The CCD chip is about as large as your fingernail. The chip inside a high performance camera, such as the 2.5 megapixel C-2500L, has over 1,700 x 1,300 ®photosensitive diodes on its tiny surface. They are arranged in columns and rows, much like the pixels of a computer monitor.
If each of the sensors on a CCD chip were to react to incoming light in the same way, a digital camera could only take black-and white pictures. However, to ensure that all colours can be captured, the sensors are covered with different colour filters. These can be either RGB (Red, Green, Blue) or CMY (Cyan, Magenta, Yellow) with additional green filters for more trueto-life results.
Of course, brightness data is also required for correct image reproduction. The light is divided up into 256 levels of intensity for each colour.
This combination yields 256 x 256 x 256 = 16.7 million possible colours for true colour reproduction. This data is transformed into digital signals so that all relevant image information can be stored by the digital camera.
Basically, there are two different kinds of CCD chips which can be used in digital cameras. The first was originally developed for television and video and has since been optimised for still cameras. Called the video, or interlaced CCD, these CCDs have the advantage of high light sensitivity. They feature either RGB or CMY and green colour filters. Olympus uses the latter filter type for its video CCDs.
Although this CCD captures the information in one shot, the data is read in two sequences. First the rows 2, 4, 6 etc. and then rows 1, 3, 5 etc. To allow uninterrupted reading, no further light is permitted inside the camera. This is ensured through the use of a mechanical shutter.
Due to their high performance, relatively simple construction and low production costs, video CCDs are now also found in ®megapixel cameras.
Another type of CCD designed especially for digital cameras, yields even better digital pictures. The progressive-scan-CCD, or simply progressive CCD, can record a number of complete images per second. Because the image is captured and read in one movement, a mechanical shutter is superfluous. This, in turn, enables extremely high shutter speeds to be achieved.
A progressive CCD is coated with RGB (Red, Green, Blue) colour filters. Since each pixel in the picture corresponds to one of the three types of CCD pixels, only one colour is recorded for each dot. Special software is used to calculate and complete the missing colour data. The better the software program, the better the resulting picture will be.
An additional improvement in picture quality is achieved by a relatively simple trick. The subject can be reproduced even more precisely when the CCD chip’s proportion of green-filtered pixels is modified to contain two green-coloured data for every red and blue-coloured data.
The reason for this: not only is the human eye more sensitive to green but the colour green also significantly influences our perception of brightness.