The invention relates to an apparatus for determining a layer thickness on a tape moved along a feed direction, comprising: a movement unit for moving the tape along the feed direction, a light generating unit for generating illumination radiation, a beam shaping unit disposed downstream of the light generating unit for shaping at least one strip-shaped illumination beam for linear illumination of the tape transversely with respect to the feed direction, a detector unit for detecting illumination radiation reflected and/or transmitted at the tape, and an evaluation device for determining the layer thickness on the basis of the detected illumination radiation. The apparatus can also be used for determining a degree of contamination of a surface of the tape, the surface being contaminated by particles, by detecting illumination radiation scattered at the tape.