Image sensor
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An image sensor is a device that converts an optical image to an electric signal. It is used mostly in digital cameras and other imaging devices. An image sensor is typically a charge-coupled device (CCD) or a complementary metal–oxide–semiconductor (CMOS) active-pixel sensor.
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[edit] 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.
A CCD 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 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. CMOS can potentially be implemented with fewer components, use less power and/or provide faster readout than CCDs. CCD is a more mature technology and is in most respects the equal of CMOS.[1][2]
[edit] Performance
There are many parameters that can be used to evaluate the performance of an image sensor, including its dynamic range, its signal-to-noise ratio, its low-light sensitivity, etc. For sensors of comparable types, the signal-to-noise ratio and dynamic range improve as the size increases.
[edit] Color sensors
There are several main types of color image sensors, differing by the means of the color separation mechanism:
- Bayer sensor, low-cost and most common, using a color filter array such as a Bayer filter that passes red, green, or blue light to selected sensels, or pixels, forming interlaced grids sensitive to red, green, and blue. The image is then interpolated using a demosaicing algorithm.
- Foveon X3 sensor, using an array of layered sensors where every pixel contains three stacked sensors sensitive to the individual colors.
- 3CCD, using three discrete image sensors, with the color separation done by a dichroic prism. Considered the best quality, and generally more expensive than single-CCD sensors.
[edit] Specialty sensors
Special sensors are used for various applications such as thermal imaging, creation of multi-spectral images, gamma cameras, sensor arrays for x-rays, and other highly sensitive arrays for astronomy.
[edit] See also
- Video camera tube
- Semiconductor detector
- Contact image sensor
- Image sensor format, the sizes and shapes of common image sensors
- Sensitometry, the scientific study of light-sensitive materials
[edit] References
- ^ CCD vs CMOS from Photonics Spectra 2001
- ^ Sensors By Vincent Bockaert
[edit] External links
- Digital Camera Sensor Performance Summary by Roger Clark.
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