What this part does
The Omron E3Z-T81K-L 2M is the emitter half of a through-beam photoelectric sensor pair. It transmits a modulated light beam to a separate receiver; the pair detects objects that break the beam. This is not a standalone sensor — the receiver (e.g., E3Z-T81-D 2M) is required for detection. If you're stocking spares, make sure both sides of the pair are in your BOM.