Abstract: A database of target objects suitable for use in detecting the presence of one or more target objects in a receptacle is provided. The database of target objects comprises a plurality of entries, each entry being associated to a respective target object whose presence in a receptacle it is desirable to detect during security screening. An entry for a given target object comprises a group of sub-entries, each sub-entry being associated to the given target object in a respective orientation. At least part of each sub-entry is suitable for being processed by a processing unit implementing an optical correlation operation to attempt to detect a representation of the given target object in an image of the receptacle. A method, an apparatus and a system for generating entries for the database of target objects are also provided.