Image Processing Reference
In-Depth Information
2
Image Acquisition
Before any video or image processing can commence an image must be captured by
a camera and converted into a manageable entity. This is the process known as
image
acquisition
. The image acquisition process consists of three steps;
energy
reflected
from the object of interest, an
optical system
which focuses the energy and finally a
sensor
which measures the amount of energy. In Fig.
2.1
the three steps are shown
for the case of an ordinary camera with the sun as the energy source. In this chapter
each of these three steps are described in more detail.
2.1
Energy
In order to capture an image a camera requires some sort of measurable energy. The
energy of interest in this context is light or more generally
electromagnetic waves
.
An electromagnetic (EM) wave can be described as massless entity, a
photon
, whose
electric and magnetic fields vary sinusoidally, hence the name wave. The photon
belongs to the group of fundamental particles and can be described in three different
ways:
•
A photon can be described by its energy
E
, which is measured in electronvolts
[eV]
•
A photon can be described by its frequency
f
, which is measured in Hertz [Hz].
A frequency is the number of cycles or wave-tops in one second
•
A photon can be described by its wavelength
λ
, which is measured in meters [m].
A wavelength is the distance between two wave-tops
The three different notations are connected through the speed of light
c
and
Planck's constant
h
:
·
c
f
,
h
c
=
=
·
⇒
=
λ
E
h
f
E
(2.1)
λ
An EM wave can have different wavelengths (or different energy levels or differ-
ent frequencies). When we talk about all possible wavelengths we denote this as the
EM spectrum
, see Fig.
2.2
.
