Scanners vary in resolution and sharpness. Most flatbed scanners have a true hardware resolution of at least 300x300 dots per inch (dpi). The scanner'sdpiis determined by two factors:
- Thex-direction sampling rate- This is determined by the number of sensors in the CCD imaging array.
- They-direction sampling rate- This is determined by the precision of the stepper motor.
Let's take a simple example: If a scanner'sresolutionis 300x300 dpi, and that scanner is capable of scanning a letter-sized (8.5x11-inch) document, then theCCDhas 2,550 sensors arranged in each horizontal row -- 8.5 (inches across) x 300 (x-direction sampling rate) = 2,550. A single-pass scanner would have three of these rows for a total of 7,650 sensors. The stepper motor in our example is able to move in increments equal to 1/300ths of an inch.
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Sharpnessdepends mainly on thequality of the opticsused to make the lens and thebrightness of thelightsource. A bright氙lamp and high-quality lens will create a much clearer and therefore sharper image than a standardfluorescentlamp and basic lens.
Of course, many scanners proclaim resolutions of 4,800x4,800 or even 9,600x9,600. To achieve a hardware resolution with an x-direction sampling rate of 9,600 would require a CCD array of 81,600 sensors, which is almost unheard of. If you look at the specifications, these high resolutions are usually labeledsoftware-enhanced,interpolated resolutionor something similar. What does that mean?
Interpolationis a process that the scanning software uses to increase theperceived resolutionof an image. It does this by creatingextra pixelsin between the ones actually scanned by the CCD array. These extra pixels are a加权平均of the adjacent pixels. For example, if the hardware resolution is 300x300, and the interpolated resolution is 600x300, then the software is adding a pixel between every two pixels scanned by a CCD sensor in each row.
Here are some interesting links:
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