Image Processing Reference
In-Depth Information
An example of the spectral response of red, green, and blue in the Bayer color filter is
shown in Figure 6.12. If it were thought of as one of a sampling means, it is quite different
from that of space and time because of the very small sampling point number of only three
and the very wide sampling widths that overlap each other. The reason is that this method
in not a kind of sampling, but a method utilizing the human eye and brain's perception of
color. Here, the color perception of the human eye is briefly mentioned. As is well known,
the human eye processes light through the retina. The retina contains two kinds of photo-
receptor cells: rods and cones. Rods only detect light intensity at a very low light level, but
do not sense color. Readers might have experienced the ability to discern the shape of a
body but not color under very low illumination. On the other hand, cones detect both light
intensity and color in relatively bright illuminance. There are three types of cones, referred
to as S, M, and L after their size.
Figure 6.13 shows the wavelength dependence of the cones' spectral response
3
: the
highly sensitive wavelength ranges of S, M, and L are 400-500, 500-600, and 550-650 nm,
respectively. In other words, S, M, and L are sensors that detect the ranges of violet to blue,
green to orange, and yellow-green to red, respectively. Because of the overlap of the highly
sensitive range, each of the three types of cones is excited by the incidence of any wave-
length in visible light and responds to generate a reference stimulus.
A set of reference color stimuli generated by each of the three types of cones by light
absorption facilitates color perception in the human brain. For example, if yellow, whose
wavelength is around 580 nm, comes into focus at the retina, the stimulus occurs at the
same level in cones L and M and at a lower level in cone S. This stimuli signal is transmit-
ted to the brain, facilitating color perception as yellow. However, if the same intensity of
green and red light is focused at the same point on the retina at the same time, the same
level of stimuli occurs at cones M and L, which is transmitted to the brain and facilitates
Red
Green
Blue
400
500
600
700
Wavelength (nm)
FIGURE 6.12
Example of spectral response of primary color filter for Bayer array.
