L/25 Cylindrical Filament Lighthouse Lamp

Illustrated here is a typical medium power lamp formerly used in English and Welsh lighthouses operated by Trinity House Corporation, and Scottish lighthouses of the Northern Lighthouse Board. Its is classified by the Lighting Industry Federation as type L/25, also known under its fuller reference BSL25 according to British Standard 1546.

It is designed for 100V circuits, a low voltage which used to be provided by diesel generators or batteries prior to the extension of the mains electricity grid to the remote and offshore locations of British lighthouses. The filament has a cylindrical shape, and consists of two filaments each folded into a semi-circular pattern and arranged back to back so as to form a complete circle when viewed from above. This helps to ensure that the lighthouse appears equally bright from all viewing angles at sea level. The light signal from such lamps used to be coded into brief pulses of precise and repetitive duration to aid identification, by a system of optical lenses rotating around the lamp, and spatial variations in luminous intensity from the filament may present an incorrect signal.

The filament assembly is mounted on a conventional hard glass stem and sealed into a spherical borosilicate bulb. The cap is of the standard P40s mogul prefocus type, individually aligned to the filament centre to avoid need for refocussing the optical assembly after replacing the lamp.

These lamps were gradually superseded by 400W MBI/BD metal halide lamps during the 1980s, and as of today there are no known installations still using lamps of this kind for British lighthouses.

Manufacturer:	Thorn E.M.I.	Type L25
Lamp Power:	1500 Watts
Lamp Voltage:	100 Volts
Lamp Current:	15.0 Amperes
Cap Type:	P40s/45	Ni plated brass & porcelain
Bulb Type:	G-150	G-48 in eighths/inch
Bulb Finish:	Clear	Borosilicate glass
Filament Type:	2C-5	Cylindrical
Filament Size:	15mm x 30mm
Atmosphere:	Argon-Nitrogen	Red phosphorus getter
Luminous Flux:	27,750 Lumens
Luminous Efficacy:	18.5 Lumens per Watt
Luminance:	550 cd/cm²
Colour Temperature & CRI:	CCT: 3000 Kelvin	CRI: Ra 100
Chromaticity Co-ordinates:	CCx: 0.452	CCy: 0.409
Rated Lifetime:	800 hours
Burning Position:	Vertical cap down ± 30°
Overall Length:	225 mm	8⁷/₈ inches
Light Centre Length:	100 mm	4 inches
Factory:	Buckie	Scotland
Date of Manufacture:	July 1985	Serial No: 43-201
Original Value:	Unknown


References:	1) Thorn EMI Lamp Catalogue, 1984/85
	2) Lighting Industry Federation Schedule of Projection Lamp Types, London, 1982.
	3) BS 1546 - British Standard Specification for Electric Lamps for Lighthouses, 1963.
	4) Twenty Years in Lighthouses 1924-1944, F.W. Cooper, Lighthouse Department, Chance Brothers Limited, 1979, pp.19-20.
	5) Private Communication, Ian Tutt, Trinity House Lighthouse Corporation Ltd, Cowes, Isle of Wight, August 1995.
	6) Private Communication, Bill Bellion, Watermarks Ltd., Hayling Island, September 1995.