Light is the visible part of the electromagnetic spectrum. It is an
analog signal. When we are talking to someone face to face, our speech is
an analog signal. If our cell phone and cell phone carrier use digital
technology, then our phone converts our analog speech into a digital signal,
which is transmitted to the recipient and their phone converts it back to an
analog signal they can hear.
When an image is captured in digital photography, the analog image is converted
into a digital signal (see Figure 1) and stored as discreet information in a
file. This discreet information is called pixels.
Pixel is short for picture (pix) element (el) and is the smallest, complete unit
of information about a single point in a digital image. In a RGB image,
one pixel contains the hue, saturation and tone information for a single point
in the photograph. Consequently, it can take thousands or millions of
pixels to make up one image.
Figure 1. Traditional film is a continuous
tone medium. Taking a picture with a digital camera or scanning film
converts an analog signal to a digital one.