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CMOS sensor

A CMOS sensor is a semiconductor chip, which behaves like the human retina: Optical signals (light) are converted into electrical ones. The first attempts to build CMOS sensors ended up in frustration. Their image quality was not worth discussing. In contrast, it was much easier to obtain a good image quality with CCD sensors. Their triumph began in the 1970s.

In the 1990s some researchers remembered the first experiments with CMOS sensors. With new ideas, they were able to build the first CMOS sensors with acceptable images. Today, more and more industrial USB cameras, FireWire cameras and GigE cameras are based on CMOS sensors.

How do CMOS sensors work?

CMOS sensors, but also CCD sensors, are based on the photoelectric effect: When light strikes a piece of matter, it creates electrons. In our case, the piece of matter is a silicon chip. This chip is composed of hundreds of thousands of capacitors laid out in a grid pattern - the pixels. They collect the released electrons.

Up to here, the structures of CMOS sensors and CCD sensors are very similar. The main differences are the physical realization of the capacitors and the transfer of the electrons to the output of the chip. The pros and cons of CMOS sensors and CCD sensors are mainly based on these differences.

How does a CMOS sensor see color?

Like the human retina, a CMOS sensor has three different types of pixels ("cones" on the retina) to see colors - one for red, one for green and one for blue. Additionally, the retina has a fourth pixel type for the perception of black and white (called "rods").

However, there are no CMOS sensors that combine all these four types in one chip. CMOS sensors have either black and white pixels or pixels that are sensitive to red, green and blue light. The first category is used to build black and white cameras, the second one is the base for color cameras.

The article Color cameras - What is the origin of the colors? describes these differences in detail.

The advantages of CMOS sensors

It is not by chance that CMOS sensors even dominate the market of semi-professional consumer cameras in the meantime. Their advantages are especially of importance in today's mobile world:

  • Low power consumption: The handling of the collected electrons makes the main difference between CMOS sensors and CCD sensors. In the case of CMOS sensors this handling requires considerably less power.
  • Binning and ROI are simple to realize: For the same reason it is easy to put pixels together to increase the sensitivity (binning) and to output only smaller parts of the image (region of interest) to increase the frame rate.
  • Small industrial cameras: CMOS sensors are "ordinary" chips and therefore, it is possible to put more than only the "light sensor" onto the sensor chip. Consequently, this leads to smaller industrial cameras.

Disadvantage of CMOS sensors

Meanwhile, some "traditional" disadvantages of CMOS sensors are outgrown or negligible in the case of normal applications. The remaining disadvantages are:

  • Rolling shutter: Most CMOS sensors are unable to expose the image in one go, but only line by line (rolling shutter). Thus, the images of moving objects are skewed.
  • Limited long-time exposure: As already mentioned, the different handling of the collected electrons in CMOS sensors and CCD sensors usually works to the CMOS sensors' advantage. In the case of long-time exposure it is vice-versa: CCD sensors provide useful images even after an exposure time of one hour and more. In contrast, the maximum exposure time of CMOS sensors is only a few minutes.
  • Short product life cycles: The short product life cycles of CMOS sensors are not due to their technical characteristics. They mostly originate from the fast moving consumer market.

Industrial cameras are a fast growing market. The manufacturers of CMOS sensors are starting to notice this development. Therefore, they now offer the first sensors especially for this market. Obviously, these sensors provide a global shutter as well as long-term availability.