|[ Team LiB ]|
How the view of the scanning sensor moves to that following line is the fundamental design difference between scanners. Somehow the long line of sensing elements must shift their attention with extreme precision over the entire surface of the image to be captured. Nearly all scanners require a mechanical sweep of the sensors across the image, although a few low-resolution scanners use video technology to sweep their view electronically.
To make a sweep in a mechanical scanner, engineers have devised two primary strategies. One requires the image sensor to move across a fixed original, like you examining a statue in a museum by walking around it. The other moves the original in front of a fixed scanner the same way you might examine an apple for intruders that have bored inside, by holding it in your hand and turning it around in front of your eyes. With a video scanner, nothing moves except an electron beam.
The very first scanners helped newspapers and wire services send images across the country with the ease of telegraphing messages. Someone in one newspaper office wrapped a photo around a metal cylinder or drum, and the scanner spun the drum around while a single light sensor checked the brightness of the photo at a single spot—which became a chain of observations as the drum continually spun the image under the watchful photo-eye. With every spin of the drum, the light detector moved slightly down the photo until it got to see the entire image. A matching spinning drum covered with light-sensitive paper (like photographic film) at the other end of the connection created the image by scanning it with a light beam.
Engineers adapted this same moving-cylinder approach into the drum scanner. Instead of a single photo-eye, the drum scanner uses a linear array so that a single spin of the drum covers the entire image.
Operationally, the drum scanner works like a printing press in reverse. You feed a piece of paper that bears the image you want to capture into the scanner, and the paper wraps around a rotating drum that spins the image past a sensor string that's fixed in place inside the machine.
The drum design lends itself to document processing. The mechanism puts the paper being scanned in motion, so adding a page feeder is a relatively simple addition. Because of their orientation to scanning printed pages, drum scanners are sometimes termed page scanners.
Today, most consumer-model drum scanners lack the drum that gave them their name. Instead of wrapping each sheet of paper around a drum, most scan each sheet as it slides through their mechanisms flat. The image sensor peers through a narrow slit across the paper path, recording line after line as the paper rushes through. Expensive, precision scanners used in the graphic arts industry still cling to the classic drum design because of its precision and simplicity.
The drum scanner mechanism imposes a stiff penalty—it allows only thin, flexible images to be scanned. In general, a drum scanner accepts only sheets of normal paper. Books (at least while intact) and solid objects are off limits. Moreover, most drum scanners accept only a few sizes of paper, typically the 8.5-by-11-inch sheets of business documents. Consequently, drum-scanning technology today is restricted to high-volume (and expensive) document processing systems used in big businesses.
The flatbed scanner takes the opposite tack. Instead of moving the paper to scan it, the flatbed scanner moves its line-up of sensors down the sheet. It earns its "flatbed" name from the flat glass surface, the bed upon which you must place the item to be scanned, face down. In most flatbeds, the linear array of scanning sensors is mounted on a bar that moves under the glass, automatically sweeping across the image. The clear glass lets the sensors see up to the image. In addition, the glass protects the sensors and gives them a target fixed in place at a preset distance from the scanner, which keeps things in focus.
Flatbed scanners have precision mechanisms that step the sensors or image a small increment at a time, each increment representing a single scan line. The movement of the mechanism, which is carefully controlled by the electronics of the scanner, determines the width of each line (and thus the resolution of the scanner in that direction).
Flatbed scanners are like copying machines in that anything you can lay flat on their glass faces can be scanned—books, magazines, sections of poster, even posteriors and other parts of your anatomy if you get imaginative, bored, or drunk. Of course, the scanned image can be no larger than the scanner bed.
In the past, the chief drawback of the flatbed scanner has been price. But manufacturers have refined flatbed technology to the point flatbed scanners are sometimes given away free to entice you to buy a particular computer. Although top-quality flatbed scanners for graphic arts professionals still demand hefty prices, you can buy an entirely satisfactory flatbed scanner for little more than $50. Those prices have made scanners that use other technologies scarce.
Hand scanners are a variation on the flatbed design (believe it or not!) that make you the motive force that propels the sensor over the image. You hold the T-shape hand scanner in the palm of your hand and drag it across the image you want to scan. A string of sensors peers through a plastic window in the bottom of the hand scanner to register the image.
