Digital Camera basics - Part 2: Image Sensors

November 12, 2007

A conventional camera and a digital camera differ in the way they capture and store an image. The earlier cameras (even now they are not completely replaced by digital cameras) use film to capture the images while a digital camera uses an image sensor instead. The image sensor is made up of millions of pixels which are photosensitive and covert the light that falls on them into electrically charged particles, i.e. electrons. In other words, the resolution of a digital camera is the number of pixels or tiny photosensitive spots present the sensor. For instance, a 5 mega pixel digital camera will have a sensor having 5 million pixels.

CCD sensors and CMOS sensors

Digital cameras use two types of image sensors to capture the images - charge-coupled devices (CCD) and Complimentary Metal Oxide Semi-conductor devices (CMOS). Both the sensors have photodiodes or photosites which capture the light and convert it into electrically charged particles (electrons). Both the technologies produce excellent pixels quality, however, they differ from each other in terms of processing the image and the hardware aspects.

CCD sensors (Charge-Coupled Devices)

kodak CCD sensor_1 Digital Camera basics - Part 2: Image Sensors

Being the older of the two technologies and long been used in various digital cameras like camcorders, compact digital cameras, scanners and astronomical telescopes, CCD is considered better suited to capturing of images in digital cameras and most digital cameras use this technology. Since CCD has passed through various advancements during its evolution, it is capable of producing better resolution of an image as it has large number of pixels that can produce much more picture details than its counterpart.

A CCD sensor, when it is exposed to light, its pixels get charged with different magnitudes depending upon the intensities of the light that strike each pixel. The register in the sensor reads the charge of the pixels row by row. The pixel values are fed into an analog-to-digital converter which convert them into digital format (in 1s and 2s strings). This is done for each and every row of pixels on the surface of the CCD chip.

CMOS Sensors (Complimentary Metal Oxide Semi-conductor devices)

kodak CMOS sensor Digital Camera basics - Part 2: Image Sensors

CMOS sensor has been recently used in digital cameras for a few advantages like lower cost of production and less consumption of power. They are made up of small circuits and other devices like transistors knitted together on a silicon chips. Since CMOS chips are used in PCs as processors and memory chips, they are generally produced in large scale resulting in lower cost of production, thus cutting the price of digital cameras when they are used instead of CCD sensors. Another significant advantage that a CMOS sensor has over CCD sensor is that it consumes less power, just one-hundredth of what a similar sized CCD chip will consume. On the negative side, CMOS sensors generally produce lesser pixels quality in comparison with CCD sensor as their photo sensitivity is much lower than CCD sensors. This is because the light strikes the network of transistors arranged near the pixels (photodiodes). CMOS sensors also produce noisy pictures as the processing circuits are present on the same chip.

Major Differences between CCD Sensors and CMOS sensors

CCD sensors are well known for their superior pixels quality. They are highly photo sensitive. In comparison, CMOS sensors are less sensitve to the light and produce slighltly inferior picture quality.
CCD sensors need a separate amplifier and Analog to Digital Converter (ADC) to process the image while CMOS sensors come with integrated transistors which amplfy the charges and hence do not need ADC.
CCD sensors generally do not produce noisy image processing while CMOS sensor are quite prone to noisy pictures.
CMOS sensors are less expensive and consume much less power. However, a CCD usually costs much higher and consumer many times the power a similar sized CMOS sensor consumes.

In spite of these differences, both CCD and CMOS sensors are being used by the leading digital camera makers. Despite their advantages and disadvantages, both produce excellent picture quality.

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