CCD and CMOS Sensors
- Tuesday, 01 July 2014
What's best for your application?
How does one select the best HD video camera and imaging sensor for professional video in applications such as life sciences, surgical imaging, microscopy, industrial imaging, and specialized point-of-view broadcasting where physical camera size is important and exceptional color video characteristics are critical?
Largely, these applications are based on dynamic, real-time, live viewing of the video image by people looking at a display and making decisions based on what they see coming from the camera. Is a 3-chip camera necessary or will a single chip camera suffice? What about sensor size, format, pixel size, and pixel density - how do these factors affect your image? This article will review and clarify key points to consider in camera selection to achieve the best outcome possible for your video application.
CCD vs. CMOS
Which is better - CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor)? It depends, as there are advantages to both sensor technologies. For most applications CMOS provides the better choice but in others, CCD continues to hold its ground. Both use semiconductors to convert light into electrical signals.
In a CMOS sensor, each pixel has a photoreceptor performing its own charge-to-voltage conversion and typically includes amplifiers, noise-correction, and digitization circuits, enabling the sensor to output digital data directly. The pixels typically don’t store any charge; they simply read how much light is hitting that pixel at a particular moment and read out, progressively from top left to bottom right, line by line while the shutter is open. In a CCD sensor, light enters the photoreceptor and is stored as an electrical charge within the sensor, then converted to voltage, buffered, and sent out as an analog signal when the shutter is closed.
A strong advantage for CMOS technology is that it provides digital output and can be controlled at the pixel level in ways that are not possible with CCDs. This provides potentially huge advantages in specialized imaging where one might want to apply partial scanning or a particular control process to only a segment of the sensor. This capability is useful for control of cameras in different imaging modes for multi-spectral imaging or binning.
CCD advantages over CMOS are the sensors’ higher quantum efficiency (QE) and generally lower noise. The proportion of each pixel dedicated to light gathering vs. being masked for other functions, is also comparatively high. However, CCD cameras generally consume more power than CMOS, which can be a consideration for certain life science applications or for cameras which are battery-powered. Blooming is an unwanted CCD-specific artifact, which appears as a vertical smear line when a bright light or saturation occurs in the image.
Global vs. Rolling Shutter
Probably the most significant issue when deciding between CCD or CMOS is global vs. rolling shutter. Most CMOS sensors today use a rolling shutter which is always active and rolling through the pixels line by line from top to bottom. CCDs on the other hand store their electrical charges and read out when the shutter is closed and the pixel is reset for the next exposure, allowing the entire sensor area to be output simultaneously. When the shutter is open, the CCD receives light and accumulates charges again.
These shutter variations impact video imaging in several ways especially when there is rotational movement, horizontal motion, laser pulse or strobe light. CCDs manage these motions and pulsed light conditions rather well as the scene is viewed or exposed at one moment in time, like a snap shot. In addition the CCD sensor (global shutter) can be more easily triggered, enabling synchronous timing of the light or motion to the open shutter phase.
With CMOS (rolling shutter), it can be managed to an extent through a combination of fast shutter speeds and timing of the light source, however, not all rolling shutter artifacts can be overcome. There are CMOS sensors available implementing global shutter capabilities, but their format and video performance characteristics aren’t yet optimal for many life science requirements.