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WHAT IS A SENSOR?

A sensoris a device that measures a physical quantity and converts itinto a signal which can be read by an observer or by an instrument.

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

For accuracy, all sensors need to be calibrated against known standards. 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. Ifsomething 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 sensorsthat 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.

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 asphotosites.

When you take a picture, the camera's shutter opens briefly and eachphoto 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 ofnumbers (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.

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What IS A PIXEL

The smallest discrete component of an image or picture on a CRT screenis 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.

FILL FACTOR

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

If circuits cover 25% of each photosite, the sensor is said to have a fillfactor 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 werebeing used.

From 1960-1975 early research and development was done in the fieldsof 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)2. Complementary Metal Oxide Semiconductor (CMOS)

CCD SENSOR HISTORY

3. The CCD started its life as a memory device and one could only "inject"charge into the device at an input register.

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

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

6. It was conceived in 1970at Bell Labs.Cmos

7. Complementary metaloxidesemiconductor (CMOS), is a major class ofintegrated circuits. CMOS technology is used in microprocessors,

microcontrollers, static RAM, and other digital logic circuits.

8. CMOS technology is also used for a wide variety of analog circuits suchas image sensors, data converters, and highly integrated transceivers for

many types of communication. Frank Wanlass successfully patented

CMOS in 1967.

9. Charge-coupled devices (CCDs) are silicon-based integrated circuitsconsisting of a dense matrix of photodiodes that operate by converting

light energy in the form of photons into an electronic charge.

10.Electrons generated by the interaction of photons with silicon atoms arestored 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 atransmission region made out of a shift register (the CCD, properly

speaking).

An image is projected by a lens on the capacitor array (the photoactiveregion), 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 theimage, while a two-dimensional array, used in video and still cameras,

captures a two-dimensional picture corresponding to the sceneprojected onto the focal plane of the sensor.

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Once the array has been exposed to the image, a control circuit causeseach 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 entiresemiconductor contents of the array to a sequence of voltages, which it

samples, digitizes and stores in some form of memory.