Scanners vary in resolution and sharpness. Most flatbed scanners have a true hardware resolution of at least 300x300 dots per inch (dpi). The scanner's dpi is determined by two factors:
- The x-direction sampling rate - This is determined by the number of sensors in the CCD imaging array.
- The y-direction sampling rate - This is determined by the precision of the stepper motor.
Let's take a simple example: If a scanner's resolution is 300x300 dpi, and that scanner is capable of scanning a letter-sized (8.5x11-inch) document, then the CCD has 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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Sharpness depends mainly on the quality of the optics used to make the lens and the brightness of the light source. A bright xenon lamp and high-quality lens will create a much clearer and therefore sharper image than a standard fluorescent lamp 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 labeled software-enhanced, interpolated resolution or something similar. What does that mean?
Interpolation is a process that the scanning software uses to increase the perceived resolution of an image. It does this by creating extra pixels in between the ones actually scanned by the CCD array. These extra pixels are a weighted average 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.
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