Page 134 - Introduction to Autonomous Mobile Robots

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Perception
You can see that the basic light-measuring process is colorless: it is just measuring the
total number of photons that strike each pixel in the integration period. There are two
common approaches for creating color images. If the pixels on the CCD chip are grouped
into 2 x 2 sets of four, then red, green, and blue dyes can be applied to a color filter so that
each individual pixel receives only light of one color. Normally, two pixels measure green
while one pixel each measures red and blue light intensity. Of course, this one-chip color
CCD has a geometric resolution disadvantage. The number of pixels in the system has been
effectively cut by a factor of four, and therefore the image resolution output by the CCD
camera will be sacrificed.
The three-chip color camera avoids these problems by splitting the incoming light into
three complete (lower intensity) copies. Three separate CCD chips receive the light, with
one red, green, or blue filter over each entire chip. Thus, in parallel, each chip measures
light intensity for one color, and the camera must combine the CCD chips’ outputs to create
a joint color image. Resolution is preserved in this solution, although the three-chip color
cameras are, as one would expect, significantly more expensive and therefore more rarely
used in mobile robotics.
Both three-chip and single-chip color CCD cameras suffer from the fact that photo-
diodes are much more sensitive to the near-infrared end of the spectrum. This means that
the overall system detects blue light much more poorly than red and green. To compensate,
the gain must be increased on the blue channel, and this introduces greater absolute noise
on blue than on red and green. It is not uncommon to assume at least one to two bits of addi-
tional noise on the blue channel. Although there is no satisfactory solution to this problem
today, over time the processes for blue detection have been improved and we expect this
positive trend to continue.
The CCD camera has several camera parameters that affect its behavior. In some cam-
eras, these values are fixed. In others, the values are constantly changing based on built-in
feedback loops. In higher-end cameras, the user can modify the values of these parameters
via software. The iris position and shutter speed regulate the amount of light being mea-
sured by the camera. The iris is simply a mechanical aperture that constricts incoming light,
just as in standard 35 mm cameras. Shutter speed regulates the integration period of the
chip. In higher-end cameras, the effective shutter speed can be as brief at 1/30,000 seconds
and as long as 2 seconds. Camera gain controls the overall amplification of the analog sig-
nal, prior to A/D conversion. However, it is very important to understand that, even though
the image may appear brighter after setting high gain, the shutter speed and iris may not
have changed at all. Thus gain merely amplifies the signal, and amplifies along with the
signal all of the associated noise and error. Although useful in applications where imaging
is done for human consumption (e.g., photography, television), gain is of little value to a
mobile roboticist.