It is customary to produce a three-dimensional image using a camera pair in order to ensure that people are isolated in a lock for separating people and to check that no more than one person at a time passes through the lock for separating people. It is an object of the invention to improve known systems and to accelerate them. To this end, a plurality of camera pairs are used according to the invention, which monitor a spatial volume which is to be monitored from several spatial directions at the same time. To this end, each camera pair monitors the space, determines a height profile and supplements shadowed points in the height profile with data of other camera pairs. Monitoring of a spatial volume, for example a passage lock.
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