Charged Coupled Device

Many image sensors: Infrared, gamma ray, x-rays etc. Focus on sensors for visible light (slightly into infrared and uv light) Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 2 CCD and CMOS sensors are normally used for visible light  Have good natural sensitivity in near infrared, usually removes that by filters  Can be specially adapted for other parts of the spectrum e.g. X- rays

The beginnings Video camera tube sensors in the 1930s.1969, George Smith ، First, CCD, Willard Boyl Fairchild's effort, led by ex-Bell researcher Gil Amelio, was the first with commercial devices, and by 1974 had a linear 500-element device and a 2-D 100 x 100 pixel device. The first KH-11 KENNAN reconnaissance satellite equipped with charge- coupled device array technology for imaging was launched in December 1976.[6] Under the leadership of Kazuo Iwama, Sony also started a big development effort on CCDs involving a significant investment. Eventually, Sony managed to mass produce CCDs for their camcorders. Before this happened, Iwama died in August 1982; subsequently, a CCD chip was placed on his tombstone to acknowledge his contribution.[7] Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 3

How to convert light to electric charge? Inner photoelectric-effect at a photodiode: Photon excites electron creating a free electron and a hole The hole moves towards the anode, the electron towards the cathode Now we have our charge! Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 4

Charge-Coupled Device (CCD) Integrated circuit Array of connected capacitors (Shift register) Charge of capacitor is transfered to neighbour capacitor At the end of chain, charge is converted into voltage by charge amplifier Transfer stepped by Clock-Signal CCD has photosites, arranged in a matrix. Each comprises a photodiode which converts light into charge and a charge holding region The charges are shifted out of the sensor as a bucket brigade The A/D conversion is done at the edge of the circuit Serial charge processing Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 5

Each capacitor is coupled with a photodiode All capacitors are charged parallelly Charges are transferred serially

CCD-Sensor But how to prevent light to charge up the capacitors while transferring? Mechanical shutter Buffer of capacitors that store the charge until it is transferred Loss of resolution or larger sensor → more expensive

One-or three-chip camerathree-chip is usually at least 3 times as expensive  The color filter matrix for one-chip, usually ”Bayer mosaic” Reduces color resolution to about half Also reduces light collection efficiency Anisotropic in x and y A new method invented by Foveon uses “vertical filters” with less resolution loss

What happens, if too much light hits the sensor ? Capacitors overload Charge “spills“ over to neighbour capacitors Blooming effect occurs

CCD: Pro: No rolling shutter Lower noise Good image quality Con: Analog device! Blooming effect Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 12 CCD vs. CMOS CMOS: Pro: No blooming Cheaper Lower power consumption Faster readout Con: Rolling shutter Variations in brightness (per Pixel)