Devil physics The baddest class on campus IB Physics

1. Devil physicsThe baddest class on campusIB Physics

2. Tsokos Lesson 8-2Digital imagining with charge-coupled devices

3. 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.

4. 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.

5. 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.

6. 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

7. Introductory Video

8. 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?

9. 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?

10. Capacitors • The difference in charge is the potential difference or potential (V) • How much of a potential is built up?

11. 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

12. Capacitors • Capacitance is charge per unit potential that can build up on a conductor • SI unit for capacitance is the farad (F)

13. Capacitors • Capacitance is based on: • Material of conductors • Surface area of the plates • Distance between the plates • Material between the plates

14. 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

15. 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

16. 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

17. 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

18. Charge-Coupled Device • Since, where c is the speed of light and λ is the wavelength of light, then

19. 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

20. Charge-Coupled Device • This will give you the relative brightness of a picture • Think of grayscale

21. 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

22. 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)

23. 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

24. 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

25. 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?

26. 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

27. 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

28. 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)

29. 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

30. 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

31. 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!

32. Summary Video – How CCDs are Made

33. 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?

34. 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.

35. 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.

36. 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.

37. Questions?

38. Homework #1-25