure 10 lg dB . E2 21 22 23 24 25 26 27 28 Image Sensorics (1) 12 Image Sensorics 34 Analogue Photography 56 Photochemical Processes: These processes dominated the longest time in the history of photography, and still play an important role. They work as follows: 78 9 10 1) Film Emulsion: The active component of most photographic films are emulsions containing grains of silver halides. At grain surface light exposure transforms the silver halide to metallic silver. 11 12 2) Development: Metalising the grains entirely followed by washing out the remaining silver halide results in a negative image with inverse contrast: regions exposed to strongest light are darkest due to the silver. 15 16 3) Photo Creation: Positive images with normal contrast are obtained from negatives by copying to another photographic emulsion (on paper). Modifications of the whole process allow the direct generation of positives. 13 14 17 18 Negative 19 20 21 22 23 24 Positive 25 26 27 28 Image Sensorics (2) Analogue Photography 12 34 History: The most probable inventor of photography is N. Ni´pce (ca. 1827). e The earliest analog photography technique which was in broader use were daguerreotypes (starting 1839, direct positive images on metal surfaces). 56 Problem: Conversion to digital images requires another image acquisition step with other sensors involved (e.g. scanning). 9 10 78 11 12 13 14 15 16 17 18 19 20 From Left to Right: (a) Nic´phore Ni´pce (1765–1833). (b) Louis Daguerre (1787–1851). e e (c) View of Saint-Loup-de-Varennes. Probably the oldest existing photograph (Ni´pce 1827). e (d) Daguerreotype of Edgar Allan Poe, 1848. (Images: Spektrum der Wissenschaft 2/1997 (a, c), Wikipedia (b, d).) 21 22 23 24 25 26 27 28 Image Sensorics (3) 12 Electrophotography 34 Alternative: Another analog process of storing image information is electrophotography, also known as xerography (1938 by C. Carlson). 56 78 • a foil is charged electrostatically 9 10 • light exposure discharges the foil locally 11 12 • remaining charge attracts powder (toner) 13 14 • toner is transferred to film or paper 15 16 • toner is then fixed by melting • used in photocopiers and laser printers Wikipedia Chester Carlson (1906–1968) Attention: Again, from the digital imaging viewpoint, xerography is just an intermediate storage procedure that must precede another imaging step. 17 18 19 20 21 22 23 24 25 26 27 28 Image Sensorics (4) 12 Semiconductor Photosensors 34 Photon Detection 56 • A photon absorbed in a semiconductor pushes one or more electrons from their places in the semiconductor crystal, leading to electron-hole pairs. • The current of generated charge pairs follows the equation e R=η Eλ R e η Eλ responsivity (A/W ) electron charge (e ≈ 1.6 × 10−19C) quantum efficiency (depends on λ) photon energy 78 9 10...
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