How Scanners Work

CCD, CIS, and all the ways scanners digitize document data

There are many types of scanners, and most capture data in the same way—be it for text documents, business graphics, photos, or film.

Here's how a scanner takes a physical document, reproduces its content, and then transfers the data into a computer file that can be uploaded and shared digitally.

A hand holidng a book facedown on a scanner
Bibica / Getty Images

Charged-Coupled Device (CCD) Array

While scanners are made up of several different parts, the core component is the charged-coupled device (CCD) array. A CCD array is a collection of light-sensitive diodes that converts photons (light) into electrons or electrical charges. These diodes are known more commonly as photosites.

Photosites are sensitive to light. The brighter the light, the greater the electrical charge. Depending on the scanner model, the scanned image or document finds its way to the CCD array through a series of lenses, filters, and mirrors. These components make up the scan head. During the scanning process, the scan head is passed over the document or object or being scanned.

Some scanners are single-pass, and some are three-pass, which means the scanner picks up the object being scanned in either one pass or three. On a three-pass scanner, each pass picks up a different color (red, green, or blue), and then software reassembles the three RGB color channels, restoring the original image. Most modern scanners are single-pass, with the lens doing the actual separating of the three color channels.

Contact Image Sensor

Another, less expensive imaging array technology is a contact image sensor (CIS). CIS replaces the CCD array. Here, the image sensor mechanism consists of 300 to 600 sensors that span the width of the platen or scanning area. While an image is scanned, the LEDs combine to provide white light, illuminating the image, which is then captured by sensors.

CIS scanners do not typically provide the same level of quality and resolution delivered by CCD-based machines. However, these scanners are usually thinner, lighter, and cheaper.

Resolution and Color Depth

The resolution you choose depends on how you plan to use the image. HD computer monitors, tablets, and smartphones can support resolutions up to 96 dots per inch (dpi). If you scan an image at a higher resolution than it can be displayed, the extra data is tossed out.

The photos in high-end brochures and print media are a different story. For the best possible results, you should always scan at least 300 dpi. More is always better, especially if you need to enlarge the image during layout.

Color depth defines the number of colors an image (or scan) contains. The possibilities are 8-bit, 16-bit, 24-bit, 36-bit, 48-bit, and 64-bit. 8-bit supports 256 colors or shades of gray, and 64-bit supports trillions of colors—more than the human eye can discern.

High resolutions and deep color depths enhance scan quality. The colors, the quality, and the detail have to be there before you scan. No matter how good your scanner is, it cannot perform miracles.