Image Sensor PPT
- 1. 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
- 2. 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.
- 3. Types of Sensors Thermal Energy Sensors Electromagnetic Sensors Mechanical Sensors Chemical Sensors Optical and Radiation Sensors Acoustic Sensors Biological Sensors
- 4. 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.
- 5. What is a Pixel? The smallest discrete component of an image or picture 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 photo site that is sensitive to light. If circuits cover 25% of each photo site, the sensor is said to have a fill factor of 75%. The higher the fill factor, the more sensitive the sensor.
- 6. 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.
- 7. Types of Image Sensors CCD: Charged Coupled Device CMOS: Complementary Metal Oxide Semiconductor
- 8. Charged Coupled Device (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.
- 9. 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.
- 10. Types of CCD Image Sensors 1. Interline Transfer CCD Image Sensor 2. Frame Transfer CCD Image Sensor
- 11. 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 photo gates 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 photo detection (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.
- 12. Complementary Metal Oxide Semiconductor (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, etc.
- 13. Basic Operation of CMOS 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.
- 14. Types of CMOS Image Sensors 1. Active Pixel Image Sensor 2. Passive Pixel Image Sensor
- 15. 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 photo site is covered with circuitry that filters out noise and performs other functions.
- 16. Sensing Color
- 17. 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.
- 18. Application of Image Sensor
- 19. Digital Cameras
- 20. Personal Digital Assistance (PDA)
- 21. Camcorders
- 22. Toys and Robots
- 27. Image sensors applications in medicine
- 28. Security Market Volume of CCD, CMOS
- 29. THANK YOU