Image Processing Reference
In-Depth Information
amplitude as well as space sampling. False signals exist due to aliasing during sampling
in the time domain as well as space sampling. For example, on TV, the phenomenon of a
rotating wheel appearing not to rotate or, inversely, appearing to rotate slowly is caused
by this mechanism based on the synchronization of periodic motion and exposure timing.
6.4 Sampling in Wavelength Domain and Color Information
If the same approach as in the case of space and temporal information is adopted to obtain
wavelength information, sampling by dividing the wavelength domain is considered. For
higher-quality wavelength information, a higher sampling frequency and a narrower sam-
pling width are required. Specifically, multiband cameras* capture images at each divided
wavelength region, as shown in Figure 6.11, and synthesize them.
Figure 6.11 shows the case of 16 bands as the wavelength area is divided into 16 parts.
In actual methods, 16 color filters, each with a spectral response that corresponds to each
divided wavelength area, are prepared, and 16 still pictures are captured by using each
color filter. Then, 16 pictures are synthesized to 1 color still picture. From the procedure, it
is clearly understood that this system can only apply to still objects. Therefore, its applica-
tion is restricted to particular kinds of tasks, such as digital archive development of art
objects. And for practical reasons, the number of bands range from around 4 to 8.
†
Thus, a
multiband camera system is inadequate for general application.
Almost all of the camera systems that are actually used are single-chip color cameras
represented by the Bayer color filter array shown in Figure 1.7. For high-quality imaging,
such as for broadcasting, professional, and high-end consumer use, the three-chip color
camera
‡
is used, in which the wavelength region is separated into three parts by a prism,
leading to the use of each corresponding sensor.
Sampling pitch
Aperture
16 band
0
400
500
600
700
Wavelength (nm)
FIGURE 6.11
Example of wavelength region division for wavelength sampling.
* Discussed in more detail in Section 7.4.2.
†
Section 7.4.2 contains an example of a 16-band camera.
‡
Discussed in more detail in Section 8.3.
