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Basics of Imaging systems

Basics of Imaging systems. Lecture 3 prepared by Rick Lathrop 9/99 revised 9/06. Framing systems Instantaneously acquire an image.

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Basics of Imaging systems

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  1. Basics of Imaging systems Lecture 3 prepared by Rick Lathrop 9/99 revised 9/06

  2. Framing systems Instantaneously acquire an image • Film Camera - uses a lens to form an image at the focal plane. A shutter opens at selected intervals to allow light to enter, where the image is recorded on photographic film or an array of detectors • Digital Camera - type of camera that records an image on an array of photosensitive electronically charged detectors that is recorded on magnetic disk

  3. Components of a framing camera • Lens - function is to gather light directed from the ground scene and bring it into focus at the focal plane • Focal length - the linear distance from the center of the lens to the focal plane • Shutter speed - various times of exposure • Diaphragm - controls the amount of light transmitted to the film when the shutter is open

  4. Components of a framing camera Graphic from http://library.thinkquest.org/16541/eng/explore/media/photos/camera_diagram.jpg

  5. Single-lens reflex camera This cross-section (side-view) of the optical components of an SLR shows how the light passes through the lens (1), is reflected by the mirror (2) and is projected on the matte focusing screen (5). Via a condensing lens (6) and internal reflections in the pentaprism (7) the image appears in the eye piece (8). When an image is taken, the mirror moves in the direction of the arrow, the focal plane shutter (3) opens, and the image is projected in the film(4) in exactly the same manner as on the focusing screen Text and graphics from http://en.wikipedia.org/wiki/Image:Slr-cross-section.png

  6. Focal length o i Graphic from http://en.wikipedia.org/wiki/Lens_%28optics%29

  7. 1/f = 1/o + 1/i • Where f = focal length o = object distance = object to lens i = image distance = lens to focal plane • f is constant, as object distance changes the image distance must change. In aerial photos, o is large, 1/o goes to zero, so i must equal f

  8. Mapping or metric camera • Single lens frame camera • High geometric quality • Film format is 230 mm (~9 in) on a side • Focal length of 152 mm common • Fiducial marks for later registration and defining principal point of the photo Keystone’s Wild RC-10 mapping camera B&W NAPP photo

  9. Large Format Camera (LFC) • Large Format Camera (LFC) flown in Space Shuttle has f = 305 mm and film size of 230x460 mm which resulted in a typical ground dimensions of 225 x 450 km (140 x 280 mi) Photos: NASA

  10. Digital Framing/Scanning Systems • Charge coupled device (CCD): electronic sensor sensitive to a particular wavelength of light, that are generally physically separate on the focal plane • RGB color image generally has separate RGB CCDs • There can be difficulty in spatial co-registering of the different wavebands for the same pixel

  11. Digital Mapping Camera: Zeiss/Intergraph Imaging • 2d CCD matrix (array) to ensure a rigid image geometry similar to a traditional precision film platen • Panchromatic 7000 x 4000 pixels • Color 3000 x 2000 pixels • Separate lens for each band • Multiple smaller camera heads to create image rather than a single, large diameter • 12 bit radiometric resolution http://imgs.intergraph.com/dmc/

  12. Digital Line Sensing Systems:Leica Airborne Digital Sensor (ADS40) • Pushbroom linear array system rather than a 2D framing system • 3 line scanners : forwards, downwards and backwards to provide for stereoscopic coverage • Three CCD sensors: B&W color (RGB) & NIR • 12,000 pixels across • RGB co-registration through special trichroid filter that splits beam from single lens, rather than 3 different lens • Field of View of 64o • Produces up to 100GB of data per hour of flight http://www.gis.leica-geosystems.com/products/ads40/

  13. Compact Airborne Spectrographic Imager (CASI) • Hyperspectral: 288 channels between 0.4-0.9 mm; each channel 0.018mm wide • Spatial resolution depends on flying height of aircraft CASI 550 For more info: www.itres.com

  14. Most aerial photo mapping missions require overlapped coverage of successive aerial photos along a flight line

  15. Pushbroom Scanning vs. 2D Framing Graphics from http://www.gis.leica-geosystems.com/products/documents/ADS40_product_description.pdf

  16. Film Exposure Overexposed Underexposed Graphics from http://www.photoretouchingsecrets.com/imagefiles/

  17. Film Exposure • Exposure, E = s * d2 * t / 4 f2 • where E = film exposure, J mm-2 s = scene brightness, J mm-2 sec-1 d = diameter of lens opening, mm t = exposure time, sec f = focal length of lens, mm

  18. Photo exposure example Case 1 Case 2 f = 40 mm if d = 10 mm d = 5 mm t = ? t = 1/125 s E1 = s1 (d1)2 t1 = s2 (d2)2 t2 = E2 4(f1)2 4(f2)2

  19. Photo exposure example Case 1 Case 2 f = 40 mm if d = 10 mm d = 5 mm t = ? t = 1/125 s E1 = s1* (5)2* 1/125sec = s2* (10)2* t2 = E2 4(40)2 4(40)2 t2 = (5)2* 1/125sec = 25/125 sec = 1 / 500 sec (10)2 100

