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

Physical phenomenon

Measurement Output

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

What is an Image Sensor?

An image sensor is a device that converts an optical image into

an electrical signal.

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 photo sites.

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.

What is a Pixel?

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

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 photo site, thesensor is said to have a fill factor of 75%. Thehigher the fill factor, the more sensitive thesensor.

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 andCMOS.

From 1975-1990 commercialization of CCD took place.

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

Types of Image Sensors

CCD: Charged Coupled Device CMOS: Complementary Metal Oxide Semiconductor

Charged Coupled Device (CCD)

Charge-coupled devices (CCDs)are silicon-based integratedcircuits consisting of a densematrix of photodiodes thatoperate by converting lightenergy in the form of photonsinto an electronic charge.

Electrons generated by theinteraction of photons withsilicon atoms are stored in apotential well and cansubsequently be transferredacross the chip through registersand output to an amplifier.

Basic Operation of a CCD

In a CCD for capturing images, there is aphotoactive region, and a transmission regionmade out of a shift register (the CCD, properlyspeaking).

An image is projected by a lens on the capacitorarray (the photoactive region), causing eachcapacitor to accumulate an electric chargeproportional 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 stillcameras, captures a two-dimensional picturecorresponding to the scene projected onto thefocal plane of the sensor.

Types of CCD Image Sensors

1. Interline Transfer CCD Image Sensor 2. Frame Transfer CCD Image Sensor

Interline Transfer vs. Frame Transfer

Frame transfer uses simpler technology (no photodiodes), and achieves higher fill factorthan 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 thecharge transfer is not subject to corruption by photo detection (can be avoided in frametransfer using a mechanical shutter).

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

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

Complementary Metal Oxide Semiconductor (CMOS)

“CMOS" refers to both a particular styleof digital circuitry design, and the familyof processes used to implement thatcircuitry on integrated circuits (chips).

CMOS circuitry dissipates less powerwhen static, and is denser than otherimplementations having the samefunctionality.

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

Basic Operation of CMOS

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

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

With each pixel doing its own conversion,uniformity is lower. As shown above, theCMOS image sensor consists of a large pixelmatrix that takes care of the registration ofincoming light.

The electrical voltages that this matrixproduces are buffered by column-amplifiersand sent to the on-chip ADC.

Types of CMOS Image Sensors

1. Active Pixel Image Sensor 2. Passive Pixel Image Sensor

CCD vs CMOS

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

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

Not only does this make the camera smaller, lighter, and cheaper; it also requires lesspower 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 lowlight conditions.

Their sensitivity to light is decreased because part of each photo site is covered withcircuitry that filters out noise and performs other functions.

Sensing Color

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 exposuretimes must be. Too low a fill factor makes indoor photography without a flashvirtually impossible.

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

Application of Image Sensor

Digital Cameras

Personal Digital Assistance (PDA)

Camcorders

Toys and Robots

Finger print Scanner

Virtual Key Board

Image sensors applications in medicine

Security Market Volume of CCD, CMOS

THANK YOU