Scan resolution (DPI)
Some important definitions are clarified here. They comprise the vital basics of image reproduction theory.

Input resolution
Resolution is a common term that refers to the number of finest elements in an image, or pixels - 'pixel' is a word made from the combination of the words picture and element - that a sensor such as a scanner or one of its components, can record or distinguish. The unit of measure is in general dpi (dots per inch) or dpcm (dots per centimeter). The higher the resolution, the greater the number of pixels that can be scanned.

Optical resolution / interpolated resolution
Resolution also defines the extent to which a digital image capture system, such as a scanner or a component thereof, can display image details.

The manufacturers of scanners usually specify the resolution of the built-in sensor as the optical resolution. This specification refers only to this one component. However, due to the complex structure of a scanner (including optics, mirrors, electrical components such as A/D converters and cables (shielding)), the actual usable resolution may be lower.

But since the actual usable or effective resolution is the one we are interested in, we define optical resolutions as resolutions that the scanner with all its components actually achieves. This resolution can be measured with our USAF 1951 Resolution Targets.

Optical resolution is also called physical resolution. It indicates how many lines or dots per inch or cm can actually be differentiated, i.e. clearly distinguished, by the CCD, the electrical circuits and the optics of the scanner. In practice, this can be seen by whether two closely spaced lines can still be recognized as separate from each other (as individual lines).

The interpolated resolution is a mathematical resolution calculated by the hardware or software, which, as we will see later, is only relevant for line reproduction, but not for grayscale reproduction.

For more detailed and legally binding information on the subject of resolution, we recommend reading the following Iso standards:

ISO 16067-1:2003
Photography — Spatial resolution measurements of electronic scanners for photographic images — Part 1: Scanners for reflective media

ISO 16067-2:2004
Photography - Electronic scanners for photographic images - Spatial resolution measurements — Part 2: Film scanners