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Display Systems and photosensors ( Part 4). LCD TFT LED-OLED CCD CMOS. Charged Coupled Device (CCD).  Many image sensors: Infrared, gamma ray, x-rays etc.  Focus on sensors for visible light (slightly into infrared and uv light).

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Display Systems and photosensors ( Part 4)

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 Many image sensors: Infrared, gamma ray, x-rays etc.

 Focus on sensors for visible light (slightly into infrared and uv light)

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

Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 2


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, Sonymanaged 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


  • 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 camera three-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 neighbor capacitors
  • Blooming effect occurs


  • CCD:
  • Pro:
  • No rolling shutter
  • Lower noise
  • Good image quality
  • Con:
  • Analog device!
  • Blooming effect
  • CMOS:
  • Pro:
  • No blooming
  • Cheaper
  • Lower power consumption
  • Faster readout
  • Con:
  • Rolling shutter
  • Variations in brightness (per Pixel)

Michael Thomas, TU Berlin, 2010 Processing Digital Camera Images, WS 2010/2011, Alexa/Eitz 12

cmos image sensors

CMOS image sensors

Presenter: Alirezaeyvazzadeh


Complementary metal–oxide–semiconductor

a technology for constructing integrated circuits

CMOS technology is used in



static RAM

other digital logic circuits

CMOS technology is also used for several analog circuits such as

image sensors

data converters


CMOS image sensors

Two most common types of sensors used in digital cameras:

CCD - Charge Coupled Device

CMOS - Complementary Metal Oxide Semiconductor

All CCD and CMOS image sensors operate by exploiting the photoelectric effect to convert light into electricity

With the CMOS imager both the 'Photon-to-Electron' conversion and the 'Electron-to-Voltage' conversion is done within the pixel


Some drawbacks of CCDs

complex clocking requirements

high power consumption

difficulty of on-chip integration of circuitry

limited frame rate

CMOS image sensors address these drawbacks by using the same technology as microprocessors and memory chips


Advantages and drawbacks of the CMOS image sensors

Low Power Consumption

one-third to more than 100 times less than that of CCDs

Lower cost compared to CCD’s technology

On chip functionality

A sensor can integrate various signal and image processing blocks such as amplifiers, ADCs, circuits for color processing and data compression, etc. on the same chip


High-speed imaging

Random access of image data

Selective read-out mechanism

Also they are still too noisy and less sensitive than CCDs


present applications of CMOS image sensors

internet camera

digital still camera

machine vision


children’s toy

medicine and dentistry

fingerprint ID



motion analysis

industrial inspection

quality control

process control

target tracking



Overall architecture

CMOS imagers architecture can be divided into four main blocks:

1. Pixel Array

2. Analog Signal Processors

3. Row and Column Selector

4. Timing and Control


Basic pixel structures

Each pixel contains a photodetector and some transistors. This area is the heart of an image sensor and the imaging quality is largely determined by the performance of this area.

active pixels (APS)

passive pixels (PPS)

Pixel circuits

An APS has three transistors in a pixel, while a PPS has only one transistor. To achieve further improvement, an advanced APS that has four transistors in a pixel, the so-called 4T-APS, has been developed.

APS are sensors that implement a buffer per pixel. Currently, APS are the predominant devices, although in some cases PPS are also used.

The in-pixel amplifier in APS enables non-destructive read of the photodiode charge at a faster speed and a generally higher signal-to-noise ratio (SNR) than PPS


Operation of a photodetector comprises:

  • generation of free electron-hole pairs due to impinging light
  • separation and collection of electrons and holes
  • (c) production of an output signal through interaction with other components

Several popular silicon-based photosensing devices are


PN and PIN photodiodes


photo gates


PN junction photodiode

an important sensor for digital imaging

The potential voltage decreases when electrons accumulate. By measuring the voltage drop, the total amount of light power can be obtained.

active pixel sensor 3t aps
Active pixel sensor, 3T-APS

First, the reset transistor MRS is turned on

PD is reset to the value Vdd −Vth

Vth is the threshold voltage of transistor MRS

MRS is turned off and the PD is electrically floated

The accumulated charge changes the potential in the PD; the voltage of the PD,VPD ,decreases according to the input light intensity

After an accumulation time, the select transistor MSEL is turned on and the output signal in the pixel is read out in the vertical output line.

When the read-out process is finished MSEL is turned off and MRS is again turned on to repeat the above process.


Sensor peripherals


In CMOS image sensors, to address each pixel, a scanner or a decoder is used

Readout circuits

The voltage of a PD is read with a source follower (SF)

a follower transistor MSF is placed in a pixel and a current load Mb is placed in each column