IMAGE SENSORS

Submitted By:Pranav Haldar (40)Sumit Srivastava (52)EN 3rd Year

Guided By:Dr. M.A. AnsariAssistant ProfessorDept. of Electrical and Electronics Engineering

Contents What is a Sensor? How to choose a sensor? Types of Sensors What is an Image Sensor? What is a Pixel? What is Fill Factor? Image Sensor History Types of Image Sensors History of CCD History of CMOS What is CCD? Basic Operation of a CCD What is CMOS? Basic Operation of a CMOS CCD vs CMOS Applications of Image Sensors Conclusion

What is a Sensor?

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.

For example, a thermocouple converts temperature to an output voltage which can be read by a voltmeter.

For accuracy, all sensors need to be calibrated

against known standards.

How to choose a sensor? Environment: There are many sensors that work well and

predictably inside, but that choke and die outdoors.

Range: Most sensors work best over a certain range of distances. If something comes too close, they bottom out, and if something is too far, they cannot detect it. Thus we must choose a sensor that will detect obstacles in the range we need.

Field of View: Depending upon what we are doing, we may want sensors that have a wider cone of detection. A wider “field of view” will cause more objects to be detected per sensor, but it also will give less information about where exactly an object is when one is detected.

Types of Sensors Thermal Energy Sensors

Electromagnetic Sensors

Mechanical Sensors

Chemical Sensors

Optical and Radiation Sensors

Acoustic Sensors

Biological Sensors

Thermal Energy Sensors

Temperature Sensors:

Thermometers, Thermocouples, Thermistors, Bi-metal thermometers and Thermostats.

Heat Sensors:

Bolometer, Calorimeter.

Electromagnetic Sensors Electrical Resistance Sensors:

Ohmmeter, Multimeter

Electrical Current Sensors:Galvanometer, Ammeter

Electrical Voltage Sensors:Leaf Electroscope, Voltmeter

Electrical Power Sensors:Watt-hour Meters

Magnetism Sensors:Magnetic Compass, Fluxgate Compass, Magnetometer, HallEffect Device

Mechanical Sensors

Pressure Sensors:Altimeter, Barometer, Barograph, Pressure Gauge, Air Speed Indicator, Rate of Climb Indicator, Variometer.

Gas and Liquid Flow Sensors:Flow Sensor, Anemometer, Flow Meter, Gas Meter, Water Meter, Mass Flow Sensor.

Mechanical Sensors:Acceleration Sensor, Position Sensor, Selsyn, Switch, Strain Gauge.

Chemical Sensors

Chemical sensors detect the presence of specific chemicals or classes of chemicals.

Examples include oxygen sensors, ion-selective electrodes, pH glass electrodes, redox electrodes.

Optical and Radiation Sensors

Radiation Sensors:

Geiger Counter, Dosimeter, Scintillation Counter.

Proximity Sensor.

Scanning laser.

Interferometry.

Acoustic Sensors

Microphone.

Hydrophones.

Seismometers.

Biological Sensors

All living organisms contain biological sensors with functions similar to those of the mechanical devices described.

These include our eyes, skin, ears and many more.

What is an Image Sensor? Unlike traditional camera, that use film to capture and store an

image, digital cameras use solid-state device called image sensor.

Image sensors contain millions of photosensitive diodes known as photosites.

When you take a picture, the camera's shutter opens briefly and each photo site on the image sensor records the brightness of the light that falls on it by accumulating photons. The more light that hits a photo site, the more photons it records.

The brightness recorded by each photosite is then stored  as a set of numbers (digital numbers) that can then be used to set the color and brightness of a single pixel on the screen or ink on the printed page to reconstruct the image.

What is a Pixel?

The smallest discrete component of an image or picture on a CRT screen is known as a pixel.

“The greater the number of pixels per inch the greater is the resolution”.

Each pixel is a sample of an original image, where more samples typically provide more-accurate representations of the original.

What is Fill Factor?

Fill factor refers to the percentage of a photosite that is sensitive to light.

If circuits cover 25% of each photosite, the sensor is said to have a fill factor of 75%. The higher the fill factor, the more sensitive the sensor.

Image Sensor History

Before 1960 mainly film photography was done and vacuum tubes were being used.

From 1960-1975 early research and development was done in the fields of CCD and CMOS.

From 1975-1990 commercialization of CCD took place.

After 1990 re-emergence of CMOS took place and amorphous Si also came into the picture.

Types of Image Sensors

An image sensor is typically of two types:

1. Charged Coupled Device (CCD)

1. Complementary Metal Oxide Semiconductor (CMOS)

History of CCD The CCD started its life as a memory device and

one could only "inject" charge into the device at an input register.

However, it was immediately clear that the CCD could receive charge via the photoelectric effect and electronic images could be created.

By 1969, Bell researchers were able to capture images with simple linear devices; thus the CCD was born.

It was conceived in 1970 at Bell Labs.

History of CMOS

Complementary metal–oxide–semiconductor (CMOS), is a major class of integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits.

CMOS technology is also used for a wide variety of analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass successfully patented CMOS in 1967.

Charged Coupled Device (CCD)

What is CCD?

Charge-coupled devices (CCDs) are silicon-based integrated circuits consisting of a dense matrix of photodiodes that operate by converting light energy in the form of photons into an electronic charge.

