Digital sensors

Digital sensors
(T.Schenk § 7)
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DIGITAL PHOTOGRAMMETRIC SYSTEM
SISTEMA FOTOGRAMMETRICO
DIGITALE
UNITA' DI
UNITA' DI
ACQUISIZIONE
RESTITUZIONE
DATA CAPTURE UNIT yields the digital image so that the right 2D fiducial coordinates is
associated to each radiometric (discretized) value, passing through pixel coordinates,
corrected for systematic errors (optical distorsions, calibration ….)
RESTITUTION UNIT allows the operator to handle all the digital images,
performing all photogrammetric operations (I.O., relative and absolute orientation,
stereoplotting…..)
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DATA CAPTURE UNIT
The digital images used in a photogrammetric process can be
obtained in TWO WAYS :
DIRECT
• images directly taken in numeric form with digital
cameras;
INDIRECT
•
traditional photograms, digitized through suitable
devices
ACQUISITION
Summary of photogram characteristics:
1. format: 23 cm x 23 cm (in satellites also 12” x 18 ”). The whole
film roll is often scanned
2. very high geometric resolution and accuracy : ∼10 micron of
pixel size and 2 micron of positioning accuracy for A.T.
3. high radiometric resolution: 10 or 12 bits (internal, output 8bit)
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DENSITOMETER
It
transforms
images
analogue to digital
from
It is an instrument built for
INDIRECT image data capture
made of:
• a calibrated source of light;
• a plane surface where the photogram is placed, with a small slit where the
source of light is;
• a sensor receiving the light after being passed through the film;
• filtres to separate three colour layers (for colour or I.R. films);
•
analogue/digital converter to give digital output data;
•
a display to show the measure result (i.e. the local density);
•
a recording system.
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The light, with known and constant intensity, is
attenuated through the film, the more where the film
is darker.
The light that arrives ton the sensor produces a
current, proportional to the light intensity
The unit area of a pixel is a square with variable side, according the desired
geometric resolution, and of capability of the densitometer (over 5000dpi)
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SCANNER
Two types of construction choices : different moving engine
• Flatbed Scanner: the sensors move on the image area
• Drum Scanner (a tamburo rotante ) : the image moves too, on a cylindric support.
Flatbed scanners use solid state sensors
Drum scanners use photo-multiplier (photodiode)
Common characteristic: a sensor detects the light, coming from a calibrated source, that
pass through a small region of the film.
Drum scanners are difficult to calibrate because of cylindric support
Flatbed scanners are more used
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FLATBED SCANNER
According to resolution and accuracies, scanners can be classified as:
Scanner
Format
cmxcm
(UNI)
Geom.
resolution
(DPI)
Radiom.
resolution
(bit)
Usage
DTP
21x30
42x30
300-1200
8-36
Desktop
publishing
Photogram.
26x26
1200-4096
8-24
Photogram
scanning
Photogrammetric scanners, have very high geometric resolution and accuracy
(typically 2-5 µm);
There is often a sw for I.O. and fiducial marks collection
The scanning time is usually long (from 15' to 45' for geometric resolution of 15 µm);
disadvantages : high costs and complex sw.
Costs: 500-5000€ for DTP, up to 150.000 € - 200.000 € for Photogrammetric.
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LH-SYSTEM DSW300
Basic Technology
Geometric Resolution
Geometric Accuracy
Scanning Format
CCD sensor
Acquiring pixel dimension
Optical resolution
Dimensions
weight
Operatine system
Software
Storage formats
Acquiring times
Fixed CCD and moving
photogram
0.5 µm
< 2 µm
260 x 260 mm2
1024x1536,
2029x2044,2056x3072
9µm
33 lp/mm, 40 lp/mm, 100 lp/mm
1238 mm x 1003 mm x 1175
mm
150 kg
PC WINDOWS
Geometric and radiometric
calibration, automatic inner
orientation
TIFF, JPEG
4 minutes in gray tones (12.5
µm) – 9.5 minutes in RGB
INTERGRAPH PhotoScan TD
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DTP SCANNERS
DeskTop Publishing scanners have not
developed for photogrammetric applications
been
They have lower costs (500, 5000€) and widespread
diffusion and their capabilities are always increasing
DTP are usually flatbed scanners and use line sensor CCD (see later)
Their geometric and radiometric resolutions are always increasing (nowaday at
least 1200 dpi, up to 24 bit)
The main drawback is the poor geometric accuracy, due mainly to mechanics,
lens distorsions, and lack of calibration sw
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DeskTop Publishing scanners can be used for photograms scanning only after a
careful calibration procedure, both geometric and radiometric
This is done using a precision grid as in traditional stereoplotter.
