CCDs for Mr Stead

1. CHARGE COUPLED
DEVICES
What are they? How do they?
Why do they do that?

2. WHAT ARE THEY?
 CCDs are used to convert light into a digital value
 Capture high quality image data in a wide range of applications

3. HOW DO THEY?
 They transfer incoming photons into an electrical charge in the CCD
chip
 The photons are received in a photoactive region of silicon
 A lens is used to focus the light onto the silicon
 A charge is accumulated in the capacitors proportional to the light
intensity
 The charge is stored in capacitors known as “bins”
 The charge is transferred between “bins” moving from one bin to its
neighbour until it reaches the last capacitor
 The charge is then dumped to a charge amplifier which converts his
charge into a voltage
 By repeating this process for many rows the charge values for the
whole array can be digitized and stored in memory

4. WHAT IS A BAYER FILTER?
 A Bayer filter is put over the image sensor
 It has three colours: green, red and blue. These are arranged in
squares in the ratio 50% green, 25% red and blue
 The reason green is more common than the others is to imitate
the biology of a human eye, as the eye is more sensitive to green
light
 It produces a colour image while only using a single chip digital
sensor such as in a digital camera or a scanner
One Pixel

5. BAYER FILTER ON A WEBCAM

6. BAYER PATTERN IMAGES
 The image produced by a Bayer filter is known as a Bayer pattern
image
 This cannot be used as the final image because it only shows
three colours
 A computer can be used to perform an algorithm on the Bayer
pattern image
 This is image will look more as we would expect to see it such as
below

7. WHY DO THEY DO THAT?
CCDs are used in a great number of applications such as:
 Digital cameras
 Telescopes
 Spectroscopy

8. PROBLEMS
 There may be what is known as “dead pixels” and “hot
pixels”, these would reduce the usefulness of the image produced
 When there is no light on the CCD there is still a current produced
known as the “dark current”
 The “dark current” changes the image because it adds charge to
the values so the light appears to be brighter than it in fact was
 To combat this the dark current charge values are taken from
those of the image produced resulting in a real image