Assembly for, and method of, detecting and signalling when an object enters a work zone

An assembly for detecting and signalling when an object enters a work zone including a plurality of transmitter-sensor pairs serially connected along a section of the work zone perimeter for detecting when an object passes therethrough. Once an object is detected, an optical warning signal is activated. The optical warning signal includes a primary strobe and a plurality of relay strobes positioned throughout the work zone. The primary strobe flashes at a predetermined flash rate which is detected by a photoelectric sensor in a nearby relay strobe. The nearby relay strobe begins flashing at the predetermined flash rate thereby activating another relay strobe in close proximity. This cascading strobe effect saturates the work zone.