Page 135 - Introduction to Autonomous Mobile Robots

Chapter 4
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In color cameras, an additional control exists for white balance. Depending on the
source of illumination in a scene (e.g., fluorescent lamps, incandescent lamps, sunlight,
underwater filtered light, etc.), the relative measurements of red, green, and blue light that
define pure white light will change dramatically. The human eye compensates for all such
effects in ways that are not fully understood, but, the camera can demonstrate glaring incon-
sistencies in which the same table looks blue in one image, taken during the night, and
yellow in another image, taken during the day. White balance controls enable the user to
change the relative gains for red, green, and blue in order to maintain more consistent color
definitions in varying contexts.
The key disadvantages of CCD cameras are primarily in the areas of inconstancy and
dynamic range. As mentioned above, a number of parameters can change the brightness
and colors with which a camera creates its image. Manipulating these parameters in a way
to provide consistency over time and over environments, for example, ensuring that a green
shirt always looks green, and something dark gray is always dark gray, remains an open
problem in the vision community. For more details on the fields of color constancy and
luminosity constancy, consult [40].
The second class of disadvantages relates to the behavior of a CCD chip in environments
with extreme illumination. In cases of very low illumination, each pixel will receive only a
small number of photons. The longest possible integration period (i.e., shutter speed) and
camera optics (i.e., pixel size, chip size, lens focal length and diameter) will determine the
minimum level of light for which the signal is stronger than random error noise. In cases of
very high illumination, a pixel fills its well with free electrons and, as the well reaches its
limit, the probability of trapping additional electrons falls and therefore the linearity
between incoming light and electrons in the well degrades. This is termed saturation and
can indicate the existence of a further problem related to cross-sensitivity. When a well has
reached its limit, then additional light within the remainder of the integration period may
cause further charge to leak into neighboring pixels, causing them to report incorrect values
or even reach secondary saturation. This effect, called blooming, means that individual
pixel values are not truly independent.
The camera parameters may be adjusted for an environment with a particular light level,
but the problem remains that the dynamic range of a camera is limited by the well capacity
of the individual pixels. For example, a high-quality CCD may have pixels that can hold
40,000 electrons. The noise level for reading the well may be 11 electrons, and therefore
the dynamic range will be 40,000:11, or 3600:1, which is 35 dB.

CMOS technology. The complementary metal oxide semiconductor chip is a significant
departure from the CCD. It too has an array of pixels, but located alongside each pixel are
several transistors specific to that pixel. Just as in CCD chips, all of the pixels accumulate
charge during the integration period. During the data collection step, the CMOS takes a new