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Devil physics The baddest class on campus IB Physics. Tsokos Lesson 8-2 Digital imagining with charge-coupled devices. IB Assessment Statements . Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs) 14.2.1. Define capacitance.

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ib assessment statements
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.1. Define capacitance.

14.2.2. Describe the structure of a charge-coupled device (CCD).

14.2.3. Explain how incident light causes charge to build up within a pixel.

14.2.4. Outline how the image on a CCD is digitized.

ib assessment statements1
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.5. Define quantum efficiency of a pixel.

14.2.6. Define magnification.

14.2.7. State that two points on an object may be just resolved on a CCD if the images of the points are at least two pixels apart.

ib assessment statements2
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.8. Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image.

14.2.9. Describe a range of practical uses of a CCD, and list some advantages compared with the use of film.

14.2.10. Outline how the image stored in a CCD is retrieved.

14.2.11. Solve problems involving the use of CCDs.

objectives
Objectives
  • Understand the definition of capacitance
  • Understand the basic operation of a charge-coupled device (CCD)
  • Define quantum efficiency, magnification, and resolution
  • Solve problems with CCDs
  • Name the applications of CCDs in medical imaging
capacitors
Capacitors
  • Any two conductors that are separated by either a vacuum or an insulator
  • When the switch is closed, does current flow?
  • What is the end result?
capacitors1
Capacitors
  • Initially, electrons will flow in a clockwise direction
  • Negative charge builds up on the bottom plate
  • Equal positive charge on the top plate
  • What is the difference in charge called?
capacitors2
Capacitors
  • The difference in charge is the potential difference or potential (V)
  • How much of a potential is built up?
capacitors3
Capacitors
  • The amount of potential is dependent on a property of the material known as capacitance
  • The amount of charge built up is proportional to the potential difference
capacitors4
Capacitors
  • Capacitance is charge per unit potential that can build up on a conductor
  • SI unit for capacitance is the farad (F)
capacitors5
Capacitors
  • Capacitance is based on:
    • Material of conductors
    • Surface area of the plates
    • Distance between the plates
    • Material between the plates
charge coupled device
Charge-Coupled Device
  • Invented at Bell Labs in 1969
  • Produces digital images in a fraction of the time needed for standard photography
  • Digital images can be easily manipulated, processed and transmitted
  • Originally designed for use in astronomy
  • Formed the basis for digital cameras, digital video recorders, digital scanners
charge coupled device1
Charge-Coupled Device
  • Consists of a silicon chip covered with light-sensitive elements called pixels
  • If your camera is 8 megapixels, the camera’s CCD has 8 x 106 pixels on its surface
  • Each pixel emits electrons when light is incident on it based on the photoelectric effect
charge coupled device2
Charge-Coupled Device
  • Think of each pixel as a small capacitor
  • Electrons released by the photoelectric effect carry a charge, Q
  • This creates a potential at the ends of the pixel, V, based on the capacitance, C, which can be measured by electrodes attached to the pixel
charge coupled device3
Charge-Coupled Device
  • Energy carried by a single photon of light of frequency f is given by,

where h = 6.63 x 10-34 J-s, Planck’s constant

charge coupled device4
Charge-Coupled Device
  • Since,

where c is the speed of light and λ is the wavelength of light, then

charge coupled device5
Charge-Coupled Device
  • The number of electrons released when light is incident on a pixel is proportional to the intensity of the light incident on the pixel.
  • Therefore, the charge produced in the pixel (capacitor) and thus the potential difference measured by the electrodes, are proportional to the intensity of light on that pixel
charge coupled device6
Charge-Coupled Device
  • This will give you the relative brightness of a picture
  • Think of grayscale
charge coupled device7
Charge-Coupled Device
  • This is a diagram of a CCD
  • When light is incident on the CCD (shutter open), charge builds up on each pixel based on the intensity of light incident on each particular pixel
  • When the shutter closes, a potential difference is applied to each row of pixels
charge coupled device8
Charge-Coupled Device
  • When the shutter closes, a potential difference is applied to each row of pixels
  • The potential difference forces the charge stored in each pixel to move to the row below (hence the name, charge-coupled, charges in one row coupled to charges in the row below)
charge coupled device9
Charge-Coupled Device
  • The potential difference forces the charge stored in each pixel to move to the row below
  • When a row of charges reaches the register, they are moved horizontally, one by one, through an amplifier and then through an analog-to-digital converter
charge coupled device10
Charge-Coupled Device
  • The ADC records two pieces of information:
    • Voltage of the pixel
    • Position of the pixel
  • The process is read until all pixels are read and stored in a file that contains all the information needed to re-create the image
charge coupled device11
Charge-Coupled Device
  • The previous discussion showed how to re-create an image based on intensity, but this would only result in a grayscale image
  • What about color?
charge coupled device12
Charge-Coupled Device
  • For color images, pixels are arranged in groups of four, as shown above
  • There are two with green filters (eyes are most sensitive to green), one with a red filter, and one with a blue filter
  • Computer algorithms compare the relative intensities for each color to create all the colors of the spectrum
quantum efficiency
Quantum Efficiency
  • Not every photon incident on a pixel will result in an electron being released
    • Some will reflect
    • Some will pass straight through
  • The quantum efficiency of a pixel is the ratio of the number of emitted electrons to the number of incident photons
quantum efficiency1
Quantum Efficiency
  • Relative quantum efficiencies:
    • Human eye – 1%
    • Photographic Film – 4%
    • CCDs – 70-80%
  • Note: Not constant at all wavelengths
  • Because of this, CCDs can measure the brightness of stars (which the HLions will learn all about in Astrophysics)
magnification
Magnification
  • Ratio of the length of an image to the actual length of the object
  • Magnification of a CCD system is dependent on the properties of the lenses used to focus the light
resolution
Resolution
  • Ability to identify two distinct objects that are close together
  • On a CCD, two points are resolved if their images are more than two pixel lengths apart
  • Higher pixel density, higher resolution
medical uses of ccds
Medical Uses of CCDs
  • Endoscopy – a CCD at the end of a long tube that can be used to create real-time images of internal organs with minimal invasiveness
  • X-Ray CCDs – use of CCDs in X-ray imagining has cut down the exposure time for patients

HLions will learn more about X-Ray imaging in Medical Physics!

objectives1
Objectives
  • Do you understand the definition of capacitance?
  • Do you understand the basic operation of a charge-coupled device (CCD)?
  • Can you define quantum efficiency, magnification, and resolution?
  • Can you solve problems with CCDs?
  • Can you name the applications of CCDs in medical imaging?
ib assessment statements3
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.1. Define capacitance.

14.2.2. Describe the structure of a charge-coupled device (CCD).

14.2.3. Explain how incident light causes charge to build up within a pixel.

14.2.4. Outline how the image on a CCD is digitized.

ib assessment statements4
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.5. Define quantum efficiency of a pixel.

14.2.6. Define magnification.

14.2.7. State that two points on an object may be just resolved on a CCD if the images of the points are at least two pixels apart.

ib assessment statements5
IB Assessment Statements

Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs)

14.2.8. Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image.

14.2.9. Describe a range of practical uses of a CCD, and list some advantages compared with the use of film.

14.2.10. Outline how the image stored in a CCD is retrieved.

14.2.11. Solve problems involving the use of CCDs.