Sounds of Old Technology

IB Assessment Statements Topic 14.2., Data Capture and Digital Imaging Using Charge-Coupled Devices (CCDs) Define capacitance Describe the structure of a charge- coupled device (CCD) Explain how incident light causes charge to build up within a pixel 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) Define quantum efficiency of a pixel Define magnification 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) Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image Describe a range of practical uses of a CCD, and list some advantages compared with the use of film Outline how the image stored in a CCD is retrieved 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

Introductory Video

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 (C)  The amount of charge built up is proportional to the potential difference

Capacitors  Capacitance is charge (coulombs) per unit potential (volt) 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 10 6 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 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  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  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!

Summary Video – How CCDs are Made

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) Define capacitance Describe the structure of a charge- coupled device (CCD) Explain how incident light causes charge to build up within a pixel 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) Define quantum efficiency of a pixel Define magnification 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) Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image Describe a range of practical uses of a CCD, and list some advantages compared with the use of film Outline how the image stored in a CCD is retrieved Solve problems involving the use of CCDs.

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