[3] The lighthouse guides passing vessels through the hazardous waters surrounding the Bill, while also acting as a waymark for ships navigating the English Channel.

[9] A pressurised vapour paraffin lamp was used, placed at the centre of a large (first-order) revolving optic; weighing 3.5 tons, this was made up of four asymmetrical catadioptric lens panels and a concave prismatic reflector.

[10] These were connected at a higher level to the sounding tanks, which fed the compressed air to the diaphone itself, mounted behind its trumpet-like emitter which protruded through the window.

The diaphone remained in regular use as an aid to navigation until 1995, when it was replaced by a high-frequency electric fog signal (sounding from another window, further down) in readiness for automation.

[10] On 18 March 1996, Portland Bill Lighthouse was demanned, and all monitoring and control transferred to the Trinity House Operations & Planning Centre in Harwich.

[12] The original Type F diaphone was decommissioned in 1996, but in 2003 Trinity House restored it to occasional use for the benefit of visitors;[13] (it was sounded regularly for half an hour on Sunday mornings, except when foggy, until 2017).

[4] In November 2018 Trinity House applied for (and obtained) planning permission to remove the lamp and optic from the lantern room as part of a programme of modernisation.