The grid is scanned, with the desired resolution , and the digital position of the crosses
are compared with the calibrated ones.
This comparison gives a calibration model to be applied to all photograms scanned.
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Three different pattern for the assembling of photosensitive sensors:
• single sensor, acquires the image (often placed on a rotating drum), pixel by pixel, line by
line;
• line array of sensors, acquires the image line after line.
Ex: PhotoScan PSi (Zeiss/Intergraph) has a line sensor with 2048 single sensors, with pixel
size of 7.5 µm
• matrix array of sensors (frame), acquires the image divided in regions. Single region are
numerically connected using a grid of crosses with known coordinates as reference points.
Linear sensors are more used because they show higher sensitivity, up to five
times the frame sensors
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every scanner presents radiometric anomalies that produces local lack of uniformity,
sometimes corrected with a local low-pass filtering
Drum scanners have more geometric accuracies problems, due to the various
moving parts.
But : for the highest geometric resolution drum scanner are used
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PHOTO-SENSORS
The role of the sensor is to ‘measure’ the film density
Two type of sensors are mainly used:
photomultiplier and solid state sensors
Photomultipliers are based on external photoelectric effect: when the light hits the cathode,
some electrons come out and can be attracted and captured from the anode, and a current
can be measured.
To increase sensitivity, the electron flux is amplified with an auxiliary cathode, therefore are
called photomultiplier.
They have very quick time response, and give the highest geometric resolutions, but cannot
use linear or frame array.
Semiconductors work with the internal photoelectric effect :
Light photons that hit a semiconductor produces pairs of negative charges (free electrons)
and positive charges (‘holes’) that can travel inside the semiconductor if a potential
difference is applied, thus producing a detectable current.
The photosensitive elements are coupled with electronic devices for the transferring and
the measure of the current CCD (Charge Coupled Devices= dispositivi a carica accoppiata).
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CCD sensors have different sensitivity in the
three color band R G and B, therefore there
are problems in alignment of the 13
three color
matrix, (lower sens.to blue light)
LIGHT SYSTEM IN A SCANNER
Direct light
Diffuse light
Direct light : a light amplifier projects the light over the whole image surface
Advantages :
- energetically cheap (lamps with low heat radiation are used, also with optical fibers)
- high field of depth (direct light has a small aperture angle)
- low sensitivity to small blurring effect
BUT : some diffraction can be seen
Diffuse light: it is obtained interposing a diffusing ground glass
The best photogrammetric scanner make use of diffuse light, because it gives best
image quality: on the image are not projected possible dust particles or scratches,
Furthermore, diffuse light produces less noise compared with direct light.
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OTHER COMPONENTS
Photo carrier.
Usually is a modified version of a stereoplotter photo-carrier, therefore it has the same
geometric accuracy : about 2 micron.
Max scanning speed is around 10-20 mm/sec, and must be constant to have a uniform
pixel size.
Images are acquired in portions, called swath, with amplitudes given by sensor
dimension (multiplied by the optical enlargement).
To avoid overlapping or shift between subsequent swaths, the positioning must be
very accurate
adjacent swaths that are not in line, or overlap or
separate, cause errors in digital images
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Different pixel size …..
Pixel size along the sensor line depends on sensor size and optical enlargement.
Pixel size in scanning direction depends on scanning speed and integration
interval.
Furthermore, output pixel size (usually square) is the result of some resampling
passage.
Pixel size of the internal sensor
between 10 and 15 micron
Scanning pixel size : projection of the pixel on the film
It is the direct (raw) output of the scanner, and depends on the optical system ,
the speed, and the integration time
‘Refined’ pixel size : final result of scanning and postprocessing
sometime is the same of raw scanning pixel size
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Geometric resolution, sampling and CCD
The sampling interval have not to be confused with the pixel size
In an ideal sampling process, they should be identical
FIGURA
Ideal case in which pixels are clearly separated and
never overlap over the same image portion
In some instrument the sampling interval is much smaller than the sensor size, in
this case pixels overlaps and blurring and smoothing effects appear.
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DIRECT ACQUISITION
Digital Cameras
The image plane contains a 2-dimensional field of sensors (frame array)
The charged state of sensors (analogical signal) is read at given times.
The analogical signals are converted into digital signals.
The most used digital cameras use solid state sensors, usually CCD
There are also cameras based on ‘photoconductor tubes’ “vidicons” (the name of most
used device).
