Image Acquisition - Introduction to Video and Image Processing - page 9

Fig. 2.4 Backlighting. The light source is behind the object of interest, which makes the object
stand out as a black silhouette. Note that the details inside the object are lost

mation of interest, then this way of illumination—denoted backlighting —can be an
optimal solution, see Fig. 2.4 . Even when the illumination is not directed toward
the camera overly bright spots in the image might still occur. These are known as
highlights and are often a result of a shiny object surface, which reflects most of
the illumination (similar to the effect of a mirror). A solution to such problems is
often to use some kind of diffuse illumination either in the form of a high number
of less-powerful light sources or by illuminating a rough surface which then reflects
the light (randomly) toward the object.
Even though this text is about visual light as the energy form, it should be men-
tioned that infrared illumination is sometimes useful. For example, when tracking
the movements of human body parts, e.g. for use in animations in motion pictures,
infrared illumination is often applied. The idea is to add infrared reflecting markers
to the human body parts, e.g., in the form of small balls. When the scene is illu-
minated by infrared light, these markers will stand out and can therefore easily be
detected by image processing. A practical example of using infrared illumination is
given in Chap. 12.

2.2 The Optical System

After having illuminated the object of interest, the light reflected from the object
now has to be captured by the camera. If a material sensitive to the reflected light
is placed close to the object, an image of the object will be captured. However, as
illustrated in Fig. 2.5 , light from different points on the object will mix—resulting
in a useless image. To make matters worse, light from the surroundings will also
be captured resulting in even worse results. The solution is, as illustrated in the
figure, to place some kind of barrier between the object of interest and the sensing
material. Note that the consequence is that the image is upside-down. The hardware
and software used to capture the image normally rearranges the image so that you
never notice this.
The concept of a barrier is a sound idea, but results in too little light entering the
sensor. To handle this situation the hole is replaced by an optical system . This section
describes the basics behind such an optical system. To put it into perspective, the
famous space-telescope—the Hubble telescope—basically operates like a camera,
i.e., an optical system directs the incoming energy toward a sensor. Imagine how
many man-hours were used to design and implement the Hubble telescope. And
still, NASA had to send astronauts into space in order to fix the optical system due