Image Processing Reference
In-Depth Information
3
Color Images
So far we have restricted ourselves to gray-scale images, but, as you might have
noticed, the real world consists of colors. Going back some years, many cameras
(and displays, e.g., TV-monitors) only handled gray-scale images. As the technology
matured, it became possible to capture (and visualize) color images and today most
cameras capture color images.
In this chapter we turn to the topic of color images. We describe the nature of
color images and how they are captured and represented.
3.1
What Is a Color?
In Chap. 2 it was explained that an image is formed by measuring the amount of
energy entering the image sensor. It was also stated that only energy within a cer-
tain frequency/wavelength range is measured. This wavelength range is denoted the
visual spectrum
, see Fig. 2.2. In the human eye this is done by the so-called
rods
,
which are specialized nerve-cells that act as
photoreceptors
. Besides the rods, the
human eye also contains
cones
. These operate like the rods, but are not sensitive
to all wavelengths in the visual spectrum. Instead, the eye contains three types of
cones, each sensitive to a different wavelength range. The human brain interprets
the output from these different cones as different colors as seen in Table
3.1
[4].
So, a color is defined by a certain wavelength in the electromagnetic spectrum as
illustrated in Fig.
3.1
.
Since the three different types of cones exist we have the notion of the
primary
colors
being red, green and blue. Psycho-visual experiments have shown that the
different cones have different sensitivity. This means that when you see two differ-
ent colors with the same intensity, you will judge their brightness differently. On
average, a human perceives red as being 2.6 times as bright as blue and green as
being 5.6 times as bright as blue. Hence the eye is more sensitive to green and least
sensitive to blue.
When all wavelengths (all colors) are present at the same time, the eye perceives
this as a shade of gray, hence no color is seen! If the energy level increases the
shade becomes brighter and ultimately becomes white. Conversely, when the energy