First CCD have been invented in 1970, the first sensor had 96 pixels.
In '80 sensor arrived to have 500x500 pixel element.
Nowaday there exist sensors with more than 5000x5000 elements (4096x4096 are
in a chip built by Ford-Aerospace, on board of espace telescope Hubble).
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Solid State Cameras
They have spectral flexibility and geometrical resolution always increasing.
One of the main defect was geometric resolution, the other is the reduced field of view
Terrestrial digital cameras
Frame sensors
Aerial digital cameras
Linear sensors (not only)
The main advantage (compared with film cameras) consists in the prompt availability
of the images, for processing and analysis, and this also allows the development of
‘real time photogrammetry’, for instance in mobile mapping vehicle or machine vision
(time delay between image capture and analysis : 20 msec)
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Basic working principle
The semiconductor chip is made of a big number
of small light sensitive elements: each element
(Sensor ELement SEL) usually rectangular, is only
few µm wide, and can record the light intensity
hitting its surface.
The sensor is attached on the image plane,
with elements (SEL) in linear or areal pattern
It is usually protected by glass or micro-lenses,
and can be fixed (conventional cameras) or can
be moved in the focal plane, to increase
resolution.
Schematic diagram of the basic element of a CCD :
a capacitor (condensatore) and a semiconductor
(usually silicon). The metal electrodes are divided
by a insulating substance (an oxide)
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When Electro-Magnetic Radiation (EMR) hits the element, photons with energy higher than the
semiconductor energetic gap can be absorbed into the ‘depletion region’ (regione di
svuotamento) creating pairs electron-hole.
By applying a positive voltage to the electrode, electrons are forced to stay in the depletion
region, while the holes (positive charges) move toward the ‘electric ground’ (fondo).
The result is a charge collection at the opposite sides of the insulating region, and charges can
be collected and measured
The actual charge is proportional to the number of absorbed photons, therefore the total
charge in each cell is proportional to radiation intensity and exposure time
The band gap energy of silicon corresponds to photons with wavelength of 1,1 µm
Higher energy photons (shorter wavelength) can be absorbed out of the depletion region; lower
energy photons do not produce electron-holes pair.
some photons do not produce current, the efficiency is less than 1
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One frame = one pixel matrix monochromatic image
Color (RGB) images ??
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An alternative solution is represented by the new sensors FOVEON
Three different layers of sensors, that work also as filters, detect independently
the three primary colours.
The final image is generated with information coming from all three layers
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Charge transfer
A CCD is structured as a charge transfer
device (CTD)
A sequence of voltage pulses moves the
charges from one sensor to the adjacent,
mantaining the ordering.
When the charge under one sensor reaches the end of the line,
it is drained, i.e. collected and measured.
The original position of cumulated charge, depends on the number of pulses used,
and can be stored together with the charge information pixel position
Furthermore : depending on the pulse phases (2, 3 or 4), the number of charged elements
drained can be changed one charge measure is the sum (integration) of more elements
charge
Therefore the single image element, the PIXEL, consists of the area covered by the
number of sensors drained.
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Charge transfer
Different techniques have been developed for a fast and precise transfer of the charge
According to the different organization of
drainage for signal transfer CCD are
classified in :
• Interline Transfer (IT)
• Frame Transfer (FT)
• Linear arrays with bilinear readout
• Time Delay and Integration transfer
(TDI)
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CMOS
Complementary Metal Oxide Semiconductor
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Time Delay and Integration transfer (TDI)
This kind of charge transfer subsitutes the FMC device.
Instead of applying a displacement to the image plane (like in
aerial film camera) the image motion is compensated by a charge
transfer.
During the integration time, the charge is moved from one pixel (or
colums) to the adjacent, where the integration continues.
This is equivalent to the motion of the sensor during exposure time.
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DIGITAL AIRBORNE CAMERAS
The ground projection of an array pixel is called IFOV
(Istantaneous Field Of View).
IFOV, integration time and charge transfer time define the
acquisition geometry in terms of effective ground pixel
dimensions
if the charge transfer interval is very short the acquisition
can be considered as continuous.
BUT if speed or direction of flight change the images can be
distorted
With LINEAR ARRAYS the main problem is the Exterior
Orientation determination: each line has to be oriented !!
For instance 300 O.E. per second !!
