Slides and negatives always have small dust particles and microscopic scratches on them, even when handled very carefully.
The OpticFilm 8200i scanner features an infrared channel that detects all the dust and scratches on the film surface. Infrared light has a very long wavelength, which allows it to pass through the emulsion of negative films and slide without resistance, unlike scratches and dust particles that reflect it. SilverFast iSRD uses this behavior as follows.
The image is scanned twice - the first is the normal scan and the second is the additional infrared scan that captures only defects like dust and scratches. Then, computational dust and scratch removal takes effect, only where the infrared channel has detected any defects without losing any significant detail.