A big thankyou to Rob Weir for writing in with this detailed explanation of Lyon Lights (and thanks to Tony for the introduction):
Each Lyon Light was a self-contained (Lister petrol engine with generator) small searchlight (about 20 -24 inch diameter) which was an integral part of Hong Kong's beach defences. The combination of Pillbox and Lyon Light were referred to officially as a Beach Defence Unit.
The light was installed in its own concrete shelter, usually no more than 30 - 50 yards to the side and behind the PB. If possible, it was elevated above the PB either by the terrain (e.g PB 30)*, or by a tall concrete base (e.g the one behind PB 16)*.
Exceptions are known; if you look at the photo of the ruined PB at Braemar Point (between pages 140 and 141 in Not The Slightest Chance), you will see the LL shelter was actually on the roof of the PB. How many were like this I don't know. The only remaining example is PB 33a on the SW coast of the D'Aguilar Peninsular.
Some of the PB's along the north coast didn't have space for dedicated shelters, but where necessary the LL's were installed in adjacent buildings. PB 59 at the Navy Yard was an odd-ball having two LL's back to back on it's roof. No other known examples.
The shelter was a small concrete structure with a curved, open front, allowing the light to be used. Outward opening steel shutters, hinged at the bottom, could be closed to protect the light when it was not in use. Immediately behind the shutters, on a concrete block, sat the light itself. Above, was a small square hole in the roof, which, through a rooftop duct to a vertical airshaft at the back, created air circulation to the light, which generated a lot of heat in operation.
The power unit sat on concrete shelves at the back, with a hole through the wall for the exhaust fumes. The light was served by two men from the PB crew, and shelters had provision for two stretcher type beds on the back wall. The entry was a normal doorway, with a steel door. Some units had a protective wall between the PB and LL (e.g. PB 30)*.
In the world of digital electronics, it is difficult to believe that communication between the PB and LL Shelter occupants was by a speaking tube. This looked for all the world like a length of 3" dia. water pipe running between the two, and some remains can still be seen. Operation of the light was controlled by the PB commander, either by voice, or a predetermined number of whistle blasts.
I have no knowledge of LL's existing without a corresponding PB. Their purpose was to illuminate an approaching landing force, at fairly short range, so the PB's could engage. eg There was a PB 4 at Telegraph Bay. I visited it in 1996, but it was within the fence of a village house, and covered in vegetation and rubbish, so I didn't get into it. It was demolished sometime in 2003 during earthworks for the Cyberport - which also wiped out PB 5 and its LL Shelter. It was to the NE of the LL i.e. more into the original bay towards the channel for the undersea cables. There is a small photo taken by Hedda Morrison of Telegraph Bay, where the PB is visible (just). The LL Shelter was still standing, although covered by vegetation, last year.
There were also separate Beach Defence Lights at a number of points around the Island, but they were much bigger and more powerful lights. There were also reports of Anti Aircraft Searchlights being moved and used as Beach Defence Lights.
* The examples given are those that still exist and are easily accessible, except PB 16. That is it's LL Shelter alongside the walking path. If you look over the concrete sea wall, just below the LL, there is a concrete block at water's edge, still with the outline of part of the PB wall on it.