@ Apparatus for detecting missing areas or folded corners in sheets, for example banknotes (14), is disclosed. Figure 1 shows a banknote (14) being scanned in a strip along one edge of the banknote. A strip light source (12) and a strip sensor (16) are disposed on opposite faces of the banknote which is caused to move perpendicularly to the light path between the source and sensor, and perpendicularly to the plane of the drawing. A photometer (18) responds to the total amount of light transmitted through the edge strip of the banknote (14) as the banknote is scanned. A filter (20) may be inter posed in the light path to improve the sensitivity of the apparatus. The total amount of transmitted light detected by the photometer (18) is integrated over a period of the scan during which the intensity differs substantially from the mean level for an unflawed portion of the banknote. Integration is thus performed only over missing or folded areas of the banknote. It may also be limited to only the leading and trailing portions of the edge strip of the banknote, corresponding to the corners of the note.
In order to compensate for the passage of a skewed banknote passing through the apparatus, integration is inhibited over the said period whenever the intensity waveform from the photometer (18) is steeper than a predetermined gradient.