How Does a Flat Bed Scanner Work?

by Ma Wen Jie


Glass is the first element in a flat bed scanner. To scan an image or document, open the scanner and place the document face down on the glass. Because the scanner uses light reflected from the document through the glass, the quality of the scanner's glass can affect the quality of the final scan. Most scanners use good quality tempered glass.

Light Source

Flat-bed scanners have a built-in light source. In many scanners, the light source is LED lights. Other scanners use fluorescent bulbs. The light source is usually connected to a ballast or other voltage regulator to ensure consistency of light over the scan pass.


The sensor in a scanner is the device that "reads" the light reflected from the document or image. Most scanners use a charge coupled device (CCD) array. A few scanners use a CMOS type sensor, and still others use a contact image sensor (CIS). Regardless of the technology used, a sensor array contains light-sensitive diodes that convert analog light waves into a digital signal. Some scanner arrays contain three rows of sensors. Each row is calibrated to measure red, blue or green light. Other, usually older, scanner models use filters placed either in front of the light source or over the sensor array to filter red, green or blue light.


The sensor and light are mounted on a carrier that moves down the length of the scanner. The carrier moves at a constant rate and is driven by an electrical stepping motor using either a drive belt or a rack and pinion system.

The Scanning Process

Once a document or image has been placed on the glass, the carriage moves to a calibration strip inside the scanner case. The sensor takes a series of measurements to calibrate color balance, contrast and brightness. Once calibrated, it moves to the start of the area of the document to be scanned and scans the selected area by taking a series of "slices" of the image. Circuitry in the scanner then assembles these slices sequentially to create the whole image for transfer to a computer.

Transferring the Scan to a Computer

Once the scan is complete, the image is transferred to a computer. This usually is done via a USB or FireWire interface. Some older scanners use SCSI interfaces for image transfer.

About the Author

Although he grew up in Latin America, Mr. Ma is a writer based in Denver. He has been writing since 1987 and has written for NPR, AP, Boeing, Ford New Holland, Microsoft, RAHCO International, Umax Data Systems and other manufacturers in Taiwan. He studied creative writing at Mankato State University in Minnesota. He speaks fluent Mandarin Chinese, English and reads Spanish.

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