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)
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, 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 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
• Capacitors overload
• Charge “spills“ over to neighbor capacitors
• Blooming effect occurs
the sensor?
CCD vs. CMOS
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
Presenter: Alireza eyvazzadeh
Complementary metal–oxide–semiconductor
a technology for constructing integrated circuits
CMOS technology is used in
Microprocessors
Microcontrollers
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 'Electronto-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
Miniaturization
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
Automotive
children’s toy
medicine and dentistry
fingerprint ID
surveillance
aerospace
motion analysis
industrial inspection
quality control
process control
target tracking
spectroscopy
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)
Pixel circuits passive pixels (PPS)
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:
(a) generation of free electron-hole pairs due to impinging light
(b) 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
Photoconductors
PN and PIN photodiodes
Phototransistors
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
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
Addressing
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
The End