Image sensor
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A CCD image sensor on a flexible circuit board
Image sensor on the motherboard of a Nikon Coolpix L2 6MP
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. Early sensors were video camera tubes but a modern one 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] CMOS sensors are less expensive than the CCD sensors when it comes to manufacturing.
Recently, however a newer architecture has been developed which can benefit from the advantages of CCDs while retaining the features of CMOS chips. These hybrid CCD/CMOS devices, currently being sold under the name "sCMOS", are comprised of CMOS Readout Integrated Circuits (ROICs) that are bump bonded to a CCD imaging substrate --- a technology that was developed for infrared focal plane arrays and now adapted to silicon-based detector technology.[3]
[edit] Performance
An infrared-blocking filter removed from a Canon EOS 350D dSLR
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 that passes red, green, or blue light to selected pixel sensors, forming interlaced grids sensitive to red, green, and blue – 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 sensors also include backside 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.[3][4]
- 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. Considered the best quality, and generally more expensive than single-CCD sensors.
[edit] Table of sensors commercially used in digital cameras
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Width Height Aspect ratio Actual pixel count Megapixels Camera examples 320 240 76,800 0.01 Steven Sasson Prototype (1975) 640 480 307,200 0.3 Apple QuickTake 100 (1994) 832 608 505,856 0.5 Canon Powershot 600 (1996) 1,024 768 786,432 0.8 Olympus D-300L (1996) 1,280 960 1,228,800 1.3 Fujifilm DS-300 (1997) 1,280 1,024 5:4 1,310,720 1.3 Fujifilm MX-700 / Leica Digilux (1998), Fujifilm MX-1700 (1999) / Leica Digilux Zoom (2000) 1,600 1,200 1,920,000 2 Nikon Coolpix 950 2,012 1,324 2,663,888 2.74 Nikon D1 2,048 1,536 3,145,728 3 Canon PowerShot A75, Nikon Coolpix 995 2,272 1,704 3,871,488 4 Olympus Stylus 410, Contax i4R (although CCD is actually square 2,272x2,272) 2,464 1,648 4,060,672 4.1 Canon 1D 2,640 1,760 4,646,400 × 3 4.7 × 3 (14.1 MP) Sigma SD14, Sigma DP1 (3 layers of pixels, 4.7 MP per layer, in Foveon X3 sensor) 2,560 1,920 4,915,200 5 Olympus E-1, Sony Cyber-shot DSC-F707 2,816 2,112 5,947,392 6 Olympus Stylus 600 Digital 3,008 2,000 6,016,000 6 Nikon D40, D50, D70, D70s, Pentax K100D 3,072 2,048 6,291,456 6.3 Canon 300D, Canon 10D 3,072 2,304 7,077,888 7 Olympus FE-210, Canon PowerShot A620 3,456 2,304 7,962,624 8 Canon 350D 3,264 2,448 7,990,272 8 Olympus E-500, Olympus SP-350, Canon PowerShot A720 IS 3,504 2,336 8,185,344 8.2 Canon 30D, Canon 1D II, Canon 1D II N 3,520 2,344 8,250,880 8.25 Canon 20D 3,648 2,736 9,980,928 10 Olympus E-410, Olympus E-510, Panasonic FZ50 3,872 2,592 10,036,224 10 Nikon D40x, Nikon D60, Nikon D3000, Nikon D200, Nikon D80, Pentax K10D, Sony Alpha A100 3,888 2,592 10,077,696 10.1 Canon 400D, Canon 40D 4,064 2,704 10,989,056 11 Canon 1Ds 4,000 3,000 12,000,000 12 Canon Powershot G9, Fujifilm FinePix F100fd 4,032 3,024 12,192,768 12.3 Olympus PEN E-P1 4,256 2,832 12,052,992 12.1 Nikon D3, Nikon D3S, Nikon D700 4,272 2,848 12,166,656 12.2 Canon 450D 4,288 2,848 12,212,224 12.2 Nikon D2Xs/D2X, Nikon D300, Nikon D90, Nikon D5000 4,368 2,912 12,719,616 12.7 Canon 5D 4,672 3,104 14,501,888 14.5 Pentax K20D 4,992 3,328 16,613,376 16.6 Canon 1Ds II 5,270 3,516 18,529,320 18.5 Leica M9 5,616 3,744 21,026,304 21.0 Canon 1Ds III, Canon 5D Mark II 6,048 4,032 24,385,536 24.4 Sony α 850, Sony α 900, Nikon D3X 7,500 5,000 37,500,000 37.5 Leica S2 7,212 5,142 39,031,344 39.0 Hasselblad H3DII-39 8,176 6,132 50,135,232 50.1 Hasselblad H3DII-50 8,956 6,708 60,076,848 60.1 Hasselblad H4D-60 8,984 6,732 60,480,288 60.5 Phase One P65+
[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
- Color filter array, mosaic of tiny color filters over color image sensors
- Sensitometry, the scientific study of light-sensitive materials
[edit] References
- ^ CCD vs CMOS from Photonics Spectra 2001
- ^ Sensors By Vincent Bockaert
- ^ a b [1]
- ^ OmniVision on Backside-illuminated CMOS Sensors
[edit] External links
- Digital Camera Sensor Performance Summary by Roger Clark.
- Image Sensors Europe 2010 Conference by IntertechPira.
- Custom Image sensors