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Sensor – The Heart of any CCTV Camera

web-post October 21, 2013

Kuber Singh clears the myth around CCD Vs CMOS sensors used in CCTV applications. Is one better than the other?

Almost all cameras today are increasingly based on two dominant image sensing technologies, namely Charge-Coupled Device (CCD) or Complementary Metal-Oxide-Semiconductor (CMOS). The security industry relies heavily on the various applications of these image sensors in manufacturing cameras for surveillance purposes.

Although CCD has been known to be the dominant technology in the market when it comes to high quality images, advances in technology have now led CMOS to be almost at par with CCD when it comes to quality. CCD has been more reliable in the industry for many years due to its higher resolution images which led to its market dominance. CMOS technology however soon caught on due to its inexpensive design, smaller package size and drastically lower power consumption.

Both technologies rely on the principle of transduction where light energy or photons are converted to electrical energy. The difference between CCD and CMOS lies in the way this conversion takes place and how an image is digitized into machine readable binary language.

In CCD devices, an array of millions of light-sensitive picture elements, or pixels, spans the surface of the sensor. After exposure to light, the accumulated charge over the entire CCD pixel array is read out at one end of the device and then digitized via an Analog Front End (AFE) chip or CCD processor. On the other hand, CMOS sensors directly digitize the exposure level at each pixel site.

CCD vs CMOS image

CCD based cameras usually require extra circuitry to convert the collected analog charge into digital format and this requires more space when it comes to packaging the camera. On the other hand, Cameras using CMOS image sensors are smaller in size since the digital conversion takes place at the pixel level where each pixel has small transistors behind it which actively contribute towards digitizing the image.

Both technologies are now rather mature in the market and command shares of the market based on their application requirements. For the security industry there has mostly been a need for higher quality imaging systems and so CCD has always been a better choice, however the need for precisely picking the right equipment for the right security need is purely application based.

Low-Light Conditions
With heavy improvements in lithography techniques for CMOS manufacturing and the relative improvement in CCD based imaging systems the sensitivity to light or the dynamic range of individual pixels has increased drastically. Dynamic range, defined as the luminance range of a scene being photographed, signifies exactly how well a camera could perform in low-light conditions, which is often a necessary requirement for many surveillance based applications.

The individual transistors for digital conversion present on a CMOS pixel have different gains that are difficult to match in practice and are often not adjustable this causes more of a pixel variation specially at low light conditions whereas CCD’s have a single transistor which results in less pixel variation.

Lower light conditions also signify a increase in noise that is riding on the original light signal which is close to both the sensors noise threshold. However CMOS sensors have individual transistors which have a lower bandwidth, meaning they perform better for lower frequency lights and act as a filter for the higher frequency noise riding on these signals on low light conditions. This in turn helps to offer a better signal-to-noise ratio (S/N) ratio in low light conditions as compared to a CCD sensor which has a higher bandwidth and in turn results in a lower signal-to-noise ratio in low lighting conditions.
Past half a decade has seen the rise of backlit illuminated CMOS sensors that provide excellent low light imaging capabilities at consumer prices. Nevertheless, a CCD sensor still offers a higher sensitivity in low-light conditions since it offers a higher fill area which is unobstructed due to the lack of individual amplifications transistors that are often present on a CMOS
Frame Rate:
The human eye and brain are compatible at discerning a video as normal and continuous at frame rates of about 24 frames per second and higher. Although both technologies are capable at providing this frame rate, CMOS is mostly certainly a clear winner when it comes to high speed motion detection applications in certain surveillance requirements.

The CCD camera is technologically restricted since all the pixels in its large array must be read and amplified by one transistor. So the larger the image, the lower will be the frame rate for a CCD. Also restricting higher frame rates in CCD is the transistor itself, where higher frame rates require a higher bandwidth amplifier which usually also signifies more noise and power consumption.
CMOS sensors are converted to digital values by the pixel and since the collected charge does not need to be transferred through the array, It is possible to achieve much higher frame rates. Also, in CMOS the charge doesn’t necessarily have to be read out from one end, unlike CCD, so its possible to read selected portions of an array at higher frame rates. In surveillance applications requiring high speed motion detection, CMOS is certainly the way to go.

Invisible Light
Electromagnetic wavelengths such as infra-red or ultraviolet are invisible to the naked eye yet they contain important information that is apparent in many security requirements. An example would be thermal imaging where light from the infra-red spectrum is captured by the image sensor. Infra-red has a higher wavelength than the visible light and this results in deeper silicon penetration which means that the individual pixels must now be made thicker to accommodate for this change, this is easily achieved in CCD technology but in CMOS the individual transistors must also be manufactured with thicker silicon substrates which adversely affects their performance.

In very rare surveillance requirements the ultra-violet light also finds uses in criminology where obstacles are marked with UV light readable-only power, homeland security with examples of man-made weapons such as surface to air missiles whose exhaust plumes emit a specially marked UV signal. Although the uses are low, imaging technology is needed to read these signals, sometimes even in broad daylight where it is often hard to read infra-red signals or even visible colours due to solar blindness. In these situations the CCD is again an optimal choice, mainly due to fabrication procedures which help produce thinner pixels for capturing the low wavelength ultraviolet light.

CCDs have the highest quality and lowest noise, but they are not power-efficient. CMOS sensors are easy to manufacture and have low power dissipation, but at reduced quality. Part of the reason for this is because the transistors at each pixel site tend to obstruct light from reaching part of the pixel. However, CMOS has started giving CCD a run for its money in the quality arena, and increasing numbers of mid-tier camera sensors are now CMOS-based.

Both technologies have certain drawbacks related to image quality. On one hand, CCD sensors are more susceptible to vertical smear from bright light sources when the sensor is overloaded. This phenomenon is called blooming and it happens when a part of the pixel array is exposed to bright light, which causes the capacitive bins to be overloaded; high-end CMOS sensors in turn do not suffer from this problem. On the other hand, CMOS sensors are susceptible to undesired effects such as wobbling, skew and smear that come as a result of rolling shutter.

CMOS technology has increasingly dominated the current generation mobile phone market and the market of smaller digital cameras where size restrictions and power considerations have clearly set this technology as the winner. In the security industry, where size is definitely not a factor since most cameras are placed selectively at stationary locations, CCD image sensor based cameras have been the market leader due to its higher resolution images, the need for which is most evident in surveillance.

In conclusion, CCD sensors deliver higher image quality while CMOS sensors are more efficient in terms of both cost and power consumption and they are also consumer friendly due to their small size. Both systems have roles to play in the security industry today, though it is arguable whether they truly compete with each other as their uses depends on the requirements for surveillance. For low-light application usage, CCD is often the clear winner while CMOS is often used for applications requiring high speed motion capture or for thermal imaging purposes.

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