General Information about Scanners
A Scanner is a device for electronical data collection. A scanner has optical sensors to scan the surface of a document, analyzing the colours at all points on the document. The data is digitized with an Analog-Digital-Converter so that they are suitable for further processing with a computer.
During the scanning process, the document is lit with a lamp from within the scanner. The light which is reflected is then collected by a rod lens. This lens focusses the light and eliminates other scattered light.
The focussed light rays are then filtered in the colour red, green and blue and are picked up by an opto-electronic (optical microelectronic) line sensor.
The sensor emits an analog current proportional to the amount of light it received. These analog signals are converted to digital signals, pixel by pixel. Thus, line by line, the digital image is created.
The resolution determines the quality and size of a scan. The unit of measurement of resolution is DPI (dots per inch). This number states how many different pixels the scanner creates from one inch of space on the original document (1 inch = 2,54 cm). The higher the DPI value, the better the quality of the scan, but the more memory is needed to store the image.
Note that not every document is suitable for a high-resolution scan, for example when the document has a bad image quality or the image is intended to be used on a website. Images are shown with a resolution of 72 dpi in the internet.
If you want to use documents for professional printouts, you need to use a high resolution. Generally, 300 dpi are advisable if the document doesn't have to be scaled.
For private use, scanners with a resolution up to 1200 dpi are usually sufficient. For professional scans, resolutions of up to 6400 dpi are possible. Care must be taken that a suitable computer with enough memory for the resulting large images is used, otherwise it may freeze or fail to store the scan.
Types of Scanners
A flatbed scanner has a glass surface on which the document is placed for scanning. The sensors are underneath the glass surface and move from end to end. This type of scanner is suitable for most types of documents, such as photos, images, texts, books. However, the document must be placed flat on the glass to achieve optimal image quality. A flatbed scanner usually supports document sizes of at least DIN A4 (21 cm x 29.7 cm).
This type of scanner is used to scan several similar documents quickly, such as stacks of letters. The documents are collected by a system of reels and moved past the sensors.
A Copier is a larger machine usually used to quickly multiply documents, but most modern devices can also be used as a scanner. They usually support automatic
document feeding and larger documents up to DIN A2 (42 cm x 59.4 cm). Most have integrated scaling and processing capabilities.
The document scanner, also called document reader, is similar to the flatbed scanner and is usually used to digitize small documents, such as bills, receipts or chip cards.