Imaging
Chapter 2,
Date 07/16/04
Imaging is the process of acquiring images, the process of sensing our surroundings and
then representing the measurements that are made in the form of an image.
We have seen , that all regions of the EM spectrum are suited to imaging.
1. Passive and active imaging
Passive imaging employs energy sources that are already present in the scene, whereas
active imaging involves the use of artificial energy sources to probe our surroundings.
Passive imaging is subject to the limitation of existing energy sources: The Sun, for
example , is a convenient source of illumination. Active imaging is not restricted in this
way, but it is more complicated and expensive procedure, since we must supply and
control a source of radiation in addition to an imaging instrument.
In remote sensing, we are predominately interested in energy that has been reflected,
scattered, or emitted.
This reflected/scattered/emitted energy can then be measured using various kinds of
remote sensing instruments. Thankfully, many earth materials have very unique spectral
signatures, almost like fingerprints.
1. 1 REMOTE SENSING INSTRUMENTS -- TYPES, CHARACTERISTICS, AND GENERAL USES
We measure earth materials remotely using many different classes or kinds of
instruments or sensors. The first distinction is between ACTIVE and PASSIVE sensors.
PASSIVE VS. ACTIVE SENSORS
What are they?
Passive sensors detect naturally reflected or radiated energy. Active sensors supply, or
send out, their own electromagnetic energy and then record what comes back to them. An
example of a passive remote sensing satellite is Space Imaging's IKONOS. A common
type of active remote sensing is radar.
Passive systems such as IKONOS have similar terrain interactions, but the sun acts as a
natural illumination source.
Typical Sensors:
Active: RADARSAT, SRTM, ERS, SIR-C
Passive: Landsat, IKONOS, HyMap,
For more information please see:
http://www.es.ucsc.edu/~hyperwww/chevron/Ikonos
2. Camera optics
A camera uses a lens to focus part of the visual environment onto a sensor.
2.1 Image Formation by a Converging Lens
The above Figure shows: two arrows, a converging lens, and rays of light being
emmitted by the red arrow. The red arrow is the object, while the green arrow is the
image that results after the rays have passed through the lens. The Figure also displays
two focus shown as blue dots.
The image formed by a converging lens can be made using only three principal rays.
going through the lens it
passes through the focal point.
through the lens. Thus any point on the object can be mapped, using the rays above, into
a corresponding point on the image. This point is located on the intersection of the rays.
The most important characteristics of a lens are its magnifying power and its light
gathering capacity.
2.2 Magnification factor is
m
image size
object size
By similar triangles , we can also say that
image size
v

u object size
where u is the distance from an object to the lens and v is the distance from the lens to the
image plane. Hence
v
( 1)
u
It is usual to express the magnifying power of a lens in terms of its focal length, f, the
distance from the lens to the point at which parallel incident rays converge.
Focal length is given by the lens equation
1 1 1
  (2)
f u v
m
From (1) and (2)
um
f 
m 1
2.3 The light gathering capacity of a camera lens is determined by its aperture.
This can be no larger than the diameter of the lens itself, and it is usually made smaller
this by means of a diaphragm – a circular hole of adjustable size, incorporated into the
lens. It is normal to express the aperture of a lens as an “f-number”- focal length is
divided by aperture diameter
F-number
A measure of the relative aperture of a lens; its light gathering capacity. It is equal to the
ratio of the focal length of the lens divided by the diameter of its limiting opening
(aperture): f-number = focal length/iris diameter. Note that the number becomes smaller
as the aperture grows larger and that it must be squared to directly measure the area
which is the light gathering capacity. Most lenses offer a sequence of fixed apertures
(f2.8, f4, f8, f11)
2.4 All lenses suffer from defects or aberrations, which can affect image quality
2.4.1 Spherical Aberration
Any spherical mirror or lens will
bring to a focus only those light rays
that emanate from the radius of
curvature.
Light from a distance source,
which is essentially parallel, will
not come to a precise focus.
Geometric distortion may also be a problem, particularly for lenses with small focal
lengths.
The effects of aberrations can be reduced by making the lens aperture as small as
possible.
3. CCD sensors
In recent years, the silicon charge-coupled device (CCD) has revolutionized both home
and studio video recording. The CCD is a solid-state chip that turns light into electric
signals.

rectangular array of photodetector sites built into silicon . A CCD comprises
an array of discrete imaging elements ( photodetector sites) , or photosites.
When light falls on a CCD , each photosite accumulates an amount of electric
charge proportional to the illumination time and the intensity of incident
illumination. A photosite has a finite capacity of about 106 charge carriers,
which places an upper limit on the brightness of objects to be imaged
 free of geometric distortion. CCD’s have become the sensor of choice in
imaging applications because they do not suffer from geometric distortion
 heat can generate thermal electrons – dark current
 Figure shows the architecture of a simple “full-frame “CCD . In order to
retrieve image data , the accumulated charge must be shifted from the
photosites in the imaging area into an output register, and thence to an
amplifier that outputs a serial video signal.
 accumulated charge is shifted out
 full-frame – shifts out one row of pixels at a time
 frame – transfer – entire contents of the imaging area are shifted into a storage
buffer
• Charge-coupled device (CCD) - CONCLUSION
Charge coupled devices rely on the chemical properties of semiconductors for their
operation. Photons of certain energies strike a layer of silicon freeing an electron from the
lattice. The CCD then collects and counts these free electrons.
CCDs are being used in a wide variety of places for a wide variety of purposes. They are
being used as high precision scientific instruments as well as low cost commercial
products. This versatility accounts for the speed at which charge coupled devices are
being adopted in the marketplace.