A standard (or optical) resolution of a scanner is determined by how many sensors are located in each row and how many small, fine pixels are used to create the larger image. This determines the level of detail of an image that will be picked up by the scanner, thus affecting the clarity of the image being scanned. The overall quality of the optics used to make the lenses and the strength of the light source will also impact the clarity and sharpness of the image. So, when you are purchasing a scanner, be sure to note optical resolution rather than interpolated resolution. Understanding the color and bit depth is also important.
You may be confused by interpolated resolution (also referred to as enhanced resolution and digital resolution), which will be significantly higher than optical resolution. This number is often advertised to mislead buyers into thinking the scanner has an incredible inherent resolution, when in reality, interpolated resolutions are generated by algorithms in software.
Basically, interpolated software adds pixels to the image after it has been scanned, filling in the gaps and making the image appear to have been scanned at a much higher optical resolution. Unfortunately, adding pixels to an original scan can have a negative effect. For instance, when the size of the image is altered, those extra pixels can cause blurriness or discoloration. Most experts advise to disregard interpolated resolution as it is not an accurate representation of the what the image will look like when printed from the digital file.
Many scanners use the TWAIN protocol, which is universal and allows scanners to operate with any image program. The TWAIN driver transfers the scanned image to memory where you can make adjustments and enhancements. Via the TWAIN software, you can set the mode (color, grayscale, line art), scan area, and desired resolution. Most standard documents, such as invoices or legal paperwork, can be scanned at a resolution of 300 or 400 dpi. (This is different than the dpi created by ink and toner). There is no need to set the scanning resolution higher than that unless a color image or graphic is being processed. In fact, higher resolutions will often create a much larger file that can be more difficult to email or share.
To reiterate, optical resolution is the actual quality in which the document is scanned and stored. This is the most important resolution. Interpolated resolution comes from enhancement software and is determined by adding pixels in between the existing pixels. Don’t let this number mislead you. When shopping for a scanner, consider the resolution you are likely to actually use on a regular basis, and whether the higher resolutions will even be necessary.