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It is possible to mount many sensors for the simultaneous acquisition
of images at different inclinations
In this way it is possible to have different independent stripes that
overlap, thus obtaining a sort of stereoscopic pair (images have to be
‘registered’ i.e. relatively oriented)
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DIGITAL AIRBORNE CAMERAS
• Camera LH-System
ADS40
• 3 linear sensors (similar
to satellite ones)
• Each linear image is
oriented by an integrated
IMU system (GPS/INS)
Line
• Simultaneous capture of
stereoscopic images
• Camera Z/I Imaging
DMC
• 4 linear CCD frame
sensors, in central
perspective
• The four images are
mosaiked into a single
image
Multi-cone
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Airborne Digital Scanner (ADS40)
• DLR e LH-Systems
• 1 lens, 3 directions of view
• along-track stereo
• airborne with GPS/INS
• different line CCD configurations in focal
plane
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In the focal plane three panchromatic lines and other
multispectral lines are mounted, with one unique optics.
ADS40 uses 8 parallel lines of sensors
The three color lines (with 12000 pixels each) are
optically overlapped during the flight by using mirrors,
(near IR is displaced)
Stereopairs are obtained taking stripes with
three different angle of view: nadir, backward,
forward
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mono-CCD multicamera.
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World-wide market distribution
Airborne mapping cameras (status Nov 2005)
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World-wide market distribution
Airborne mapping cameras (forecast 2010)
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SATELLITE PHOTOGRAMMETRY
DIFFERENCES WITH AERIAL PHOTOGRAMMETRY
- DIFFERENT SENSORS
- DIFFERENT PROCESSING
- DIFFERENT RESOLUTIONS
NOW AERIAL AND SATELLITE IMAGES CAN BE
USED FOR SIMILAR PURPOSES
-> SIMILAR SENSORS AND GEOMETRY
-> HIGH RESOLUTION
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IKONOS
Agency:SpaceImaging (USA)
Launch: December 1999
Orbit: sun-synchronous, quasi polar, height 680km
Resolution: 1m PAN, 4m MS
Stripe width: 11 km
Rivisiting time: 2-3 days
QUICKBIRD
Agency:EarthWatch, Digital Globe (USA)
Launch: October 2001
Orbit: sun-synchronous, quasi polar, height 600km
Resolution :
0.8m PAN, 3.2m (MS)
Stripe width : 22 km
Rivisiting time: 1-5 days
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EROS
Agency: ImageSat (Israele)
Launch: 5 December 2000
Orbit: sun-synchronous, quasi polar, height 480km
Resolution:
1.8m PAN (0.5 -0.9 m)
Stripes width : 12.5 km
Rivisiting time: 1 day
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HOW TO HAVE STEREOPAIRS ??
TWO SOLUTIONS : CROSS TRACK e ALONG TRACK
CROSS TRACK
CCD
mirror
filter
1 - A single line of CCD (one or
more segments) combined with a
rotating mirror
Fl
igh
td
ire
cti
on
focusing system
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CROSS TRACK - 2
- A single track is acquired
- Stereo pairs come from stripes acquired in two different orbits
Flight direction
Orbit 2
Nadir
Orbit 1
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ALONG TRACK
1 - Stereo images acquired in the same orbit , with small time delay
(seconds)
Orbit
wi
ng
Time t3
v ie
vie
d
wa
r
rd
wa
P
ng
wi
Ba
ck
r
Fo
Nadir viewing
Time t2
Time t1
P
- Same geometry of AIRBORNE digital cameras
- Optical system: ONE or MORE lenses
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ALONG TRACK - 2
- Sensori Multi Lines
- CCD lines simultaneously acquire images in various directions (nadir,
off-nadir)
- CCD lines are usually parallel and orthogonal to flight direction
Direzione di volo
Direzione di volo
Nadir
rd
Ba
ck
αF
wa
For
wa
rd
αB
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ALONG TRACK - 3
- Possibility of rotating the view
-
syncron acquisition
or
asyncron
Flight direction
Costant angle of view during
image acquisition
Flight direction
Variable angle of view during
image acquisition
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ORIENTATION OF LINEAR CCD
• Image is formed by a sequence of lines acquired independently
Flight direction
IMAGE FORMATION
• FOR EACH LINE 6 E.O. parameters are UNKNOWN !!!
Traditional orientation is not feasible
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Orientation
=
Geometrical correction of high
resolution satellite imagery
Rigorous models
Non rigorous models
Acquisition geometry reconstruction
Co-linearity equations
Effemerides
Angular attitude
Focal, FOV
Atmospheric refraction
Black box models =
generalized models nondependent on acquisition
geometry
Polinomial function
DLT
Rational Function Model – RFM
Neural Net
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