CCD sensor
CCD sensors are semiconductor chips that are similar to the human eye. Both convert optical signals (light) into electrical ones. The first CCD sensors entered the market in the 1970s. Step by step, they replaced the pickup tubes of video cameras and industrial cameras, as well as the film of photo cameras.
Today, CCD sensors are being more and more replaced by CMOS sensors. Nevertheless, many industrial USB cameras, FireWire cameras and GigE cameras are still fitted with CCD sensors.
How does a CCD sensor work?
CCD sensors and also CMOS sensors are based on the photoelectric effect: Light encounters a piece of silicon and creates free electrons in it. These electrons are then collected in capacitors. The capacitors are the pixels of the image sensor.
Thus, the basic idea of CCD sensor and CMOS sensors is identical. They "only" differ in the realization of the capacitors and the removal of the free electrons for further use. These differences are responsible for the advantages and disadvantages of both sensor types.
How does a CCD sensor perceive color?
Metaphorically speaking, the retina of a human eye also consists of pixels: the rods are in charge of the perception of black and white, while three different types of cones are sensitive for red, green and blue light.
CCD sensors are also based on these four types of pixels. However, there are no CCD sensors on the market that consist of all four types. The manufacturers of industrial cameras have to decide between CCD sensors with black and white pixels (for black and white cameras) or those with red, green and blue pixels (for color cameras).
The article Color cameras - What is the origin of the colors? describes these topics in detail.
The advantages of CCD sensors
Today, even high-end consumer cameras are no longer fitted with a CCD sensor, but a CMOS sensor. However, in the context of industrial cameras they still have some advantages:
- Global shutter: CCD sensors have a global shutter "by nature" and therefore they have no problems taking images of moving objects.
- Long product life cycles: Most of the CCD sensors used in industrial cameras have been developed for the capital goods industry. Therefore, they remain much longer on the market than those sensors that are used in consumer cameras.
- Long exposure times: Without any special additional measures, CCD sensors are able to provide useful images even after an exposure time of one hour or more. Therefore, in contrast to CMOS sensors, CCD sensors are able to capture very dim objects.
The disadvantage of CCD sensors
In addition to economical reasons, there are two other typical disadvantages of CCD sensors that lead to their replacement by CMOS sensors:
- Sensitivity to overexposure: If light intensity is very high in certain parts of the image, the above mentioned capacitors are full in these parts before the end of the exposure time. In the case of CCD sensors the capacitors run over and flood neighboring pixels. In contrast to this, CMOS sensors have a "natural protection" against this so-called "blooming" effect.
- Complicated binning and ROI: Most CMOS sensors provide a readout mechanism for small image parts (region of interest). Most of them also allow a merging of pixels directly on the chip (binning) to increase its sensitivity. Both effects are hard to realize with CCD sensors.
