An image sensor or imaging sensor is a sensor that detects and conveys the information that constitutes an image. It does so by converting the variable attenuation of waves (as they pass through or reflect off objects) into signals, the small bursts of current that convey the information. The waves can be light or other electromagnetic radiation. Image sensors are used in electronic imaging devices of both analog and digital types, which include digital cameras, camera modules, medical imaging equipment, night vision equipment such as thermal imaging devices, radar, sonar, and others. As technology changes, digital imaging tends to replace analog imaging.
Early analog sensors for visible light were video camera tubes; currently used types are semiconductor charge-coupled devices (CCD) or active pixel sensors in complementary metalâ€"oxideâ€"semiconductor (CMOS) or N-type metal-oxide-semiconductor (NMOS, Live MOS) technologies. Analog sensors for invisible radiation tend to involve vacuum tubes of various kinds; digital sensors include flat panel detectors.
CCD vs CMOS
Today, most digital still cameras use either a CCD image sensor or a CMOS sensor. Both types of sensor accomplish the same task of capturing light and converting it into electrical signals.
Each cell of a CCD image sensor is an analog device. When light strikes the chip it is held as a small electrical charge in each photo sensor. The charges are converted to voltage one pixel at a time as they are read from the chip. Additional circuitry in the camera converts the voltage into digital information.
A CMOS imaging chip is a type of active pixel sensor made using the CMOS semiconductor process. Extra circuitry next to each photo sensor converts the light energy to a voltage. Additional circuitry on the chip may be included to convert the voltage to digital data.
Neither technology has a clear advantage in image quality. On one hand, CCD sensors are more susceptible to vertical smear from bright light sources when the sensor is overloaded; high-end CMOS sensors in turn do not suffer from this problem. On the other hand, cheaper CMOS sensors are susceptible to undesired effects that come as a result of rolling shutter.
CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors. CCD is a more mature technology and is in most respects the equal of CMOS. CMOS sensors are less expensive to manufacture than CCD sensors.
Another hybrid CCD/CMOS architecture, sold under the name "sCMOS," consists of CMOS readout integrated circuits (ROICs) that are bump bonded to a CCD imaging substrate â€" a technology that was developed for infrared staring arrays and now adapted to silicon-based detector technology. Another approach is to utilize the very fine dimensions available in modern CMOS technology to implement a CCD like structure entirely in CMOS technology. This can be achieved by separating individual poly-silicon gates by a very small gap. These hybrid sensors are still in the research phase and can potentially harness the benefits of both CCD and CMOS imagers.
The newer sensor technology is Back-side illuminated CMOS (BSI-CMOS) which uses less electricity than traditional CMOS with better performance than CCD, so lower end cameras still use CCD sensors such as those implemented by Fujifilm in its Bridge cameras. CCD sensors are rarely used in new models, except for very high pixel count, big sensor cameras which still use CCDs.
Performance
There are many parameters that can be used to evaluate the performance of an image sensor, including dynamic range, signal-to-noise ratio, and low-light sensitivity. For sensors of comparable types, the signal-to-noise ratio and dynamic range improve as the size increases.
Color separation
There are several main types of color image sensors, differing by the type of color-separation mechanism:
- Bayer filter sensor, low-cost and most common, using a color filter array that passes red, green, or blue light to selected pixel sensors, forming interlaced grids sensitive to red, green, and blue â€" the missing color samples are interpolated using a demosaicing algorithm. In order to avoid interpolated color information, techniques like color co-site sampling use a piezo mechanism to shift the color sensor in pixel steps. The Bayer filter sensors also include back-illuminated sensors, where the light enters the sensitive silicon from the opposite side of where the transistors and metal wires are, such that the metal connections on the devices side are not an obstacle for the light, and the efficiency is higher.
- Foveon X3 sensor, using an array of layered pixel sensors, separating light via the inherent wavelength-dependent absorption property of silicon, such that every location senses all three color channels.
- 3CCD, using three discrete image sensors, with the color separation done by a dichroic prism.
Specialty sensors
Special sensors are used in various applications such as thermography, creation of multi-spectral images, video laryngoscopes, gamma cameras, sensor arrays for x-rays, and other highly sensitive arrays for astronomy.
Until now all digital cameras use a flat sensor, but in 2014 Sony has a prototype of a curved sensor which will reduce/eliminate Petzval field curvature that occurs with a flat sensor. Although it is a prototype design, it is possible the curved sensor will be implemented in Sony's new fixed lens camera at the end of 2014. Use of a curved sensor allows a shorter and smaller diameter of the lens with reduced elements and components with greater aperture and reduced fall light-off at the edge of the photo.
Sensors used in digital cameras
Companies
The largest companies that manufacture imaging sensors include the following:
- Agilent
- Aptina (formerly division of Micron Technology) - Now part of ON Semiconductor
- Canesta
- Canon
- Caeleste
- CMOSIS
- Dalsa
- Eastman Kodak
- ESS Technology
- Fujifilm
- MagnaChip
- Matsushita
- MAZeT GmbH
- Mitsubishi
- Nikon
- OmniVision Technologies
- ON Semiconductor (formerly Cypress Semiconductor)
- PixArt Imaging
- Pixim
- Samsung
- Sharp
- Sony
- STMicroelectronics
- Toshiba
- TowerJazz
- Town Line Technologies
- TransChip
- Trusight
- Trusense imaging - Now part of ON Semiconductor
See also
- Video camera tube
- Semiconductor detector
- Full-frame digital SLR
- Image sensor format, the sizes and shapes of common image sensors
- Color filter array, mosaic of tiny color filters over color image sensors
- Sensitometry, the scientific study of light-sensitive materials
- History of television, the development of electronic imaging technology since the 1880s.
- List of large sensor interchangeable-lens video cameras
- Oversampled binary image sensor
References
External links
- Digital Camera Sensor Performance Summary by Roger Clark.