A scanner is an input device that can scan different types of documents like photographs or a text page. While the process of scanning goes on the document is converted into a digital format which creates an electronic version of that document. This document can hence be viewed and edited on a computer. There are different types of scanner available in the market. Each type has its pros and cons.
How does a scanner work?
Scanners operate by projecting light at an object or file being which has to be digitized and directing the reflected light through a series of mirrors and lenses onto a photosensitive element. Usually, in most scanners, the sensing medium is an electronic integrated circuit or an IC which can sense light also known as a charged coupled device (CCD). Light-sensitive photosites which are arrayed along the CCD can convert levels of brightness into corresponding electronic signals that are then processed into a digital image.
Different Types of scanners
Scanners can be categorised into two categories i.e.
- Based on technology used
- Based on the structure
1. Based on technology used
The most common light-sensing technology used in modern scanners is CCD. Other technologies being CIS (Contact Image Sensor) and PMT (photomultiplier tube) CCD are found in the low-end scanners whereas CIS can be found in high-end scanners. CIS is a newer technology that can enable scanners to be smaller and lighter, but by sacrificing dynamic range, depth-of-field, and resolution. Another sensing technology, CMOS which is Complementary Metal Oxide Semiconductor are found primarily in low-end, hand-held digital cameras. Its features include low cost, low power consumption and easier component integration which permits smaller, less expensive designs.
2. Based on the structure
Based on the structure, a scanner can be of the following types
- Flatbed scanner
- Film scanner
- Roller scanner
- 3-D scanner
- Planetary scanner
- Hand document scanner
- Portable scanner
- Embedded scanner
Flatbed scanner
These types of scanners have a flat scanning surface which is ideal for photographs, magazines, and various text documents. These types of flatbed scanners have a cover that lifts up so that books or other bigger objects can also be scanned.
Film scanner
This type of scanner is also called a slide or transparency scanner. It works by focussing a narrow beam of light through the film and reading the intensity and color of the light that emerges. Uncut film strips of up to six frames or four mounted slides are inserted inside a carrier which is then moved by a stepper motor across a lens and CCD sensor inside the scanner. Film scanners can vary a great deal in price and quality.
Roller scanner
These types of scanners have a flat sheet over the scanning element between rotating rollers. These types of scanners can be used for scanning single sheets up to a specified width typically about 210 mm which is the width of many printed letters and documents. Some scanners can be portable, powered by batteries and then with their own storage, can transfer stored scans to a computer using USB or other interfaces.
3-D scanner
It is one of the types of scanner that can scan 3 D objects.
Planetary scanner
This type of scanner is typically used for scanning delicate objects without any actual physical contact.
Hand document scanner
These are hand-held document scanners which are manual devices and need to be dragged across the surface of the image to be scanned by hand. So a steady hand is required for scanning documents in this manner as an uneven scanning rate can produce a distorted image. This type of scanner has a “start” button, which should be held by the operator for the duration of the scan. There are also some switches to set the optical resolution and a roller that generates a clock pulse for synchronization with the computer. Older types of hand scanners were monochrome which produced light from an array of green LEDs to illuminate the image. Modern handheld scanners scan in monochrome or color, whatever required. A hand scanner can have a small window opening through which the document being scanned could be viewed.
Portable scanner
Image scanners are usually used together with a computer that controls the scanner and stores scans. Small portable scanners can be either roller-fed or “glide-over” hand-operated, operated by batteries and with storage capability.
Embedded scanner
It is a document scanner that is embedded inside a computer keyboard which can be made available when required without taking any extra space since it is built inside the computer keyboard.
Parameters defining scan quality
Color depth
High color depth of a scanner is defined by its array characteristics. A normal scanner. A normal scanner can have 24 bits of color depth but a high-quality model can have 36-48 bits of color depth.
Resolution
It is another important qualifying parameter of a scanner which is measured in pixels per inch (ppi), sometimes it is also referred to as Samples per inch (spi). Instead of using the scanner’s true optical resolution manufacturers like to refer to the resolution as defining the quality of a scan. A high-end flatbed model can scan up to 5400 ppi while drum scanners provide an optical resolution of 3,000 – 24,000 ppi.
Dynamic range
It is the third important parameter for a scanner. It is also referred to as Drange. A scanner having a high-density range means it is able to record shadow details and brightness details in one scan. The density of film can be measured on a log scale with base 10. This measurement can vary between 0.0 (transparent) and 5.0. Dmin and Dmax measurement denotes where the least dense and most dense measurements are on a negative or positive film. Dmax measurement will be the densest part on slide film for shadows and densest on negative film for highlights.
Connection of scanner with a computer
Scanners must be connected with computer software programs that can import data from the scanner. Most scanners have inbuilt basic scanning software which allows the user to configure, initiate, and import scans. Some Scanning plug-ins are also available which can also be installed which may allow software programs to import scanned images directly.
Softwares like Photoshop can edit scanned images while some programs like Acrobat and OmniPage can actually recognize scanned text. This technology is known as optical character recognition or OCR.
