Resolution and Interpolation

Resolution and Interpolation

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 the number of sensors in a single row (x-direction sampling rate) of the CCD or CIS array by the precision of the stepper motor (y-direction sampling rate).

For example, if the resolution is 300x300 dpi and the scanner is capable of scanning a letter-sized document, then the CCD has 2,550 sensors arranged in each horizontal row. 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. Likewise, a scanner with a resolution of 600x300 has a CCD array with 5,100 sensors in each horizontal row. get your scanner spares from parts warehouse, its great

Most scanners have a scan area that is either letter size (8.5x11 inches, 21.6x27.9 centimeters) or legal size (11x14 inches, 27.9x35.6 centimeters).

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 a x-direction sampling rate of 9,600 would require a CCD array of 81,600 sensors. 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 an 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 one scanned by a CCD sensor in each row.

Another term used when talking about scanners is bit depth, also called color depth. This simply refers to the number of colors that the scanner is capable of reproducing. Each pixel requires 24 bits to create standard true color and virtually all scanners on the market support this. Many of them offer bit depths of 30 or 36 bits. They still only output in 24-bit color, but perform internal processing to select the best possible choice out of the colors available in the increased palette. There are many opinions about whether there is a noticeable difference in quality between 24-, 30- and 36-bit scanners.


Image Transfer

Scanning the document is only one part of the process. For the scanned image to be useful, it must be transferred to your computer. There are three common connections used by scanners:

    Parallel - Connecting through the parallel port is the slowest transfer method available.
    Small Computer System Interface (SCSI) - SCSI requires a special SCSI connection. Most SCSI scanners include a dedicated SCSI card to insert into your computer and connect the scanner to, but you can use a standard SCSI controller instead.
    Universal Serial Bus (USB) - USB scanners combine good speed, ease of use and affordability in a single package.
    FireWire - Usually found on higher-end scanners,FireWire connections are faster than USB and SCSI. FireWire is ideal for scanning high-resolution images.

On your computer, you need software, called a driver, that knows how to communicate with the scanner. Most scanners speak a common language, TWAIN. The TWAIN driver acts as an interpreter between any application that supports the TWAIN standard and the scanner. This means that the application does not need to know the specific details of the scanner in order to access it directly. For example, you can choose to acquire an image from the scanner from within Adobe Photoshop because Photoshop supports the TWAIN standard.

In addition to the driver, most scanners come with other software. Typically, a scanning utility and some type of image editing application are included. A lot of scanners include OCR software. OCR allows you to scan in words from a document and convert them into computer-based text. It uses an averaging process to determine what the shape of a character is and match it to the correct letter or number.

The great thing about scanner technology today is that you can get exactly what you need. You can find a decent scanner with good software for less than $200, or get a fantastic scanner with incredible software for less than $1,000. It all depends on your needs and budget.

For more information on scanners and related topics, check out the links on the next page.
Did You Know?

TWAIN is not an acronym. It actually comes from the phrase "Never the twain shall meet" because the driver is the go-between for the software and the scanner. Because computer people feel a need to make an acronym out of every term, TWAIN is known as Technology Without An Interesting Name!