Active pixel sensor
lastmod 2019-03-05created 2019-03-054 edits
An active-pixel sensor (APS) is an image sensor where each picture element ("pixel") has a photodetector and an active amplifier. There are many types of integrated circuit active pixel sensors including the complementary metal–oxide–semiconductor (CMOS) APS used most commonly in cell phone cameras, web cameras, most digital pocket cameras since 2010, in most digital single-lens reflex cameras (DSLRs) and Mirrorless interchangeable-lens cameras (MILCs). Such an image sensor is produced using CMOS technology (and is hence also known as a CMOS sensor), and has emerged as an alternative to charge-coupled device (CCD) image sensors.
Pros
A CMOS image sensor is a solid-state image sensor that uses CMOS (complementary metal–oxide–semiconductor). Like the CCD image sensor it uses photodiodes, but the manufacturing process and the way signals are read out differ.
Each unit cell has its own amplifier, and a characteristic is that less electrical noise is generated when reading out the photoelectrically converted electrical signal. Because it can be mass-produced as an application of the CMOS logic LSI manufacturing processor, it has the advantages of lower manufacturing cost compared with CCD image sensors, which have high-voltage analog circuits, and lower power consumption thanks to the small device size.
In addition, logic circuits can be fabricated in the same process to put image processing circuits On-Chip, and this has been researched for application in image recognition devices and artificial vision devices, some of which have been commercialized. Because of this, it is sometimes called an artificial retina chip.
Cons
Although it has several advantages over CCD, the device tends to become unstable easily in low-light conditions and a lot of noise tends to appear in captured images. Also, because a fixed amplifier is assigned to each pixel, it has the drawback of fixed-pattern noise caused by differences in amplifier characteristics, so a circuit to correct this is required. Recently, the signal-to-noise ratio has improved dramatically thanks to various improvements such as higher output and lower noise of the PC, improved charge transfer efficiency from the PD to the amplifier, and sharing transistors across multiple pixels to relatively enlarge the light-receiving area of the PD.
Also, due to the structural problem that charge conversion cannot be performed simultaneously, it has the drawback that when shooting a fast-moving object, the image wobbles in the direction of motion. This can be overcome by dividing a single CMOS into blocks. However, because this technical approach offsets CMOS's advantage of low cost, CMOS is rarely used in compact digital cameras. On the other hand, when the image sensor is large, as in DSLR cameras, CCD is at a disadvantage in terms of power consumption.
See also
- Sensor
- Active pixel sensor (CMOS sensor)
- Charge-coupled device (CCD)