CCD vs CMOS: Digital Sensors Compared
The more types of sensors used in photography
The heart of every digital camera is the digital sensor chip that is the deputy to convert light waves into electrical charges, then to turn them into digital information.
The sensor can be a CCD (Charge-Coupled Device) or CMOS (Complementary Metal-Oxide-Semiconductor).
CMOS sensors have almost completely taken the place of the CCD in the latest generation of digital SLR cameras and digital backs are still used in high performance digital cameras used on medium format (Hasselblad, Phase One, Mamiya, etc).
We try to explain the basics of the two technologies in a few words, highlighting the differences between the two systems.
Electrical characteristics and output
Both sensors have as a basic element of the photodiode, the photosensitive element, struck by a beam of light generates an electrical charge.
In the case of the CCD, the electrical charge is transferred through a few “nodes” of output, and then be converted into and exit from the voltage sensor as an analog signal. All the photodiodes are dedicated exclusively to the reading of the uniformity of the light signal generated is high – it is an important feature for image quality.
In a CMOS sensor, each photodiode is accompanied by a converter that converts light energy into voltage, and often signal amplifiers, reducing noise and digitization circuits, so that the output signal from the sensor is in digital format. These other circuits around the photodiode reduces the space devoted to capturing light. Furthermore, when each photodiode performs its own conversion, uniformity of the signal is lower.
Differences and strengths of the two types of sensor
Each of the two technologies therefore strengths and weaknesses that make them more suitable for certain applications and not others.
The main differences can be summarized as follows:
- The CCD creates an image of high quality and low noise level, while the CMOS is more susceptible to noise
- The CCD consumes a large amount of energy, and saves the CMOS
- The CCD sensors are more expensive than CMOS
- CMOS sensors have a more complex design of the CCD
Both the CCD sensors to CMOS can offer excellent picture quality, well designed.
CCD sensors have so far been the benchmark for performance in the field of photography, scientific, industrial applications requiring the highest image quality possible, even at the expense of system size.
CMOS sensors offer more integration (more functions within the chip), lower power dissipation and the possibility of a smaller – that is why we find them on small consumer cameras or camera phones.
The efforts of the designers of CMOS have been directed to achieving a higher image quality, while the makers of CCD have been working to reduce energy consumption and the size of the pixels.
The result is that while a few years ago there was a clear division between the areas of CMOS sensors, linked to consumer cameras and camcorders and mobile phones, and CCD found on professional SLR cameras and digital backs, this is no longer the case. While digital backs medium format CCD sensors fitted exclusively to the greater color accuracy and image quality at the expense of size and consumption, the world of prosumer and professional digital SLR cameras has seen a proliferation of digital cameras with CMOS sensors that mount on the side of those CCD.
CMOS sensors, which have suffered so many deployments in recent years, requiring fewer parts and consume less energy, but generally require the presence of a chip that optimizes image quality, thus increasing the cost and reducing the consumption advantage.
Features CCD CMOS
Output of the photodiode electric charge voltage
Output of the chip voltage (analog) bit (digital)
Output of the camera bit (digital) bits (digital) Presence of noise
Low Moderate Complexity of the sensor Low High
Dynamic Range Moderate Large High uniformity of low to moderate Speed burst Moderate to High High Colour accuracy High Media
Response in low light conditions
Under dim light, in which the variations due all’amplificazione are the greatest, the CCD sensors generate a pixel response more uniform than what is happening in CMOS: the mono amplifiers generate value-pixel CMOS uneven lighting and noise in the case of CCDs, the information of all the photodiodes are amplified by the same instrument and then the change in values between a pixel and the other will be smaller, and will result in more consistent output.
Dim light conditions also means that the signal resulting from lighting and noise level near the base of the sensor.
The amplifiers mono-pixel CMOS sensors have less bandwidth than a broadband amplifier for CCD, having to amplify the signal of a single pixel and not the entire sensor. This allows the amplifier to acquire a higher voltage CMOS before the noise levels become intolerable, maintaining a ratio signal / noise greater.
This is why the apparent sensitivity CMOS sensors offer higher ISO – the task of signal amplification is easier than a CCD, at the expense of color accuracy than in the CCD is increased instead.
By contrast, the CCD sensors have better coverage of the photosensitive surface area than the total of the sensor, which means that each pixel captures more light and generates an electric charge greater than the pixels of a CMOS.
Acquisition speed (frame rate)
A second consideration when choosing the sensor is on the frame rate (number of acquisitions per second) or burst.
For the CCD is the speed of transport of data from individual pixels to put the maximum frame rate. This is because the CCD sensor to transfer the data of all pixels to empty its transmission logs, before they can accept the next image. If each pixel transmits at the same speed, a more extensive and require more air time then the frame rate will be lower.
In a CMOS sensor instead, each pixel has a voltage converter, then the amplifier of the pixel does not need to be super to support a high frame rate. In this way, the CMOS sensors can more easily reach speeds of more than gust CCD.
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