Analog Images Versus Digital Images

When we talk about digitizing a signal from a digital radiographic unit, we are talking about assigning a numerical value to each signal point, either an electrical impulse or a light photon. As humans, we experience the world through analog vision. We see our surroundings as infinitely smooth gradients of shapes and colors. Analog refers to a device or system that captures or measures a continuously changing signal. In other words, the analog signal wave is recorded or used in its original form. A typical analog device is a watch in which the hands move continuously around the face and is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example).

Traditionally, radiographic images were formed in an analog fashion. A cassette, containing fluorescent screens and film sensitive to the light produced by the screens, is exposed to radiation and then processed in chemical solutions. Today, images can be produced through a digital system that uses detectors to convert the x-ray beam into a digital image.

In an analog system such as film/screen radiography, x-ray energy is converted to light, and the light waves are recorded just as they are. In digital radiography, analog signals are converted into numbers that are recorded. Digital images are formed through multiple samplings of the signal rather than the one single exposure of an analog image.