Film scanner

Even small specks of dust, invisible to the naked eye, can obscure a cluster of several pixels.

The simplest is the median filter, often called despeckle in many graphic manipulation programs, e.g. in Adobe Photoshop and the GIMP.

Photographic film is mostly transparent to infrared radiation (no matter what the visible image contains) but dust and scratches aren't, so they show up in the IR channel.

This information can then be used to automatically remove the appearance of dust and scratches in the visible channels and replace them with something similar to their surroundings.

LaserSoft Imaging developed the iSRD dust and scratch removal, on which among others Plustek is relying.