Image Processing Reference
In-Depth Information
Fig. 2.1 Overview of the typical image acquisition process, with the sun as light source, a tree as
object and a digital camera to capture the image. An analog camera would use a film where the
digital camera uses a sensor
In order to make the definitions and equations above more understandable, the
EM spectrum is often described using the names of the applications where they are
used in practice. For example, when you listen to FM-radio the music is transmitted
through the air using EM waves around 100
10 6 Hz, hence this part of the EM
spectrum is often denoted “radio”. Other well-known applications are also included
in the figure.
The range from approximately 400-700 nm ( nm
10 9 )isde-
noted the visual spectrum. The EM waves within this range are those your eye (and
most cameras) can detect. This means that the light from the sun (or a lamp) in prin-
ciple is the same as the signal used for transmitting TV, radio or for mobile phones
etc. The only difference, in this context, is the fact that the human eye can sense
EM waves in this range and not the waves used for e.g., radio. Or in other words, if
our eyes were sensitive to EM waves with a frequency around 2
10 9 Hz, then your
mobile phone would work as a flash light, and big antennas would be perceived as
“small suns”. Evolution has (of course) not made the human eye sensitive to such
frequencies but rather to the frequencies of the waves coming from the sun, hence
visible light.
2.1.1 Illumination
To capture an image we need some kind of energy source to illuminate the scene.
In Fig. 2.1 the sun acts as the energy source. Most often we apply visual light, but
other frequencies can also be applied, see Sect. 2.5 .
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