Understanding Image Sensors

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As light passes through a lens, it is focused on the camera’s image sensor. An image sensor is made up of many photosites and each photosite corresponds to a picture element, more commonly known as “pixel”, on an image sensor. Each pixel on an image sensor registers the amount of light it is exposed to and converts it into a corresponding number of electrons. The brighter the light, the more electrons are generated.

When building a camera, there are two main technologies that can be used for the camera’s image sensor:

 

CCD (charge-coupled device)

CMOS (complementary metal-oxide semiconductor)

 

Megapixel sensors

While CCD and CMOS sensors are often seen as rivals, each has unique strengths and weaknesses that make it appropriate for different applications. CCD sensors are produced using a technology that has been developed specifically for the camera industry. Early CMOS sensors were based on standard technology already extensively used in memory chips inside PCs, for example. Modern CMOS sensors use a more specialized technology and the quality of the sensors is rapidly increasing.

 

CCD technology

CCD sensors have been used in cameras for more than 30 years and present many advantageous qualities. Generally, they still offer slightly better light sensitivity and produce somewhat less noise than CMOS sensors. Higher light sensitivity translates into better images in low light conditions. CCD sensors, however, are more expensive and more complex to incorporate into a camera. A CCD can also consume as much as 100 times more power than an equivalent CMOS sensor.

 

CMOS technology

Recent advances in CMOS sensors bring them closer to their CCD counterparts in terms of image quality. CMOS sensors lower the total cost for cameras since they contain all the logics needed to build cameras around them. In comparison with CCDs, CMOS sensors enable more integration possibilities and more functions. CMOS sensors also have a faster readout (which is advantageous when high-resolution images are required), lower power dissipation at the chip level, as well as a smaller system size. Megapixel CMOS sensors are more widely available and are less expensive than megapixel CCD sensors.

 

Megapixel sensors

For cost reasons, many megapixel sensors (i.e., sensors containing a million or more pixels) in megapixel cameras are the same size as or only slightly larger than VGA sensors that provide a resolution of 640×480 (307,200) pixels. This means that the size of each pixel on a megapixel sensor is smaller than on a VGA sensor. For instance, a megapixel sensor such as a 1/3-inch, 2-megapixel sensor has pixel sizes measuring 3 μm (micrometers/microns) each. By comparison, the pixel size of a 1/3-inch VGA sensor is 7.5 μm. So while the megapixel camera provides higher resolution and greater detail, it is less light sensitive than its VGA counterpart since the pixel size is smaller and light reflected from an object is spread to more pixels.

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