basics of imaging systems lecture 3 prepared by rick lathrop 9/99 revised 9/06
TRANSCRIPT
Basics of Imaging systemsLecture 3
prepared by Rick Lathrop 9/99
revised 9/06
Learning objectives• RSS concepts:
– Basic components/mechanics of image framing vs. scanning systems
– Concept of focal length– Variables affecting image exposure– Image scale and ground coverage and their relationship with
flying height and focal length
• Math concepts:– Scale equation(s) as fundamental to all of geomatics– Basic application of trigonometry
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
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
Components of a framing camera
Graphic from http://library.thinkquest.org/16541/eng/explore/media/photos/camera_diagram.jpg
Single-lens reflex camera
Text and graphics from http://en.wikipedia.org/wiki/Image:Slr-cross-section.png
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
Focal length
o i
Graphic from http://en.wikipedia.org/wiki/Lens_%28optics%29
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
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
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
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
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/
Digital Line Sensing Systems:Leica Airborne Digital Sensor (ADS40)
http://www.gis.leica-geosystems.com/products/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
Compact Airborne Spectrographic Imager (CASI)
• Hyperspectral: 288 channels between 0.4-0.9 m; each channel 0.018m wide
• Spatial resolution depends on flying height of aircraft
For more info: www.itres.com
CASI 550
Most aerial photo mapping missions require overlapped coverage of successive
aerial photos along a flight line
Pushbroom Scanning vs. 2D Framing
Graphics from http://www.gis.leica-geosystems.com/products/documents/ADS40_product_description.pdf
Film Exposure
Graphics from http://www.photoretouchingsecrets.com/imagefiles/
Overexposed Underexposed
Image Exposure
• Exposure, E = s * d2 * t / 4 f2
• whereE = image 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
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
Image exposure exampleCase 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
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
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
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
F/STOPEach 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
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
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
Example: F/Stop effect on depth of field
F/22
F/8
F/4
http://en.wikipedia.org/wiki/Depth_of_field
F/STOP increases, Depth of Field increases
The range of distance over which objects are in focus
Remote Sensing Platforms• Geostationary
• Polar orbit
• manned space
• High altitude aircraft (U-2)
• Jets
• low alt. aircraft
• Platforms
• In-situ/ground
36,000km
900 km
200-300 km
90,000 ft
10-30,000 ft
500-10,000 ft
10-100 ft
0-5 ft
Image Scale
• Scale = f /H’ = d/D • where
f = focal lengthH’ = height above terraind = image distanceD = ground distanceh = terrain elevationH = flying height (h + H’)
H’
f
D
d
h
H
Example: Image Scale vs. flying height example
• 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
If I want a ground coverage of 5km, what flying height should I use?
Example: Image Scale vs. flying height example
• Scale = 1 /RFd = 1/50,000 = f /H’ • where
f = 152 mm
H’ = ?
H’ = f x RFd = 152mm x 50,000 = 7600m 1 1
If I want a scale of 1/50,000, what flying height should I use?
Effect of flying height on ground coverage
x
Adapted from Lillesand & Kiefer, 2nd edition
H’1
H’2
H’1 > H’2
D1 > D2
D2
D1
Effect of focal length on ground coverage
x
Adapted from Lillesand & Kiefer, 2nd edition
H’1
f1 > f2
D1 < D2
f1
f2
D1
D2
Ground Coverage
• Ground coverage, D, of photo frame varies with f and H’
• as f decreases, ground coverage increasese.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
Ground Coverage example
Case 1 Case 2 film size = 9.0” = 230mm film size = 9.0” f1 = 210 mm f2 = 152 mm H’ = 12,200 m H’ = 12,200 m Scale = ? Scale = ? D = ? D = ?
Ground Coverage example
Case 1 film size = 9.0” = 230mm
f1 = 210 mm H’ = 12,200 m Scale = f / H’ = 210mm / 12,200m = .210m / 12,200m = 1 / 58,000 Scale = d / D D = d * Scale RFd D = 230mm x 58,000 = 13340000 mm = 13.34 km
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
Ground Coverage for Scanning Systems
• W = 2 H’ tan where W = swath width
H’ = flying height above terrain
= one half total field of view of scanner
H’
W
Hint: remember your trigonometry
Tangent of a right angleopposite
adjacent
Opposite = tan * adjacent
opp
adj
Ground Coverage for Scanning Systems
• W = 2 H’ tan • Example: Leica ADS40 = 64o
if H’ = 2880 m
W = 2 * 2880m * tan32o = 3600m
H’
W
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?
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 • W = 2 * 450km * tan (1.06o)
= 900km * 0.0185
• W = 16.65km