How Scanners Work

And What a CCD Is

A woman’s hand holidng a book facedown on a scanner

Bibica/Getty Images

Yes, there are many types of scanners, but most of them (except for, perhaps, the drum scanners used in the publishing industry) “capture” data—be it text documents, business graphics, or photos, including film, transparencies, slides, and negatives—the same way, which is the topic of this article. Just how does a scanner take a hard copy page, reproduce its content, and then transfer that data to a computer file that you and I can do with as we please?

Charged-Coupled Device (CCD) Array

While scanners are made up of several different parts, such as mirrors, lenses, motors, and more. On most of today’s scanners, though, the core component is the charged-coupled device (CCD) array. A collection of light-sensitive diodes that converts photons (light) to 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 model of the scanner, 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 target (the object being scanned).

Depending on the scanner, some are single-pass and some are three-pass, which means they pick up the object being scanned in either one pass or three, respectively. 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.

Nowadays, most scanners are single-pass, with the lens doing the actual separating of the three color channels, without the user being any the wiser.

Contact Image Sensor

Another, less expensive imaging array technology to gain some ground recently is contact image sensor (CIS). CIS replaces the CCD array, with its arrangement of mirrors, filters, lamp, and lenses, with rows of red, green, and blue (RGB) light emitting diodes (LEDs). 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 being 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, but then the former are usually thinner, lighter, and cheaper.

Resolution and Color Depth

Which resolutions you should scan at depends on where you plan to use the image. Computer monitors, tablets, and smartphones really can’t display resolutions beyond about 72 dots per inch (dpi), with HD monitors supporting 96dpi. The only thing that happens when you scan an image at higher resolution than it can be displayed at, the extraneous data is simply tossed out, which, of course, takes time.

The photos in your high-end brochures and other media, on the other hand, are a different story. For the best possible results, you should always scan them at least 300dpi, and higher, much higher, if possible—just in case you need to enlarge the image during layout.
Color depth defines the number of colors a particular image (or scan) contains. The possibilities are 8-bit, 16-bit, 24-bit, 36-bit, 48-bit, and 64-bit, with the former, 8-bit, supporting 256 colors or shades of gray and 64-bit supporting trillions of colors—far more than the human eye can discern.

Obviously, within reason, high resolutions and deep color depths enhance scan quality, with reason, of course. The colors, the quality, and the detail have to be there before you scan. No matter how good your scanner, it can perform miracles.