CHARGE COUPLED DEVICESWhat 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 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

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

BAYER FILTER ON A WEBCAM

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

WHY DO THEY DO THAT?

CCDs are used in a great number of applications such as: Digital cameras Telescopes Spectroscopy

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