The Basics
A home photo scanner is an evolution of the photocopier. The two work the same way, by taking a picture of whatever is on the glass plate, and reproducing it. Photocopiers, of course, originally were far more crude, and it took a while to create a usable photo scanner. Essentially, a photo scanner takes an electronic "photo" of the picture, converting the analog image into ones and zeros to make it digital.
The Works
Inside a typical flatbed scanner is a moving element that takes this "photo" by "seeing" lines of data at a time. Early scanners made three passes---one each for red, green and blue---then combined this separate files into one full-color image. The technology has improved, so most modern scanners collect the image in one pass. The scan element collects data at different speeds depending on the quality of the resolution you want. For a low-quality (about 100 dots per inch), the scan is fast. For 1,200 dpi, it takes a bit longer.
Values
The scan element projects light at the photo as it rests face-down on the glass plate that is the surface of the scanner. This part essentially is just like how traditional photography works in that film records light and dark areas of a scene at a variety of "values." Pure white becomes pure black on a negative. Colors in between black and white have different values. The scanner changes this values into digits and creates the digital image that can be seen and manipulated in the computer via software. You must adjust the scanner for varying quality photos because the scanner doesn't always "see" everything in a poor or very old photo. Most scanners come with software that allows some basic control over light and dark, contrast and color. This further can be manipulated in software such as Adobe Photoshop or Photoshop Elements.
Tags: glass plate, light dark, photo scanner, scan element