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Capturing the image CCD and CMOS sensors

There are two types of sensor units that can be used in digital cameras.

CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor) units have one main feature in common. Both use an array of millions of tiny photo sensors. Each sensor creates an electrical current when exposed the light. The strength of the current is proportional to the brightness of the light. But the way in which this electrical data is captured and turned into an image file is very different.

The CMOS unit from the EOS-1Ds Mark II.

One charge at a time

The tiny photo sensors create only a minute electrical current. This must be amplified before it is of any use in creating an image. Some CCDs have a single amplifier. This deals with the current from each sensor in turn. The amplifier is placed at one corner of the sensor where it reads and amplifies the charge from the nearest sensor in the first row. The charge in this sensor is then released, leaving it ‘empty’. All the charges from each sensor in the first row now move along by one sensor, so that the amplifier can read and process the next charge. This continues until the amplifier has dealt in turn with the charges from each of the sensors in the first row.

At this point, all the sensors in the first row are ‘empty’ of charges. This allows all the charges from the sensors in the second row to move down to the first row. The second row is now empty, and is filled with the charged from the third row. In this way, all the charged drop down a row, leaving the top row empty.

The whole process then repeats itself, with the amplifier dealing with the charges that were originally in the second row.

You can guess the rest. The charges originally in the third row move to the first row, where they are fed one by one into the amplifier. This continues row by row, charge by charge, until the charge from the far end of the top row finally reaches the amplifier.

Of course, the word ‘finally‘ is relative. The whole process takes place very quickly, often in a fraction of a second.

Each charge from the sensor array is ‘tagged’ by the amplifier unit before is passed on to the camera’s microcomputer so that each piece of data can be reassembled in exactly the same sequence to produce the image.

Only five EOS digital cameras use a CCD image sensor, though four of these are not strictly Canon cameras. The EOS DCS3 (July 1995), DCS1 (December 1995), D2000 (March 1998) and D6000 (December 1998) were all produced in collaboration with Kodak. Canon provided the body, but the image sensor and electronics came from Kodak.

There is only one EOS digital camera that uses a CCD in which both body and electronics designed and built by Canon - the EOS-1D (September 2001) - and even here the CCD sensor is outsourced.

The charges (brightness data) from a CCD move across the array photo sensor by photo sensor until they reach the external amplifier unit(s).

The CMOS advantage

The CMOS unit takes a different approach to processing the charges from the millions of photo sensors. Instead of one amplifier at the side of the array, each individual pixel has its own personal amplifier. This means that all the charges can be processed at the same time, clearing the sensors for the next exposure.

Canon has concentrated all its research and development on the CMOS unit, rather than the CCD. It not only designs and makes all its own CMOS units, but also designs and manufactures the equipment that makes the CMOS units. The first EOS digital camera with a Canon CMOS sensor was the D30 May 2000). All later EOS digital cameras, with the exception of the EOS-1D, also feature a Canon CMOS image sensor.

To the outside world, Canon’s concentration on CMOS seemed a little perverse. In the late 1990s, images from CCDs were of significantly higher quality than those from CMOS units. There were two main reasons for this. CCDs are less prone to ‘noise’ - a grain-like pattern that appears in the image, especially at higher equivalent ISO speeds. Also, the light sensitivity of a CMOS unit was lower than that of a CCD. This required greater amplification of the signal - leading to more ‘noise’.

But Canon recognised that CMOS units offered long-term benefits, and set about overcoming the disadvantages.

One advantage is that CMOS units need a lot less power to operate than CCDs. A digital camera makes many demands on a battery to operate autofocusing, autoexposure, shutter control and - in some cameras - built-in flash. Any technique that helps to reduce the drain on the battery is a good thing.

Also, CMOS units are much cheaper to manufacture than CCDs. This might seem strange considering each sensor needs its own amplifier, but all this is done with microelectronics directly on the sensor unit. The CCD needs a separate amplifier and other associated electronics that add to the cost of the already expensive unit.

The lower cost of CMOS units is important to Canon because it is selling its consumer range of digital camera to a mass market. The use of CMOS was influential in the marketing of the first sub-€1,500 digital SLR - the EOS 300D - in September 2003.

Overcoming the problems

CMOS sensors were known for their ‘noise’. In simple terms, this is electrical activity that does not play any part in forming a true image of the subject. One cause of this noise is the individual amplifiers linked to each photo sensor. Inevitably, there are slight variations in output of the millions of amplifiers across the array, and this creates a noise pattern across the image.

Canon has overcome this with lateral thinking. Rather than trying to equalise the amplifiers, it has accepted that there will always be a noise pattern. So every time you take a picture, you actually make two exposures - one of the subject, and one with the shutter closed. This second exposure only captures the noise pattern. If the values of this pattern are subtracted from the first exposure, you obtain an image with little or no noise.

The second main problem with CMOS sensors is also related to the amplifiers, though in a different way. On a CCD, almost all the surface of the unit is occupied by photo sensors. On a CMOS unit, space is needed for amplifier circuitry, so less of the space is given over to the photo sensors. This means that some of the light reaching the unit hits areas between the sensors and is lost, reducing the overall sensitivity of the unit.

There are two solutions to this problem. First, the circuitry can be made smaller and the photo sensors bigger. But there is a limit to this reduction - and an escalation in manufacturing cost - so there will always be space between the sensors. To counteract this, millions of microlenses are used, one over each photo sensor. The microlens covers the photo sensor and the circuitry. Rays of light hitting the edge of the micro lens, which would normally be wasted, are now focused on the photo sensor. The effect is to increase the sensitivity of the sensor unit.

The result is that Canon CMOS sensor units can compete with, and even outperform, CCD sensor units. All current EOS professional digital cameras use CMOS sensor units, and reviews regularly comment on the lack of noise, even at high ISO settings.

Photo sensors on a CMOS unit are positioned inside ‘wells’. Between the wells is circuitry associated with the amplifier units. Rays of light reaching the spaces between the wells are wasted, reducing the efficiency of the CMOS unit.

Placing a micro lens over each sensor and its circuitry captures the light that would normally be wasted and redirects it to the photo sensors.