CCD vs. CMOS cameras
Circuitry
these devices are pixelated image sensors used to convert an optical
image into an electrical signal; they accumulate signal charge in each
pixel proportionate to the local illumination intensity
|
CCD
charge coupled device
|
CMOS
complementary metal-oxide semiconductor
|
after exposure is complete, each pixel transfers the charge it has
collected to one output node where the charge is converted to a voltage,
buffered, and sent to the chip |
after exposure is complete, charge-to-voltage conversion
and amplification takes place at each pixel |
Responsivity
the amount of signal the sensor delivers per unit of input optical
energy
|
| CCD |
CMOS |
amplification of the signal usually has significant power
penalty |
each pixel has its own amplification
electronics, therefore low-power, high-gain amplification is possible |
Dynamic range
the ratio of a pixel's saturation level (maximum signal strength) to
its signal threshold (minimum signal strength); a camera with a high
dynamic range is more capable of imaging a wide range of intensities of
light
|
| CCD |
CMOS |
twice as great of a dynamic range
capability than typical CMOS cameras |
more noise from on-chip circuitry (not ideal for
low-light imaging) |
Uniformity
the consistency of response for different pixels under identical
illumination
|
| CCD |
CMOS |
since each pixel uses the same
charge-to-voltage converter,
there is greater consistency |
since each pixel of the CMOS has its own charge-to-voltage
converter, uniformity is constrained by the uniformity of these
charger converters and amplifiers |
Shuttering
the ability to arbitrarily start and stop exposure
|
| CCD |
CMOS |
able to achieve uniform
shuttering with little fill factor compromise since the entire
image is captured in one frame store; superior for imaging
objects in motion |
uniform shuttering ability depends
on the type of CMOS imager (see below) |
Uniform shuttering for CMOS
imagers
|
| line-scan |
area-scan |
electronic shuttering does
not compromise fill factor because shutter transistors can be
placed adjacent to the active area of pixel;
higher line
rate and ultra-low noise as compared to CCD line-scan imagers |
shutter transistors are placed in what would have been an
optically sensitive area;
(a) rolling shutter: only a portion of the sensor is exposed at a
given time
(b) non-rolling shutter: all pixels are exposed at once
|
Windowing
the ability to read only a portion of the signal from the sensor in an
area of interest (AOI); when imaging a smaller area, it is possible to
attain higher imaging speeds
|
| CCD |
CMOS |
the signal is read sequentially |
the signal can be read from
only a portion of the whole sensor |
Antiblooming
blooming is when an overexposed pixel causes nearby pixels to appear
overexposed as well
|
| CCD |
CMOS |
requires specific engineering (larger buffer between pixel
rows) to drain overexposure
without affecting neighboring pixels, which reduces fill factor |
not susceptible to blooming since each
pizel does its own charge-to-voltage conversion
|
Applications
|
CCD
superior image quality, but larger
system
|
CMOS
superior power dissipation and
system
size, but low image quality
|
suitable for high-end imaging applications: e.g. digital
photography, broadcast television, high-performance imaging,
scientific and medical applications |
suitable for high-volume, space-constrained applications: e.g.
security cameras, PC and handheld videoconferencing, bar code scanners,
fax machines |
Resources
Litwiller, D. “CCD vs.
CMOS: Facts and Fiction.” Photonics Spectra (Jan 2001).
Thor Labs: CCD
vs. CMOS
Basler Vision Technologies: CMOS high-speed line scan cameras
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