Image Processing Reference
In-Depth Information
0
-0
-2
S
M
L
-3
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400
500
600
700
Wavelength (nm)
FIGURE 6.13
Relative sensitivity of cones. (Reprinted with permission from Wandell, B.A.,
Foundations of Vision,
Sinauer
Associates, Inc., 1995.)
the color perception. This is the the same as in the case of yellow incident light, as the brain
accepts only the stimuli. Therefore, the brain perceives yellow in both cases.
When colors A and B are focused at the same point on the retina, the human color vision
senses a different color C. It is not sensed as a chord-like sound. The human eye and brain
detect a stimulus of a wavelength and sense as a color based on a set of stimuli at cones S,
M, and L. Therefore, overlapping of the spectral response is necessary to reproduce a hue.
Display devices made up of only three primary colors can depicted as a wide range of
colors because of the mechanisms of the human eye and brain.
Actually used systems are real time, such as the single-sensor camera equipped with
color filters of primary or complementary colors, and the three-sensor camera. Thus,
what occurs in nature is “wavelength,” not “color.” Color cannot be discussed exclusive
of human perception. At the stage that the parameter that is a physical quantity, “wave-
length,” is substituted by human perception, “color,” physically objective affirmation of
information accuracy becomes impossible. Indeed, the colors used in filters are not com-
pletely the same, such as the case of red, green, and blue based on the primary colors and
the other case of cyan, magenta, yellow, and green based on the complementary colors
4
.
Various kinds of light sources can be captured, such as natural sunlight, fluorescent light,
incandescent light, and light-emitting diodes (LEDs). For these reasons, there is great dif-
ficulty in physically reproducing precise color. Therefore, what is aspired to is inevitably
subjective color reproduction such as perceptually equivalent color, memory color, and
preferred color.
The color information of the images obtained by the method utilizing human perception
has limited effectiveness for applications that human eyes do not view.
Thus, a precision signal ratio of R, G, and B is necessary for precision color informa-
tion. A less-accurate ratio signal causes color error at the pixel level. Figure 6.14 shows the
impact on color error caused by a random noise change according to the light intensity.
The light volume increases from left to right. The signal electron numbers at the high-
lighted parts (forehead of doll) are 90, 180, and 300 electrons, respectively. The bottom
images are expanded portions of the darker areas of the above images. While strong color
error is seen in the image of 90 signal electrons, the impact of the color error decreases by
signal-to-noise ratio (SNR) improvement with increasing illuminance.
