Drum scanners:are used by the publishing industry to capture incredibly detailed86
images. They use a technology called a photomultiplier tube (PMT). In PMT, thedocument to be scanned is mounted on a glass cylinder. At the center of the cylinderis a sensor that splits light bounced from the document into three beams. Each beamis sent through a color filter into a photomultiplier tube where the light is changedinto an electrical signal.A drum scanner uses PM (Photo-Multiplier) tube which is a light sensing device. Itoffers a high sensitivity and good signal-to-noise ratio(SNR). The image to bescanned is mounted on a spinning drum. Even shadow information that is notvisible to human eye can be captured by the drum scanner and transformed to thevisible region which improves the images. The drum scanner is a high-end productin terms of quality and price.Construction and working of Flat-bed Scanner87
How Scanner Works?1.The image to be scanned is placed on top of the scanner’s glass plate.2.The computer sends instructions to the logic board about how far the motor is to run and how fast.3.The logic board instructions place the scanning unit into an appropriate position to begin scanning.4.The scanning unit moves across the image to be scanned at a speed designated by the logic board instruction.5.As the scanning unit moves across the image, a light source shines on the Image.6.The light strikes the image, reflects, and is then reflected by a series of mirrors to the scanner lens.7.The light passes through the scanner lens and reaches the CCD sensors8.CCD sensors measure the amount of light reflected through the image and converts the light to an analog voltage.9.The analog voltage is changed to digital values by an ADC– analog to digital Converter.10.The digital signals from the CCDs are sent to the logic board and transmitted back to the computer.88
Working of flat bed scanner:The following steps are involved to scan any type of document:1. A light source illuminates a source page to be scanned is placed face down On a glass surface called as flatbed. The scanning head is located just below this bed.2. A motor moves the scan head beneath the source page. As it moves, the scan head captures light reflected from individual areas of the page.3. The light from the page is reflected through a system of mirrors called reflectors. A lens focuses the beam light onto light-sensitive diodes array or CMOS / CIS Image sensor that translate the amount of light into electrical current.4. The more light that's reflected, the greater the voltage or the current. White spaces reflect more light than black or colored letters or images.5. An analogue to digital (A-D) converter converts each analog reading of voltage as digital data of pixel representing the scanned area. A-D converter on a monochrome scanner stores only 1 bit per pixel, either on or off, representing black or white.