  20. F/STOP • F/STOP = relative aperture or lens opening • F/STOP = f/d = lens focal length/ lens opening diameter • F/STOP increases, d decreases, E decreases • must change F/STOP and exp. time, t, together. As F/STOP increases, t increases • E = s * t / 4 F2

  21. F/STOP example Case 1 Case 2 F/8 if F/STOP = F/4 t = 1/125 s t = ? E1 = s1 t1 = s2 t2 = E2 4(F1)2 4(F2)2

  22. F/STOP example Case 1 Case 2 F/8 if F/STOP = F/4 t = 1/125 s t = ? E1 = 1/125s = t2 = E2 4(8)2 4(4)2 t2 = 1/125s * (4)2= 16/125 sec = 1/500 sec (8)2 64

  23. F/STOP Each F/STOP changes the amount of light E by a factor of 2 F/STOP Shutter Speed 22 all equal 1/4 2x exp 16 2x exp 1/30 at F/8 1/8 11 1/60 at F/5.6 1/15 8 1/125 at F/4 1/30 5.6 another example 1/60 4 1/125 at F/8 1/125 2.8 1/250 at F/5.6 1/250 2 1/500 at F/4 1/500 1.4 1/1000

  24. F/STOP Interrelationships • Double the focal length, quadruple the time • 1/4 the time, double the diameter or 1/2 F/STOP • double the f, double the diameter

  25. Lens speed • The larger the lens diameter at full aperture, the more light the lens will admit in a given time interval • lens speed = F/STOP at full aperture • the smaller the F/STOP, the faster the lens F/2 has double the aperture diameter as F/4 • fast speed lenses are needed for low light conditions

  26. Example: F/Stop effect on depth of field F/22 The range of distance over which objects are in focus F/8 F/STOP increases, Depth of Field increases F/4 http://en.wikipedia.org/wiki/Depth_of_field

  27. Remote Sensing Platforms 36,000km • Geostationary • Polar orbit • manned space • High altitude aircraft (U-2) • Jets • low alt. aircraft • Platforms • In-situ/ground 900 km 200-300 km 90,000 ft 10-30,000 ft 500-10,000 ft 10-100 ft 0-5 ft

  28. Photographic Scale d • Scale = f /H’ = d/D • where f = focal length H’ = height above terrain d = image distance D = ground distance h = terrain elevation H = flying height (h + H’) f H’ H D h

  29. Example: Photographic Scale vs. flying height example If I want a ground coverage of 5km, what flying height should I use? • Scale = 1 /RFd = f /H’ = d/D • where f = 152 mm D = 5000m d = 230mm H’ = ? H’ = f x D = 152mm x 5000m = 3304 m = 3300m d 230 mm

  30. Example: Photographic Scale vs. flying height example If I want a scale of 1/50,000, what flying height should I use? • Scale = 1 /RFd = 1/50,000 = f /H’ • where f = 152 mm H’ = ? H’ = f x RFd = 152mm x 50,000 = 7600m 1 1

  31. Effect of flying height on ground coverage H’1 > H’2 D1 > D2 H’1 H’2 x D2 D1 Adapted from Lillesand & Kiefer, 2nd edition

  32. Effect of focal length on ground coverage f1 f2 f1 > f2 D1 < D2 H’1 x D1 D2 Adapted from Lillesand & Kiefer, 2nd edition

  33. Ground Coverage • Ground coverage, D, of photo frame varies with f and H’ • as f decreases, ground coverage increases e.g. f1 = 1/2 f2 D1 = 2D2 A1 = 4A2 • as H’ increases, ground coverage increases e.g. H’2 = 2H’1 D2 = 2D1 A2 = 4A1

  34. Ground Coverage example

  35. Ground Coverage example

  36. National High Altitude program (NHAP) • Flying Height, H’ = 12,200 m • color IR camera f = 210 mm scale 1:58,000 area per frame 13.3 x 13.3 km • panchromatic camera f = 152 mm scale 1:80,000 area per frame 18.4 x 18.4 km

  37. Ground Coverage for Scanning Systems • W = 2 H’ tan q/2 where W = swath width H’ = flying height above terrain q/2 = one half total field of view of scanner Hint: remember your trigonometry Tangent of a right angle = opposite adjacent Opposite = tanf * adjacent H’ q f adj W opp

  38. Ground Coverage for Scanning Systems • W = 2 H’ tan q/2 • Example: Leica ADS40 q = 64o if H’ = 2880 m W = 2 * 2880m * tan32o = 3600m H’ q W

  39. Extra Puzzler • The Quickbird satellite is flown at an altitude of 450 km, with a total angular field of view of 2.12o. What is the swath width?

  40. Extra Puzzler • The Quickbird satellite is flown at an altitude of 450 km, with a total angular field of view of 2.12o. What is the swath width? • W = 2 H’ tan q/2 • W = 2 * 450km * tan (1.06o) = 900km * 0.0185 • W = 16.65km

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