Understanding How Scanners Work

In digital photography, flat-bed and film scanners are the types most often used. In principle, they work in the same way as a digital camera - sensors convert differences in light values into electrical signals. The key difference is that a scanner provides its own source of light to illuminate the object, which is usually a print or film.

Inside the scanner's casing, a line of sensors covers the width of the bed holding the object to be scanned, and when scanning begins the sensors sweep along the entire length of the object. To direct light from the light source - usually a cold-cathode or set of LEDs (light-emitting diodes) - to the sensors that read light values, various optical devices, such as mirrors and lenses, are used. And the quality of a scanner depends greatly on the quality of the optical components.

The amount of light reflected by or transmitted through the print or film is picked up by a sensor, which produces an electrical signal that is then converted into a stream of digital information. This analogue-to-digital conversion is highly susceptible to electrical interference and the scanning process itself is sensitive to vibration and noise.

The most common type of sensor is the CCD (Charge Coupled Device); another type is the CIS (Contact Image Sensor), a technology that uses tightly packed LEDs and with sensors placed very close to the object. This arrangement allows extremely compact scanners to be made.

TWAIN driver
TWAIN is a standard for devices that acquire an image, such as scanners and digital cameras, allowing them to transfer their data to software applications. It enables software to work with these devices without the software having to know a thing about the device itself: any TWAIN-compliant device should be able to work with any TWAIN-compliant software application without any compatibility issues ever arising.

Since it is possible to connect multiple TWAIN-compliant acquisition devices to a computer, each device will have its own TWAIN module. Your software application must allow you to select which TWAIN device to use. For example, in Photoshop, the "Acquire" option in the File menu lists all the devices installed on the system.


Steve J. Wilson is living in the United States and is a freelance writer since 2006, in digital photography related topics. If you are interested in some useful Digital Photography Classes, than you are in the right place.

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