and CCD Technology Compared
The two most common sensor technologies used today in industrial digital cameras are
Charged Coupled Device (CCD) sensors and Complementary Metal-Oxide Semiconductor (CMOS)
sensors. Both CCD and CMOS sensors are manufactured in a silicon foundry and the
manufacturing equipment used is similar. But different manufacturing processes make these
sensors very different in capability and performance.
CCDs were developed in the 1970s and 1980s and were specifically optimized for the best
possible optical qualities and image quality. A CCD typically includes photosites (pixels)
arranged in an X-Y matrix of rows and columns. Each pixel contains a photodiode and an
adjacent charge holding region. The photodiode collects light photons and converts them
into charge (electrons). The number of electrons collected is proportional to the light
intensity. Typically, light is collected over the entire surface of the sensor for a
period of time and then the charge from each pixel is transferred to the adjacent charge
holding region. Next, the accumulated charges are read out of the sensor. Each row of
charge data is first moved to a horizontal charge transfer register. The charge packets
for each row are then read out serially, are changed from a charge to a voltage, and are
This architecture produces a low-noise, high-performance imager, however, it makes
integrating other electronics onto the sensor impractical. It also means that operating
the CCD requires application of several clock signals, clock levels, and bias voltages.
This complexity increases the difficulty of camera design and increases power consumption,
system size, and cost.
A CMOS sensor, on the other hand, is made with standard silicon processes in high-volume
chip foundries. Peripheral electronic devices such as digital logic, clock drivers,
amplifiers, and analog-to-digital converters can be readily integrated into the sensor
using standard manufacturing processes.
This integration can be achieved because a CMOS sensor's architecture is arranged more
like a flat-panel display. Each pixel contains a photodiode that converts light to
electrons, a charge-to-voltage converter, a reset and select transistor, and an amplifier.
A metal grid overlays the entire sensor to apply timing and read out signals and an array
of column output signal interconnects. The column lines connect to multiplexing
electronics that are arranged by column outside of the pixel array. This architecture
allows signals from the entire array, a portion of the array, or a single pixel in the
array to be read out by simple X-Y addressing.
So given this difference in architecture, what are the advantages of CMOS sensor
Lower Power Consumption - Since many of the supporting external electronic
components required by a CCD sensor can be fabricated directly onto a CMOS sensor, the
overall power consumption of the system can be reduced. This results in lower power
requirements for your camera and lower operating temperatures.
Increased Speed - Since charge conversion and
analog-to-digital conversion are accomplished at the pixel level, complex shifting schemes
are not required and frame read out can be accomplished much more quickly.
Access Flexibility - The simple X-Y pixel
addressing method used in CMOS sensors allows direct access to a single pixel or to a
group of pixels. This results in extremely high frame rates when working with smaller
"areas of interest" on the sensor.
Low Smear and Blooming - The charge holding
regions used in CCD sensors are subject to leakage between pixels. This can result in a
condition called "blooming". Also, the nature of the charge holding regions and
of the shift registers used in CCD sensors can cause unwanted streaks in the acquired
images. Because CMOS sensors convert charge to voltage directly in the pixels and because
they don't use shift registers, CMOS sensors have inherent anti-blooming and anti-smearing