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What are they? How do they?
Why do they do that?
WHAT ARE THEY?
CCDs are used to convert light into a digital value
Capture high quality image data in a wide range of applications
HOW DO THEY?
They transfer incoming photons into an electrical charge in the CCD
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
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
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
It produces a colour image while only using a single chip digital
sensor such as in a digital camera or a scanner
BAYER PATTERN IMAGES
The image produced by a Bayer filter is known as a Bayer pattern
This cannot be used as the final image because it only shows
A computer can be used to perform an algorithm on the Bayer
This is image will look more as we would expect to see it such as
WHY DO THEY DO THAT?
CCDs are used in a great number of applications such as:
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