difference between ccd and cmos
CCD chips and CMOS chips are the two types of image sensors that are used in Digital cameras.
Both the image sensors were invented in the late 1960s and 1970s. CCD became leading just
because they gave far better image quality with the fabrication technology available on the other
hand CMOS image sensors required more consistency and smaller features than silicon wafer
could deliver at the time.
In CCD (Charge coupled device) digital imaging, when the light waves enter the camera they focus
on the sensor that converts light into the electrical charge and the image is formed. It is simple:
the more light that hits the photodiode, the greater is the charge. But how does this process
separate the colors? The light entering the camera is the normal white light that contains all the
wavelengths, so inside the mechanism these wavelengths are separated by filters that are based
on the basic RGB (red-green-blue) principle. The information is read row by row and pixel by
pixel, therefore, the necessary processing time is a bit longer, but it is very accurate.
A CMOS (Complementary Metal Oxide Semiconductor) sensor, instead of converting the light
wave into an electrical charge on a different chip, it converts the photons into electrons by
processing the data at this point (and not on another chip). By using amplifiers, these sensors are
faster than CCDs.
Responsivity: It is the amount of signal that the sensor can deliver per unit of input optical
energy. The CMOS imagers are slightly more superior to CCDs because it is easier to place gain
elements on CMOS sensor because the transistors allow low- power high- gain amplifiers
whereas in CCD the amplification generally comes at a cost of power consumption.
Uniformity: It is the stability of response for different pixels under equal clarification
conditions. It is important to differentiate between uniformity under illumination and regularity
near dark. CMOS imagers were usually much worse under both conditions. As each pixel has an
open loop output amplifier and the offset and gain varies noticeably making both dark and light
non uniformities than those in CCDs.
Shuttering: It is the ability to start and stop contact randomly. CCDs can deliver better
electronic shuttering with little fill-factor cooperate, even in small-pixel image sensors as
compared to CMOS. As to implement uniform electronic shuttering in CMOS imagers require a
number of transistors in each pixel. To overcome this drawback a non uniform shutter called the
rolling shutter is used which exposes different lines of an array at different times due to which
the number of in-pixel transistor is reduced which improves the fill factor. This may be sometimes
helpful at the cost of customer but when high performance rate is required it is not the better
option as the motion of the object is visible.
Speed: It is the place where CMOS imager has a better advantage over CCDs because all the
camera functions in CMOS can be placed on image sensor. With a single die, signal and power
trace distances can be shorter with less capacitance, inductance and propagation delays.
Windowing: One of the distinct ability of CMOS technology is the ability to read out a
portion of the image sensor as this allows the important frame or line rates for small region of
interest also to be read. CCDs usually have limited capabilities in windowing.
Antiblooming: It is the ability to gracefully consume restricted overexposure without
cooperating with the rest of the image in the sensor. CMOS generally have this ability while CCDs
on the other hand require some specific engineering to achieve this task.
Reliability: Both the image sensors are equally reliable in most of the consumer and
industrial applications. Then too CMOS imagers have advantage because all the circuit can be
placed on the single chip which minimizes the leads and solder joints. This means that CMOS
based cameras can be considerably smaller in size as compared to CCD based cameras.
Cost: The CMOS may be less expensive at the system level as compared to CCDs when the
cost of the circuit functions like timing generation, biasing, analog signal processing, digitization
and feedback circuit are considered. But it is not cheaper at the component level for the pure
image sensor function itself.
For a moment both the technologies remain complementary to each other as one can do the
things uniquely that the other cannot do. But in the coming future CMOS technology will start
consuming more and more applications of CCDs.
Canon Inc. announced that it has successfully developed an APS-H-size*1 CMOS image sensor
that delivers an image resolution of approximately 120 megapixels (13,280 x 9,184 pixels), the
world’s highest level*2 of resolution for its size.
Source : http://www.udaipurtalents.com/technical-learning/difference-betweem-ccd-and-cmos
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