C.A.S.Ls. (Coast Artillery Searchlights), have been referred to as Electric Lights (original name) and D.E.L.s (Defence Electric Lights). They were searchlights used in an anti-shipping role.
Two types of searchlight were employed. A narrow, long range beam of 3°, called a ‘Fighting Light’ for illuminating targets for Close Defence Batteries (6 Inch Guns in the case of Siloso), and wide angle of 16°, 30° or 45° to illuminate a wide area AMTB Batteries. The lights were normally 90 cm in Diameter.
Despite the British using Imperial Measurements, searchlights were measured in centimetres as they were classed as optical equipment because of the concave light reflector. They were therefore measured in the continental manner for optical equipment.
A C.A.S.L. produced a beam of up to two million candlepower. The beam was not produced by a bulb, but by arcing (jumping) an electric current across a narrow gap between two carbon rods. The rods burnt down in use, so the light operator had to keep the gap between the rod carefully adjusted to achieve optimum performance. Heat produced from the arc was vented out into the open air. A C.A.S.L. was never switched on. The terminology used was that it was ‘exposed’.
There were seven searchlight emplacements constructed at Fort Siloso. The first Electric Light Emplacement overlooked the harbour. It provided illumination for the Submarine Mining Post. Two more were constructed during an upgrade to the fort. One at Siloso Point, and the other on the cliff south of the 6 Inch QF Gun Emplacement. The emplacement at Siloso Point was destroyed in the late 1930s in order to construct the A.M.T.B. Battery there. A new emplacement was built on the south west side of the fort to replace the one destroyed. At the Point, three emplacements were constructed to provide illumination for the A.M.T.B. Battery.
All except the first Siloso Point Light can be found. The first Electric Light Emplacement is in a ruinous condition, and not very safe to visit.