Hand scanners must cope with the vagaries of the sweep of your all-too-human hand. If you move your hand at a speed other than that at which the scanner expects, lines will be scanned as too wide or too narrow, resulting in image distortion—at best the aspect ratio may be off, at worse the scanned image will look as wavy as the Atlantic under the influence of an errant typhoon. To avoid such disasters, the hand scanner uses a feedback mechanism that tracks the position of the image. Most have a roller that presses down against the image you're scanning to sense how fast you drag the scanner along. The rate at which the roller spins gives the scanner's electronics the feedback it needs about scanning speed. From this information, the software that controls the hand scanner can give each scanned dot its proper place.
At one time, hand scanners were a low-cost alternative to flatbed designs. Because they omitted the most expensive parts of most scanners—the precision mechanism for moving the paper or sensor—they had an automatic edge in price. With the plummet in prices of flatbed scanners, however, hand scanners were hard-pressed to keep up. A low-cost flatbed is now likely to be less expensive than a hand scanner. Consequently, few hand scanners are left on the market.
Those remaining have survived because of the chief remaining advantage of the hand scanner—portability. Hand scanners are compact and easy to carry. You could plug one into your notebook computer and carry the complete system to the neighborhood library to scan from books in its collection.
In addition, using a hand scanner can be quicker than using a flatbed because you can make fast sweeps of small images instead of waiting for the lumbering mechanism of a flatbed to cover a whole sheet. Hand scanners may also adapt to some nonflat surfaces and three-dimensional objects. For example, most will easily cope with the pages of an open atlas or gothic novel—although few can do a good job on a globe or watermelon.
On the downside, the small size of the hand scanner means a single pass of the scanner will cover an image no more than about four inches wide. Although that's enough for a column of text (and most scanners offer a means of pasting together parallel scans of larger drawings and photos), the narrow strips of scan make dealing with large images inconvenient. On the other hand (and in the other direction), because a hand scanner is not limited by a scanning mechanism, it can allow you to make absurdly long scans, typically limited only by the scanning software you use.
Note that hand-scanning is like typing—it's a learned skill. To use a hand scanner effectively, you'll have to practice until you learn to move the scanner smoothly and at the proper speed, which means very slowly at high resolutions.
A video scanner is the electronic equivalent of a photographic copy stand. That is, the scanner operates like a camera, taking in a view of the entire image in a single look. That makes a video scanner fast—capturing an image takes a fraction of a second. You'll spend more time setting up the image or object to be scanned than the scanner needs to scan.
Typically a video scanner uses a conventional video camera to capture an image. Most video scanners permanently mount the camera on a stand and give you a stage on which you put the item to be scanned. The stage may have a backlight to allow you to scan photographic slides or negatives, or it may be a large bed for sheets of paper or even three-dimensional objects.
Video scanners avoid all the problems and inaccuracies imposed by mechanical scans. They have the potential for the greatest precision. Typically, however, video scanners yield the lowest quality. Like a video camera, video scanners require a CCD element for every pixel they scan, and affordable two-dimensional CCD arrays have only a few hundred thousand pixels. Because video scanners use the same CCD arrays as video cameras, they have the same resolution as video cameras, not measured in dot per inch but in pixels across the entire image. They are suited to snapshots and catalog illustration but not high-quality scans.
As their name implies, photo scanners are special-purpose devices aimed at capturing digital images from photographic prints. Most use an adaptation of drum-scanner technology. They move the original rather than the sensor, sliding the photo past the image sensor, flat.
Photo scanners can be quicker to use because you only need to slide your snapshots in like you're feeding dollar bills into a vending machine. Because their mechanisms are inherently less complex than those of flatbed scanners, at one time dedicated photo scanners had a price edge. With the current generation of low-cost flatbed scanners, however, that advantage has vanished. A flatbed can do everything a photo scanner can—and more—but if all you need to scan is photos, you'll have a quicker and easier time with the dedicated device.
The slide scanner is not a special technology but rather a special implementation of flatbed or video scanner technology. A slide scanner is a transmissive scanner rather than reflective. That is, it registers the light that is transmitted through an image rather than the light reflected from the image. The source of illumination is on one side of the image, and the image sensor is on the other. The image must be on a transparent medium.
A flatbed-style slide scanner is optimized for the higher-resolution needs of scanning small photographic transparencies or negatives but relies on a modified flatbed scanner mechanism. It needs only more precision in its control because of the smaller size of the scanned area of negatives and slides (and the correspondingly high resolution required). A video slide scanner is subject to the same limitations as any video scanner—chiefly, low resolution—but gives you an inexpensive means of capturing limited-resolution snapshot-quality images from slides and negatives.
|[ Team LiB ]|