A digital image sensor uses an image sensor array of millions of tiny pixels in order to produce
a photographic image. Click the camera shutter button and the exposure begins, each of these pixels has a "photosite" which is uncovered to collect and store photons in a cavity. Once the exposure finishes, the camera closes each of these photosites, and then
attempts to assess how many photons fell into each. The relative quantity of photons in each cavity are then sorted into various intensity levels, whose precision is determined by bit depth (0 - 255 for an 8-bit image).
 CCD A image sensor on a flexible circuit board
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.
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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.
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.
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.
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.
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
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