14.2 Data capture; Digital imaging
using charge-coupled devices
Data Capture; Digital Imaging Using CCDs
Methods of Capturing Images
Modern 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
CCD: An array of photodiodes creating ‘pixels’.
Each grain or pixel changes in response to photons
of light hitting it.
The quality of the image is governed by the same
factors for both pixels on a CCD and grains on a
- sensitivity to light (‘quantum efficiency’)
Large grain / pixel: Small grain / pixel:
- Allows low sensitivity - Needs high sensitivity
(collects lots of light) (collects very little light)
- Poor resolution - 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 statements into
• Measures only certain wavelengths
• Gives a linear response to brightness headed ‘CCD’ and
• 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
This is a measure of the sensitivity of a grain / pixel:
Quantum efficiency = no of photons detected
no of incident photons
In a CCD the photons cause photoelectric emission
of electrons. Thus we can say for a single pixel...
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 is the ability to store charge, measured
in Farads. A capacitor is an electrical component
that can store charge.
The capacitance of a
electrons component is the charge
stored per unit potential
difference across the
+ + + + + component.
- - - - -
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
The colour at this point is
recorded as an average
of the four pixels that
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.
Astronomy Image Capture
CCD image of Arp 188 (The Tadpole Galaxy)
taken by the Hubble telescope
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
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
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.
The ratio of the height of the image to the height of
the object is called magnification:
Magnification = Height of image
Height of object
M = hi
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
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
If the resolution of the
output device (e.g. LCD
screen) is poorer than
that of the CCD then
quality is lost.
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