Color digital images take up an extremely large amount of memory when they are stored. Methods, such as optical media, have been developed to overcome this storage problem. Optical media is very suitable for storing digital photographs. Some examples of optical media include the following: Write Once, Read Many (WORM) disks, erasable optical disks, and optical memory cards. An example of nonerasable memory is the Kodak Photo CD; this CD allows high-quality color images to be stored for archival and retrieval purposes.

One Kodak Photo CD can store up to 650 megabytes. This equates to 100 high-resolution, color digital images when stored in compressed form. These images are stored at five different resolution levels, ranging from 128 by 192 pixels for a proof, or thumbnail sketch, to a high resolution 2,084 by 3,072 pixels (compressed) full-color image. These images can be imported using photo software packages, then they can

FiPure 3-9.-Cross section of a CRT.

be manipulated, printed, or placed in various layout applications.

Image compression makes it possible to take a large color-image file and reduce its size. This reduces the amount of memory required to store it or decreases the time required to transmit it. Compression can reduce the amount of memory needed by a factor from five to one hundred. Various compression-decompression chips, add-on boards, and software are available in the commercial market.

Image compression is made possible because in a typical digitized image, the same information appears several times. For example, areas of the same color in different parts of the image or straight lines contain the same information. This duplication of information values, or REDUNDANCY, can be identified in three types as follows:

Spatial redundancy. This results from dependence among neighboring pixel values.

Spectral redundancy. This results from an association of color (RGB) planes.

Temporal redundancy. This results from a correlation between different frames in a sequence of images.

The most common compression program has been formed by the Joint Photographic Experts Group (JPEG). The technique used in JPEG compression allows the user to select the compression ratio.

High-compression ratios generally result in low image quality. This low image quality is a result of avoiding the risk of losing data as the image compression ratio is increased. The amount of image compression depends on the amount of redundancy that exists in an image. When a compressed image is reconstructed (uncompressed) and the pixel values are identical to the original image, the compression is known as lossless. When discrepancies occur between the original and the reconstructed image, the compression is called lossy. Lossless compressions can be achieved with compression ratios of up to 5 to 1. Files that are compressed may be identified by the file extension ".JPG."