Active-pixel sensor

An active-pixel sensor (APS) is an image sensor made up of tiny parts called pixels. Each of these pixels has a special part called a photodetector, usually a pinned photodiode, and one or more tiny electrical switches called transistors that help it work. These sensors are found in many devices we use every day, like the cameras in our cell phones, web cameras, small digital cameras, and bigger cameras such as digital single-lens reflex cameras and mirrorless interchangeable-lens cameras.

In most APS sensors today, something called MOS field-effect transistors, or MOSFETs, are used to make the signals stronger. There are different kinds of APS sensors. One of the earliest types used NMOS technology, but now the most common type uses something called complementary MOS, or CMOS. This type is often just called a CMOS sensor. CMOS sensors became very popular because they are a good alternative to another kind of sensor called a charge-coupled device, or CCD. By the middle of the 2000s, CMOS sensors were being used more than CCD sensors.

Sometimes the term active pixel sensor refers to just one of these tiny pixel parts, not the whole group of them together. When we talk about the whole group, we might call it an active pixel sensor imager or active-pixel image sensor. These sensors help us capture clear pictures and videos in many of the devices we use.

History

Further information: Image sensor § History

While studying a special kind of technology called metal-oxide-semiconductor (MOS), researchers Willard Boyle and George E. Smith discovered that tiny bits of electric charge could be stored. This discovery led to the invention of a device called the charge-coupled device (CCD) in 1969.

One challenge with CCD technology was that it needed perfect charge transfer during reading the image. This made it hard to use in low light, create large images, or work well with other electronics. It also had trouble working in very cold temperatures or very fast speeds.

Later, scientists created a new type of image sensor called the pinned photodiode (PPD). This improved many problems with earlier sensors and became common in consumer video and digital cameras.

The active-pixel sensor (APS) was developed to solve problems with older sensors. Each pixel in an APS contains tiny amplifiers that help make the image clearer and reduce noise. This idea was first suggested in 1968 and became practical in the 1980s thanks to better technology for making tiny electronic parts.

By the early 1990s, a new type called the CMOS sensor was created. This used a different kind of technology and became very popular because it used less power and could be made more easily. Today, CMOS sensors are used in most digital cameras, phone cameras, and computer webcams. They helped make it easy for everyone to share photos online and changed how many people communicate and express themselves.

Comparison to CCDs

Active-pixel sensors (APS) solve speed and size problems that older sensors had. They use less power, show fewer blurry images, and are easier to make than older sensors called CCDs. APS sensors can do both capturing and processing images in one tiny chip. This makes them very popular in devices like camera phones, security cameras, and optical mice. Companies such as Canon, Samsung, and Sony make these sensors.

One big plus of APS sensors, especially those made with CMOS technology, is that they cost less to produce. They also handle bright spots better, preventing colours from spreading to nearby areas. However, they can sometimes create a skew effect in moving scenes. Despite this, modern APS sensors have become just as good as CCD sensors at reducing unwanted noise in pictures.

Architecture

The active-pixel sensor (APS) is a special type of image sensor used in many cameras and phones. Each tiny part of the sensor, called a pixel, has a part that catches light and a few tiny parts that help read the light signal.

These sensors can be made in different ways. Some have parts for catching light and parts for reading the signal side by side, which makes making them easier. Others stack these parts, which can make the sensor smaller. Some APS sensors even use special transistors for big jobs like taking pictures using X-rays.

Design variants

Many different designs for pixel sensors have been created. A basic design uses the fewest wires and transistors, leaving more space for the light sensor. Using too many transistors can reduce the amount of the pixel that can detect light. Designers must balance pixel size with qualities like reducing noise (which affects how accurately light is measured) and preventing image lag (when parts of an old picture remain in a new one).

Some designs use a "hard reset" method, which stops image lag but can add noise. Others combine hard and soft reset methods to balance noise and lag. More advanced designs, like the active-reset pixel, can lower noise even more but require more complex setups or larger pixels.