Digital Technology

14.2 Data capture; Digital imaging using charge-coupled devices

(CCDs)

Data Capture; Digital Imaging Using CCDs

Methods of Capturing ImagesModern cameras use a device called a Charge Coupled Device (CCD) to capture photographic images. Until recently, photographic film was used.

Photographic film: A sheet of plastic with small ‘grains’ of a photo-sensitive compound e.g. Silver Bromide

CCD: An array of photodiodes creating ‘pixels’.

Each grain or pixel changes in response to photons of light hitting it.

Image Quality

The quality of the image is governed by the same factors for both pixels on a CCD and grains on a photographic film:

- sensitivity to light (‘quantum efficiency’)- resolution

E.g.

Large grain / pixel:- Allows low sensitivity

(collects lots of light)- Poor resolution

Small grain / pixel:- Needs high sensitivity

(collects very little light)

- High resolution

• Typical grain/pixel size about 2 µm and decreasing• Typical grain/pixel size about 0.5 µm

• Can have a very large total area – thus increasing resolution• Have a fairly small total area – thus limiting resolution

• Detect all photons within defined range of λ – up to 100% quantum efficiency• Cannot detect all the photons and has a quantum efficiency of about 2%

• Slow - Takes a long time to produce an image and then needs processing.• Fast - Produces near instant images and can be processed immediately.

• Needs replacing after each image• Never needs replacing

• Measures a wide spectrum of wavelengths• Measures only certain wavelengths

• Gives a linear response to brightness• Gives a non linear response to brightness

• The data can be kept for a long time• It is uncertain how long the data can be kept

Place these statements into two columns headed ‘CCD’ and ‘Photographic Film’

Quantum Efficiency

This is a measure of the sensitivity of a grain / pixel:

In a CCD the photons cause photoelectric emission of electrons. Thus we can say for a single pixel...

Quantum efficiency = no of photons detected no of incident photons

Quantum efficiency = no of electrons emitted no of incident photons

Image of the whirlpool galaxy on left taken with photographic film (1hr) and on the right with a CCD (6mins). Both detecting the same range of wavelengths. Which has the higher quantum efficiency?

How does a CCD work?

Capacitance

Capacitance is the ability to store charge, measured in Farads. A capacitor is an electrical component that can store charge.

The capacitance of a component is the charge stored per unit potential difference across the component.+

-+ + + + - - - -

electrons

Thus...

C = Q V

Structure of a CCD

Each pixel on the surface of a CCD is a silicon photodiode. This is essentially a capacitor which becomes charged by photoelectric emission within the silicon. Thus a p.d. develops across the photodiode.

The ratio is 2 green : 1 blue : 1 red. This is because the eye is more sensitive to green light.

The colour at this point is recorded as an average of the four pixels that surround it.

The pd that develops across each pixel is measured by a ‘serial shift register’ at one side of the CCD. After an image has been taken the register reads the pd’s on the bottom row pixels. Then the pd’s are shifted down and the next row is read and so on until all of the pixel information (including positions) is stored in the memory as binary data.

CCD image of Arp 188 (The Tadpole Galaxy) taken by the Hubble telescope

Astronomy Image Capture

Q1. A typical SLR digital camera CCD size is 16 x 24 mm, calculate the average pixel size on a 6 Megapixel camera. Assume square pixels.

Q2. A typical serial register reads at 10 MHz. How long does it take to read all the pixels on the above CCD?

Q3. A video camera takes 30 pictures per second. If the camera has 1.2 Megapixels, calculate the speed of the serial register in MHz.

Q4. When photons fall on the CCD of black and white camera, data for an image can be stored and later reproduced.

1. How is light converted to pd’s on the CCD?

- Photons cause photoelectric emission on each photodiode, thus creating pd’s across them. Each pd represents an intensity of light. The pattern of pd’s represents the object.

2. How is image data transferred to the memory?

- By shifting all pd data to the serial register, the pd’s can be converted to (binary) digital data and stored.

3. How can the data be used to produce an image?

- Through a screen / printer / projector etc.

X –Ray Image Capture

CCDs can be used to detect both soft (lower energy) X rays and hard (higher energy) X rays.

- Soft X rays can be detected directly by the photodiode (pixel). E.g. Medical Imaging

- Hard X rays are converted into visible photons upon collision with a phosphor screen. The photons are then detected by the photodiodes on the CCD.

Magnification

The ratio of the height of the image to the height of the object is called magnification:

As most CCDs are quite small (e.g. 8.8 x 6.6 mm for a compact digital camera), the image height must be very small too. A larger CCD would have more pixels and would require less magnification by the lens.

Magnification = Height of image Height of object

M = hi

ho

Resolution

If the resolution of the output device (e.g. LCD screen) is poorer than that of the CCD then quality is lost.

Two points on an object can just be resolved by a CCD if the distance between the two points on the CCD is at least two pixels.

E.g.

A digital camera is used to photograph an object. Two points on the object are separated by 0.0060 cm. The CCD in the camera has area 60 cm2 and contains 12 megapixels. The magnification of the camera is 0.80.

Can the images of the points be resolved?

hi = 0.8 x 0.0060 = 0.0048 cm

Area per pixel = 60 / 12 x 106 = 5 x 10-6 cm2

Length of pixel (assuming square) = √5 x 10-6 = 0.0022 cm

Two pixels = 0.0044 cm

Can be resolved as length of 2 pixels < hi

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