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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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### Tsokos Lesson 8-2Digital 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.

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 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 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
• 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
• 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?
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?
Capacitors
• The difference in charge is the potential difference or potential (V)
• How much of a potential is built up?
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
Capacitors
• Capacitance is charge per unit potential that can build up on a conductor
• SI unit for capacitance is the farad (F)
Capacitors
• Capacitance is based on:
• Material of conductors
• Surface area of the plates
• Distance between the plates
• Material between the plates
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 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 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 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 Device
• Since,

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

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 Device
• This will give you the relative brightness of a picture
• Think of grayscale
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 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 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 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 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 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
• 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 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
• 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
• 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
• 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!

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 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 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 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.