Scanning laser vibrometry is a widely used tool to observe Lamb wave fields for structural health monitoring (SHM) purposes. Lamb waves propagate over long distances in thin-walled structures and interact with structural inhomogeneities, for example, damages, in spite of wavelengths several times of the damage size. In SHM of sheets and glass- or carbon-fibre-reinforced plastic plates, this effect is used for determining the position as well as the size of structural faults. With the often employed one-dimensional vibrometry, a geometrically induced, systematic error occurs when measuring oblique-angled motion. This error can be, in the specific case of Lamb waves, of a non-negligible quantity. The nature of this geometrical measurement error in general and concerning Lamb waves in special is discussed analytically for both amplitude and phase data. It is shown that this matter should be taken into account in some applications. © 2012 Wiley Publishing Ltd.