As part of the modernisation of South Stack lighthouse in 1999, the size of the main lamp was reduced from 1000 watts to 150 watts, though the range of the light was only reduced from 25 miles to 20 miles.

The mechanism that supports the lighthouse prism is divided into two sections. The outer section is a stationary bath while the inner section is the float. In the narrow gap between the bath and the float is a layer of mercury which supports the whole weight of the lens and float. This creates an almost frictionless bearing. Indeed, although the lens and the float weigh several tons, they could be turned with the pressure of a single finger.