Image Processing Reference
In-Depth Information
7.4.2 Multiband Camera System
While the color information of a pixel in a single-chip color system is signified by a set sig-
nal of R, G, and B, a multiband camera can obtain much more detailed color or wavelength
information, as mentioned in Section 6.4.
As shown in Figure 7.13, multiple color filters attached to a turret are set in front of a
photographic lens in a multiband camera system, and still images are captured through
each color. It has been clarified that, in principle, 99% of color information can be obtained
by three kinds of color filters.18
18
However, as it is quite difficult to realize an ideal combina-
tion of color filters, a practical solution was proposed that 99% of color information can be
achieved by using five kinds of commercially available pigment color filters. While this
system was developed for up to 8-band cameras, there is an example of a 16-band camera.
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7.4.3 Hyperspectral Imaging System
A hyperspectral imaging system provides a thorough viewpoint of a multiband camera. While
the objective of a multiband camera with a smaller band number is to obtain higher color
information, that of a hyperspectral imaging system is precise and wide-ranging wavelength
information. A hyperspectral imaging system can be considered as a camera with a spec-
troscopic device such as built-in grating
20
rather than a multiband camera with a restricted
number of color filters. Hyperspectral imaging systems with a 5 nm bandwidth and around
100 band numbers have been developed and are considered the ultimate multiband camera.
The operational principle of hyperspectral imaging is shown in Figure 7.14. A linear por-
tion of a whole image is passed through an optical slit and is dispersed by a spectroscopic
device in a vertical direction to the incident light of the image sensor. Focusing on one
vertical line on the sensor, the spectrum can be obtained from the longest wavelength at
the top edge pixel to the shortest wavelength at the bottom pixel. On completion of the
readout of one image on an image sensor, the linear image part is shifted to the next one to
be captured in the following step and in the same manner. By scanning a two-dimensional
image from the top line to the bottom line, a hyperspectral image is obtained. Since the
principle is the same with a multiband camera, it is not suitable for real-time reproduction.
Because of the principle, a target is not limited to the visible region. While an optics system
and sensor must be chosen, their objectives are wide from UV, visible, near-infrared, infrared,
to far-infrared imaging. Among these functions, hyperspectral imaging is applied in a wide
Light source
Digital camera
Color filters
attached to a turret
Object
FIGURE 7.13
Schematic diagram of multiband camera.