Electrons generated by the interaction of photons with silicon atoms are stored in a potential well and can subsequently be transferred across the chip through registers and output to an amplifier.

Basic Operation of a CCD In a CCD for capturing images, there is a photoactive region,

and a transmission region made out of a shift register (the CCD, properly speaking).

An image is projected by a lens on the capacitor array (the photoactive region), causing each capacitor to accumulate an electric charge proportional to the light intensity at that location.

A one-dimensional array, used in cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures a two-dimensional picture corresponding to the scene projected onto the focal plane of the sensor.

Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbor.

The last capacitor in the array dumps its charge into a charge amplifier, which converts the charge into a voltage.

By repeating this process, the controlling circuit converts the entire semiconductor contents of the array to a sequence of voltages, which it samples, digitizes and stores in some form of memory.

Transformation of an image using a CCD array

1- CCD camera, 2- CCD detector, 3- Reading, 4- Amplifier, 5- A/D converter, 6- Digitization , 7- Download

Types of CCD Image Sensors

1. Interline Transfer CCD Image Sensor

2. Frame Transfer CCD Image Sensor

Frame Transfer CCD Image Sensor Top CCD array used for photodetection (photogate) and vertical

shifting.

Bottom CCD array optically shielded – used as frame store.

Operation is pipelined: data is shifted out via the bottom CCDs and the horizontal CCD during integration time of next frame.

Transfer from top to bottom CCD arrays must be done very quickly to minimize corruption by light, or in the dark (using a mechanical shutter).

Output amplifier converts charge into voltage, determines sensor conversion gain.

How CCD works?

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Vertical shift

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Image pixel

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Interline Transfer vs Frame Transfer

Frame transfer uses simpler technology (no photodiodes), and achieves higher fill factor than interline transfer.

Interline transfer uses optimized photodiodes with better spectral response than the photogates used in frame transfer.

In interline transfer the image is captured at the same time (`snap shot' operation) and the charge transfer is not subject to corruption by photodetection (can be avoided in frame transfer using a mechanical shutter).

Frame transfer chip area (for the same number of pixels) can be larger than interline transfer.

Most of today’s CCD image sensors use interlines transfer.

Complementary Metal Oxide Semiconductor (CMOS)

What is CMOS?

“CMOS" refers to both a particular style of digital circuitry design, and the family of processes used to implement that circuitry on integrated circuits (chips).

CMOS circuitry dissipates less power when static, and is denser than other implementations having the same functionality.

CMOS circuits use a combination of p-type and n-type metal–oxide–semiconductor field-effect transistors (MOSFETs) to implement logic gates and other digital circuits found in computers, telecommunications equipment, and signal processing equipment.

Basic Operation of CMOS

In most CMOS devices, there are several transistors at each pixel that amplify and move the charge using wires.

The CMOS approach is more flexible because each pixel can be read individually.

In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and the sensor often also includes amplifiers, noise-correction, and digitization circuits, so that the chip outputs digital bits.

With each pixel doing its own conversion, uniformity is lower.

As shown above, the CMOS image sensor consists of a large pixel matrix that takes care of the registration of incoming light.

The electrical voltages that this matrix produces are buffered by column-amplifiers and sent to the on-chip ADC.

Interline Transfer CCD Image Sensor

Photodiodes are used.

All CCDs are optically shielded, used only for readout.

Collected charge is simultaneously transferred to the vertical CCDs at the end of integration time (a new integration period can begin right after the transfer) and then shifted out.

Charge transfer to vertical CCDs simultaneously resets the photodiodes, (shuttering done electronically for `snap shot' operation).

Types of CMOS Image Sensors

1. Active Pixel Image Sensor

2. Passive Pixel Image Sensor

Active Pixel Image Sensor

3-4 transistors per pixel.

Fast, higher SNR, but

Larger pixel, lower fill factor.

Lower voltage and lower power.

Passive Pixel Image Sensor

1 transistor per pixel.

Small pixel, large fill factor, but

Slow, low signal to noise ratio (SNR).

CCD vs CMOS

CMOS image sensors can incorporate other circuits on the same chip, eliminating the many separate chips required for a CCD.

This also allows additional on-chip features to be added at little extra cost. These features include image stabilization and image compression.

Not only does this make the camera smaller, lighter, and cheaper; it also requires less power so batteries last longer.

CMOS image sensors can switch modes on the fly between still photography and video.

CMOS sensors excel in the capture of outdoor pictures on sunny days, they suffer in low light conditions.

Their sensitivity to light is decreased because part of each photosite is covered with circuitry that filters out noise and performs other functions.

The percentage of a pixel devoted to collecting light is called the pixel’s fill factor. CCDs have a 100% fill factor but CMOS cameras have much less.

The lower the fill factor, the less sensitive the sensor is and the longer exposure times must be. Too low a fill factor makes indoor photography without a flash virtually impossible.

CMOS has more complex pixel and chip whereas CCD has a simple pixel and chip.

APPLICATIONSOF

IMAGE SENSORS

Digital Cameras

Personal Digital Assistance (PDA)

Camcorders

Toys and Robots

Fingerprint Scanner

Some other applications

Biometrics.

Optical Mouse.

Video Conferencing.

Conclusion

Image sensors are an emergent solution for practically every automation-focused machine-vision application.

New electronic fabrication processes, software implementations, and new application fields will dictate the growth of image-sensor technology in the future.

Thank You Very Much For Your Attention