When an image is being captured by a network camera, light passes through the lens and falls on the image sensor. The image sensor consists of picture elements, also called pixels that register the amount of light that falls on them. They convert the received amount of light into a corresponding number of electrons. The stronger the light, the more electrons are generated. The electrons are converted into voltage and then transformed into numbers by means of an A/D-converter. The signal constituted by the numbers is processed by electronic circuits inside the camera.
Presently, there are two main technologies that can be used for the image sensor in a camera, ie, CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor). Their design and different strengths and weaknesses will be explained in the following sections.
Image sensors register the amount of light from bright to dark with no colour information. Since CMOS and CCD image sensors are ‘colour blind’, a filter in front of the sensor allows the sensor to assign colour tones to each pixel. Two common colour registration methods are RGB (red, green, and blue) and CMYG (cyan, magenta, yellow, and green). Red, green, and blue are the primary colours that, mixed in different combinations, can produce most of the colours visible to the human eye.
The Bayer array, which has alternating rows of red-green and green-blue filters, is the most common RGB colour filter. Since the human eye is more sensitive to green than to the other two colours, the Bayer array has twice as many green colour filters. This also means that with the Bayer array, the human eye can detect more detail than if the three colours were used in equal measures in the filter.
Another way to filter or register colour is to use the complementary colours – cyan, magenta, and yellow. Complementary colour filters on sensors are often combined with green filters to form a CMYG colour array. The CMYG system generally offers higher pixel signals due to its broader spectral band pass. However, the signals must then be converted to RGB since this is used in the final image, and the conversion implies more processing and added noise. The result is that the initial gain in signal-to-noise is reduced, and the CMYG system is often not as good at presenting colours accurately.
The CMYG colour array is often used in interlaced CCD image sensors, whereas the RGB system primarily is used in progressive scan image sensors. For more information about interlaced CCD image sensors and progressive scan image sensors, see the links in the box.
In a CCD sensor, the light (charge) that falls on the pixels of the sensor is transferred from the chip through one output node, or only a few output nodes. The charges are converted to voltage levels, buffered, and sent out as an analogue signal. This signal is then amplified and converted to numbers using an A/D-converter outside the sensor.
The CCD technology was developed specifically to be used in cameras, and CCD sensors have been used for more than 30 years. Traditionally, CCD sensors have had some advantages compared to CMOS sensors, such as better light sensitivity and less noise. In recent years, however, these differences have disappeared.
The disadvantages of CCD sensors are that they are analogue components that require more electronic circuitry outside the sensor, they are more expensive to produce, and can consume up to 100 times more power than CMOS sensors. The increased power consumption can lead to heat issues in the camera, which not only impacts image quality negatively, but also increases the cost and environmental impact of the product.
CCD sensors also require a higher data rate, since everything has to go through just one output amplifier, or a few output amplifiers.
Early on, ordinary CMOS chips were used for imaging purposes, but the image quality was poor due to their inferior light sensitivity. Modern CMOS sensors use a more specialised technology and the quality and light sensitivity of the sensors have rapidly increased in recent years.
CMOS chips have several advantages. Unlike the CCD sensor, the CMOS chip incorporates amplifiers and A/D-converters, which lowers the cost for cameras since it contains all the logics needed to produce an image. Every CMOS pixel contains conversion electronics. Compared to CCD sensors, CMOS sensors have better integration possibilities and more functions. However, this addition of circuitry inside the chip can lead to a risk of more structured noise, such as stripes and other patterns. CMOS sensors also have a faster readout, lower power consumption, higher noise immunity, and a smaller system size.
Calibrating a CMOS sensor in production, if needed, can be more difficult than calibrating a CCD sensor. But technology development has made CMOS sensors easier to calibrate, and some are nowadays even self-calibrating.
It is possible to read individual pixels from a CMOS sensor, which allows ‘windowing’, which implies that parts of the sensor area can be read out, instead of the entire sensor area at once. This way a higher frame rate can be delivered from a limited part of the sensor, and digital PTZ (pan/tilt/zoom) functions can be used. It is also possible to achieve multiview streaming, which allows several cropped view areas to be streamed simultaneously from the sensor, simulating several ‘virtual cameras’.
HDTV and megapixel sensors
Megapixel and HDTV technology enables network cameras to provide higher resolution video images than analogue CCTV cameras, ie, they improve the possibility to see details and to identify people and objects – a key consideration in video surveillance applications. A megapixel or HDTV network camera offers at least twice as high a resolution as a conventional, analogue CCTV camera. Megapixel sensors are key components in HDTV, megapixel and multimegapixel cameras and can be used to provide extremely detailed images and multiview streaming.
Megapixel CMOS sensors are more widely available and generally less expensive than megapixel CCD sensors – even though there are plenty of examples of very costly CMOS sensors.
It is difficult to make a fast megapixel CCD sensor, which of course is a disadvantage, and which makes it difficult to build a multimegapixel camera using CCD technology.
Many sensors in megapixel cameras are generally similar in size as VGA sensors with a resolution of 640×480 pixels. Since a megapixel sensor contains more pixels than a VGA sensor, the size of each pixel in a megapixel sensor becomes smaller than in a VGA sensor. As a consequence, a megapixel sensor is typically less light sensitive per pixel than a VGA sensor, since the pixel size is smaller and light reflected from an object is spread to more pixels. However, technology is rapidly improving megapixel sensors, and the performance in terms of light sensitivity is constantly improving.
A CMOS sensor incorporates amplifiers, A/D-converters and often circuitry for additional processing, whereas in a camera with a CCD sensor, many signal processing functions are performed outside the sensor. CMOS sensors have a lower power consumption than CCD image sensors, which means that the temperature inside the camera can be kept lower. Heat issues with CCD sensors can increase interference, but on the other hand, CMOS sensors can suffer more from structured noise.
A CMOS sensor allows ‘windowing’ and multiview streaming, which cannot be performed with a CCD sensor. A CCD sensor generally has one charge-to-voltage converter per sensor, whereas a CMOS sensor has one per pixel. The faster readout from a CMOS sensor makes it easier to use for multimegapixel cameras.
Recent technology advancements have eradicated the difference in light sensitivity between a CCD and CMOS sensor at a given price point.
CCD and CMOS sensors have different advantages, but the technology is evolving rapidly and the situation changes constantly. Hence, the best strategy for a camera manufacturer – and the one that Axis Communications adheres to – is to continually evaluate and test sensors for each camera that is developed. The question whether a chosen sensor is based on CCD or CMOS technology then becomes irrelevant. The only focus is if the sensor can be used to build a network camera that delivers the image quality needed and fulfils the customers’ video surveillance requirements.
Helpful links and resources
For more information, see the following:
* Axis Communications – Technical guide to network video: www.axis.com/files/brochure/bc_techguide_ 33334_en_0811_lo.pdf
* Axis Communications – Image sensors: www.axis.com/products/video/about_networkvideo/image_sensors.htm
* Wikipedia – CMOS: http://en.wikipedia.org/wiki/CMOS
* ‘Intelligent Network Video: Understanding modern surveillance systems’, by Fredrik Nilsson and Axis Communications (ISBN 1420061569)
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