The practice of using closed circuit television systems for surveillance such as, protecting safes, watching for unwarranted intruders, detecting fires, etc., has in recent years become common place. However, the disadvantage of this is that an operator is required to watch one or more monitors at all times. This can be come very tedious or strenuous, depending on the number of monitors and could result in an increase in the number of operators required for this task and hence, a rise in cost.
"Multi Video Sensor", Proc. SPIE 0033, Solving Problems in Security, Surveillance and Law Enforcement with Optical Instrumentation, (20 February 1973); doi: 10.1117/12.953622; https://doi.org/10.1117/12.953622