Optical imaging apparatus and a machine vision camera system for an alarm protecting a person's hands when working at a motorized cutting or abrading machine, uses trains of sets of mirrors to define four spaced-apart viewpoints and transfer non-overlapping scenes on to an image sensor of a single camera. An optical axis from each viewpoint allows a cuboidal protected volume to be created. The cuboid may have vertical sides. Recognizable objects inside that volume cause the machine to stop, while objects near but outside the volume cause the apparatus to raise an alarm.