In this world of laser beams and ultrasonic transducers, we sometimes lost sight of simplicity.
Remember- the simplest solution that works is probably the best. A good visual control should tell the operator, immediately, if a process is going beyond the specified parameters.
Ideally the process would be stopped automatically, however a clear signal to stop, given in time to avoid a more serious problem, is adequate.
So, in that spirit I give you (from Gizmodo) the following example: