Is this what you are referring to?
http://www.gatestore.com/gate_parts/index.php?main_page=product_info&products_id=661
Seems pricey at $320.
I would try a range finder first. You can use these to measure the distance to a known object. If the distance is shorter, then an object is passing buy.
Of course there may be other considerations like how fast the conveyor belt is moving, etc...