•1D or 2D array of photosensors can record optical images projected onto it by lens system.

•Individual photosensor in an imaging array is called pixel (picture element).

•High image resolution requires small pixels.

Two strategies for pixel operation:

• Continuous-time mode

Conversion of photogenerated charge into steady-state photocurrent

• Integration mode

Collection of photogenerated charge on capacitor until readout, followed by reset

1. FILL FACTOR

2. CONVERSION GAIN

3. SENSITIVITY

4. FRAME RATE

5. FIXED PATTERN NOISE

6. BLOOMING

Image Sensors:

1. Charge Coupled Devices (CCD’s)

2. Complementary Metal Oxide Semiconductors (CMOS)

* Passive Pixel Sensor (PPS)

* Active Pixel Sensor (APS)

3. Photogate Active Pixel Sensor (APS + CCD)

CCD Image Sensors

 A CCD is a dynamic analog (charge) shift register implemented using closely spaced MOS capacitors clocked using 2, 3, or 4 phase clocks -- capacitors operate in deep depletion regime when clock is high

 Charge transfer (from one capacitor to the next) must occur at high enough rate to avoid corruption by leakage, but slow enough to ensure high charge transfer efficiency

Advantages of CCDs :

1. optimized photodetectors

2. very low noise

3. very low fixed pattern noise

Disadvantages of CCDs :

1. cannot integrate other analog or digital circuits.

2. highly nonprogrammable.

3. high power.

4. limited frame rate.

Each pixel in an APS consists of a photosensitive part (photodiode) and a signal conditioning part

Transistors amplify the current produced by the photodiode and connect it to a common readout structure

For each pixel there is a sensor containing typically three transistors.

The conversion of light energy (photons) to a voltage or current (electrons) is realised within the sensor.

1. Can be produced in standard CMOS technology.

2. Pixels offer random access.

3. Pixel readout is non destructive & is repeatable.

4. Exposure time can be programmed electronically

5. Dissipates less electric power, Less blooming.

6. More robust under Xrays

DISADVANTAGES:

1. Reduced optical fill factor.

2. Increased offset noise.

3. Impossible to perform correlated double sampling for noise reduction.

SUMMARY

1. Pixel & two stragedies for pixel operation.

2. Types of Image sensors.

3. CCD image sensors, its merits & demerits.

4. CMOS image sensors, its merits & demerits.

5. Applications of APS.