Science & Tech

How does a scanner work

By George Garza
Info Guru,

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A flatbed scanner.
This flatbed scanner copies images and documents.
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Learn about the process used to scan documents.

How does a scanner work? Here's what you need to know.

What is a Scanner?

Scanners are hardware appliances that allow a PC to convert a picture or graphical object into digital code that allows the computer to display and use an image. A scanner has the ability to translate an unlimited number of analog voltage levels into digital values. Analog is like a clock with an hour and minute hands; they sweep around the face. But a digital clock jumps from one time frame to the next. The computer is not able a graphic as it is; essentially it is in analog form, but it must be converted into digital form. Scanners can come with specialized software called optical character recognition (OCR). This software can read text as printed or written. The information can then be manipulated in the computer. The analog form that the picture is in gets converted into digital form.


Another important feature is the resolution. This shows how sharp and detailed the scanner can read. The scans are measured in dots per inch (dpi). With higher resolutions, the sharper the image, but more memory is required to scan the file. If you want to produce a file for computer screen output, you should scan at 72 dpi because that is the best resolution that the monitor can display. It is the same of the printed output. If you only have a 300 dpi printer, do not scan the file at a higher resolution since the printer will only print it at that resolution.

Types of Scanners

Flatbed scanners have a glass window where the item to be scanned is placed on top of while the head moves past the item. This method is similar to a xerox machine.

Handheld scanners are small, portable scanners that depend on a human operator to move the head across the object or image to be scanned.

How does a Scanner Work?

Flatbed Scanners

A light source underneath the picture or document illuminates the image. White or blank spaces reflect more light than inked and colored areas. A motor moves the scan head underneath the page. When the scan head is moving, it captures light that was reflected from individual areas. Light from this page is bounced around through an intricate system of mirrors that continually pivot to keep the light beams aligned with a lens. A lens focuses the beams of light into light sensitive diodes that modify the amount of light into an electric current. The amount of the current depends on the amount of light reflected.

The analog to digital (AD) converter stores each analog reading of voltage as a digital pixel representing either a black or white area. Scanners that are more sophisticated can translate the voltage into shades of gray. In a color scanner, the scan head makes three passes under the image; the light on each pass is directed through a red, green or blue filter before it strikes the original image.

The digital information is sent to the PC where it is translated into a format that a graphics program can read.

Handheld Scanner

On the majority of handheld scanners, when you press the scan button a light-emitting diode (sometimes called a LED) illuminates the image below the scanner. An inverted, angled mirror that sits right above the scanner's window reflects the image onto a lens in the back of the scanner.

The lens focuses a single line of the image onto a CCD (charge coupled device), which is a component designed to detect subtle changes of voltage. As the light shines onto several rows of light detectors located on the CCD, each registers the amount of light as a voltage level that equals to black, white or gray.

Special analog chips receive light voltage generated by the CCD for gamma correction. This process enhances the black tones so that the eye will be able to recognize the shades of the image easier.

The line of the image is moved to the analog-digital converter. In a gray scale scanner, the converter assigns 8 bits to each pixel, or 256 shades of gray.

As the disk turns, a light shines through the slits and is detected by a photo-micro sensor on the other side of the disk. When light strikes the sensor, it throws a switch that sends a signal to the A-D converter. This signal tells the converter to send the line of bit generated to the computer. Then the converter clears itself of the old data. The computer then moves to the next line.

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