DEVIL PHYSICSTHE BADDEST CLASS ON

CAMPUSIB PHYSICS

TSOKOS LESSON 8-2DIGITAL IMAGINING WITH CHARGE-COUPLED DEVICES

Sounds of Old Technology

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

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

CVQ

Capacitors

Capacitance is charge (coulombs) per unit potential (volt) that can build up on a conductor

SI unit for capacitance is the farad (F)

CVQ

VCF 11

Capacitors

Capacitance is based on: Material of conductors Surface area of the plates Distance between the

plates Material between the

plates

CVQ

VCF 11

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

CQV

CVQ

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

hfE

Charge-Coupled Device

Since,

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

hfE

cf

hcE

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

hcE

Charge-Coupled Device

This will give you the relative brightness of a picture

Think of grayscale

hcE

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)

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.

QUESTIONS?

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