Hmmm, the shut down function could 'or' up multiple algorithms. The motion detection on a wide view could reveal how fast a hand is approaching, if too fast to always avoid a descending tool, shut down. A stationary hand holding the material would be much more common though. Many materials, textiles for example, are punched with a foot operated switch without clamping and both hands are often in danger. Even the operators, who's limbs are in danger, despise safety devices that interfere with their work. I can't count the times that I've seen disabled safety devices. That's what I like about an out of the way camera or a light level detector being the decision maker.