The
main performance parameters of a CCD device are its speed, sensitivity,
resolution and cost. However, the choice of architecture is dependent
on the application of the devices. While astronomy requires that maximum
light being captured, price seldom being a constraint, full frame
read-out architecture CCD devices are utilized. But in case of the point
and shoot photography cameras where shutter speed and cost counts the
most, interline architecture is the most popular.
CCD
arrays are only sensitive to intensity and not color. Thus, in order to
obtain a color image, filters are used. Colored images may be obtained
by using either the Bayer Pattern or 3CCD and a dichroic beam splitter
prism.
Bayer Pattern
is a special color grid which is placed over the imaging array. Human
eyes being most sensitive to green color, two out of four grids have a
green filter. The remaining have blue and red, comprising the RGB color
pattern. A digital signal processor interpolates the two missing colors
from the values of the neighboring pixels. Bayer pattern though simple
and cheap, throws information away compromising on image resolution.
Also, when used in places with abrupt changes in light intensity Bayer
pattern creates artifacts. Some cameras may also use different color
patterns for color generation too. Resolution can be improved in
specific applications by Microscanning Technology.
A dichroic beam splitter prism
splits the image into red, green and blue components which can then be
incident separately on three-CCD devices. This offers higher quantum
efficiency over Bayer Mask as most of the light is captured by the
sensors and not absorbed by intermediate layers of the Mask. Such
devices have been used in professional video camcorders.
A
CCD Device despite of all the good features it has to offer does have a
few drawbacks too. Coupling of the charges along a row of many pixels
results in the loss of considerable amount of charge resulting in
fading. Improper shutter control, or over bright conditions leading to
too many photons reaching the CCD element results in the leakage of
charge to the neighboring pixel, which causes blooming.
Further,
if photons strike sensors while charge is being transferred due to
improper clocking, data loss in the form of smears can happen. The
sensitivity of a CCD device is often a function of the temperature of
operation. As the temperature increases, the leakage dark current also
increases, thereby reducing sensitivity. Noise is inherent to every
electronics equipment and in case of devices based on CCD, the SNR
degradation may take place due to photon noise, dark noise or read noise
or the combination of them. However, intensive research work in this
field has yielded positive results and the development of many
improved variants. Devices like Electron Multiplying CCD (EMCCDs) which
incorporate on-chip multiplication gain have helped achieve single
photon detection sensitivities without compromising on resolution or
efficiency of CCD structure while reducing costs. These features are
otherwise typical of the costly Electron-Bombarded CCDs which utilizes
an image intensifier with a CCD device.