The CMOS sensor : what is essential to know

Le capteur CMOS : l’essentiel à connaître

Are you interested in movie cameras ; and you hear here and there of CCD sensors and CMOS sensors ; and you you ask questions on them ? In this article, check out :

  • What is a sensor,
  • What makes the difference between a CCD sensor and a CMOS sensor,
  • The advantages and disadvantages of a CMOS sensor,
  • Some examples of cameras equipped with CCD sensors and a few examples of cameras with CMOS sensors.

A sensor, what is it for??

A sensor is an electronic component photosensitive (reacts to light) of photosites rectangular, square or octagonal arranged in the manner of a checkerboard.

A sensor is used to convert electromagnetic radiation (IR, visible, UV) light to an electrical signal analog. It is this signal which is amplified, digitized and processed to obtain a digital image.

Note that the size of a sensor has an impact on depth of field : the more the sensor is large, the smaller the depth of field.

There are two main families of sensors :

  • The CCD sensors,
  • The CMOS sensors

The different types of sensors : the CCD sensor and the CMOS sensor

Invented at Bell laboratories in 1969, per Willard Boyle and George E. Smith, the sensor Charge-Coupled Device (CCD) is known in French under the name of ” Device to Transfer Charge (DTC) “.

There are three types of CCD sensors : CCD Full-Frame (full-frame), the CCD Full-Frame Transfer (frame transfer), and the CCD ” Interline “.

As for the sensor, Complementary Metal Oxide Semi-Conductor commonly referred to as CMOS sensor, it was invented at about the same time. But it has not been immediately adopted. Because, at the time, the image quality of CCD sensors was better ; this is no longer the case today.

Further, you will know more about the difference between the two types of sensors. In the meantime, be aware that the cameras are equipped with a sensor sort CCD/CMOS or sensor mono CCD/CMOS.

In a sensor mono CCD/CMOS, the photosites are covered with a blue filter (blue, red, or green. But the sensitivity of the photosite to the green is much less important ; this requires 2 photosites for the green.

Therefore, to obtain a color, it takes 4 photosites : 2 for green, one for red and one for blue.

The greater the amount of photosites on a sensor, the greater the number of pixels. However, for price reasons, in the entries array, it reduces the size of the sensors. Therefore, when there is an increase in the number of photosites on a smaller surface area of sensors, there is a decline in the quality of the image.

But a sensor sort CCD/CMOS is composed of three sensors, which capture each one independently one of the colors : red, blue, green. Consequences : more precise definition, image is better restored.

But how does a CCD sensor ?

As you may know, when we press the shutter of a camera, the diaphragm opens and allows the passage of a certain amount of light is composed of photons.

Each photon carries a specific energy that is absorbed by a silicon atom, liberates an electron.

All of the electrons thus obtained constitutes a load that is going to be isolated, and then driving to an amplifier that takes care then of the conversion charge/voltage.

In a CCD sensor, the amplifier is disposed at the end of a line of pixels. It then retrieves on the fly the whole of the accumulated charges in that line to each pixel. This recovery is thanks to an electric field which assists in the transport of loads pixel by pixel up to the amplifier.

The cameras equipped with a sensor CDD are not intended for use in industrial, medical, and / or monitoring. There are dedicated to the cinema. Here are some examples :

  • The Sony F-35, which is equipped with a CCD sensor, Super 35 mm, offering a full resolution of 1920*1080 for a frequency up to 50 fps in progressive mode,
  • The HSC 100-Sony which has three CCD Power HAD FX 2/3’ full resolution 1920 x 1080,
  • The PANASONIC AK-HC3500A which uses three CCD sensors 2/3’ 2.2 M pixels,
  • Etc.

But what you need to know is that the CCD sensors and the CMOS sensors do not work the same way.

In a CMOS sensor, each pixel contains a photo-diode (which may be made of a material other than silicon) and its own amplifier. The photo-diode will be used to convert photo-charge and storage charges. Regarding the amplifier, it will help with the conversion of loads in tension in the pixel itself.

What is the advantage ? A reading time is strongly reduced because the information of each pixel can be read without the need to make the transfer !

But this is not the only advantage of CMOS sensors. There are many others that you will discover below.

In the meantime, here are a few examples of digital cameras for cinema that use CMOS sensors :

  • Cameras Cinema EOS in Canon (EOS C500 PL, EOS C300 PL, XC10, etc) with large sensors CMOS Super 35 mm,
  • The RED camera WEAPON that has a sensor CMOS Full-Frame (Vista Vision) of 40.96 mm x 21.6 mm capable of providing images of 8K 8192 x 4320 pixels, or even the RED DRAGON is equipped with a CMOS sensor for 6K which offers an image resolution of 6144*3160 for a frequency of 82 fps at 6K, 120 fps at 5K. Hold on, this sensor is even capable of capturing 20 stops of dynamic range up to 2000 ISO native.
  • The Alexa 65 sensor with the CMOS 6.5 K allows you to shoot in full resolution-6560 x 3102 px from 20 to 27 fps.
  • The Sony pxw subwoofers-FS7 is equipped with a sensor Exmor CMOS type Super35 single chip and able to take charge of the shooting in 4K** 60p or Full HD at a frequency of 60 fps in 4k and 180 fps in HD.

Here, now, on to the benefits and drawbacks of the CMOS sensor.

The advantages and disadvantages of the CMOS sensor

Today, CMOS sensors have almost no disadvantages compared to CCD sensors.

The CCD sensors had the reputation of providing better image quality in high-and low-sensitivity and better performance stability.

But CMOS sensors of today are, in terms of readout noise, a lot more efficient. For example, recently, Adimec, a producer of monitoring devices and machine vision, has been compared to CCD and CMOS : at the level of read-out noise, especially when the light is low, then a Leica M Monochrom equipped with a CCD full-frame, without the Bayer filter caps to 10,000 Iso, a Nikon D4s or a Sony Alpha 7S reached the 400,000 Iso. The difference is remarkable !

In addition, there has been tremendous progress with regard to the sensitivity on the sensors of dslrs (as evidenced by the Nikon D90), the dynamic, the quality in the low sensitivity, and the reproduction more reliable colors.

But what is most impressive is the current efficiency (measured in terms of quantum efficiency/readout noise) of CMOS compared to CCD. While the CCD has almost not progressed since 2011, the CMOS have in 4 years quadrupled their performance and are now the best, and this even at the level of the infrared.

In addition, the CMOS sensors have substantially increased the level of their abilities to retain the details of a very good purpose : they had once a crosstalk reducing slightly the micro-contrast, today they are masters at their disposal for large wavelengths. But that’s not all !

The CMOS sensors also offer speed, low energy consumption, elimination of the balancing taps, and less electronic components, and they are less expensive ; which does not spoil anything !

Another thing : from a technical point of view, the CMOS sensor and the electronic circuitry can be mounted in a single system. This allows to simplify the design of the device and to integrate more functions.

In addition, the camera equipped with CMOS sensors is much more tolerant to changes in light. This is due to two factors : better unloading power in a saturation, and a wide dynamic range.

The only thing that one might still complain that the CMOS sensors, this is their rotating shutter that reads line by line. This is the origin of the delay between the top and bottom of the image. Consequences : deformation of the vertical lines on panning movements and the ” Jello effect “.

But with the launch of the first Sony Pregius, industrial cameras now have CMOS sensors with global shutter the brand, this problem will gradually disappear. As the said shutter uses, such as CCD, or a memory buffer. Therefore, they ensure the simultaneous capture of the whole surface.

That is, you’re already used to cameras equipped with CMOS sensors ? If yes, share your experiences with us in the comments below !

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