On what basis are light dues payable at UK ports?

This is a specified number of pence per ton of net tonnage as shown on the ship's International Tonnage Certificate (1969), and changes periodically. From 1 April 2010, light dues for an ordinary cargo- carrying merchant ship are payable on nine voyages per year at the rate of 41 pence per ton, subject to a minimum charge of £60 and a maximum charge of £16,400 per voyage. A "voyage" is reckoned from port to port.