Digital images have become an important aspect of everyday life, from sharing family vacation pictures to capturing images from space. Thanks to the successful design of most digital cameras, ordinary photographers do not think about the chain of events that creates the image; they just push the button and the camera does the rest. However, engineers and scientists labored over the design of the camera and placed a lot of thought into the process that creates the digital image. So what exactly is a digital image, and what is the physical chain of events (called the imaging chain) that creates it (Fig. 1.1)?
A digital image is simply an array of numbers with each number representing a brightness value, or gray-level value, for each picture element, or pixel (Fig. 1.2). The range of brightness values, called the dynamic range, is typically eight bits, giving 28 = 256 possible values with a range of 0-255, with zero being black and 255 being white. Three arrays of numbers representing red, green, and blue brightness values are combined to create a color image. When displayed, this array of numbers produces the image that we see.
The array of numbers that makes up a digital image created by a camera is the result of a chain of physical events. The links in this chain impose physical limitations that prevent the camera from capturing a "perfect" image, i.e., an image that is an exact copy of the scene information. For example, a digital image will not continue to display higher details in the scene as we view the image under higher and higher magnification (Fig. 1.3). Most of us have seen a television show or movie where a digital image is discovered that might contain the critical information to catch the bad guy if they could only zoom in and see better detail. Along comes the brilliant scientist who, with a simple click of a button, magnifies the image to an amazing quality, revealing the information that leads straight to the culprit! This is great stuff for a crime thriller, but we know that the real world is not so kind.