1. An apparatus for defect detection in a printing system, comprising: a sensor device for sensing a surface of a sheet to be printed and for generating data representing a height map of the sheet; and a processor device for processing the data from the sensor device, wherein the processor device is configured: to analyse the height map to detect a presence of a deformation in the sheet; when a deformation is detected, to determine at least one property of the deformation; to classify the deformation with respect to a plurality of deformation classes by means of the at least one property of the deformation; to determine a suitability of the sheet for printing based on the deformation class in which the deformation has been classified.