L/5 Bunch Filament Lighthouse Lamp

Illustrated here is an early form of low power lamp formerly used in English and Welsh lighthouses operated by Trinity House Corporation, and Scottish lighthouses of the Northern Lighthouse Board, and whose equivalents also find applications in wider international lighthouses. Its is classified by the Lighting Industry Federation as type L/5, also known under its fuller reference BSL5 according to British Standard 1546, and is identical to the old B1/7 incandescent floodlighting lamp.

It is designed for operation directly on the 240V mains supply, which indicates that it would have been used for a land-based lighthouse close to areas of civilisation because many of the more remote and offshore lightouses employed low voltage lamps driven from batteries or diesel generators.

The filament construction is the standard bunch design as employed in Class B1 floodlighting lamps, having a semi-cylindrical shape. This delivers a fairly uniform luminance around the lamp, to maintain a consistent brightness of the signal when viewed from different directions. The standard bunch-type filaments were satisfactory for lower power lamps having limited signalling range, although higher power required improved uniformity and tended to employ double filaments having a full cylindrical or cruciform shape.

The filament is mounted on a conventional soft glass stem, supported on heavy molybdenum wires fixed to a glass button. It is sealed into a spherical bulb blown in soda-lime glass. The cap is of the standard P28s medium prefocus type, individually aligned to the filament centre to avoid need for re-focussing after replacing the lamp.
Manufacturer: Victory Lighting Type L/5
Lamp Power: 250 Watts
Lamp Voltage: 240 Volts
Lamp Current: 1.04 Amperes
Cap Type: P28s/25 Ni plated brass & Vitrite
Bulb Type: G-80 G-25 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Filament Type: C-5 Bunch
Filament Size: 12mm x 16mm
Atmosphere: Argon-Nitrogen Red phosphorus getter
Luminous Flux: 3,125 Lumens
Luminous Efficacy: 12.5 Lumens per Watt
Luminance: 150 cd/cm²
Colour Temperature & CRI: CCT: 2900 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.444 CCy: 0.406
Rated Lifetime: 800 hours
Burning Position: Vertical cap down ± 30°
Overall Length: 130 mm 51/8 inches
Light Centre Length: 55.5±0.5 mm 23/16 inches
Factory: Wembley England
Date of Manufacture: January 1991 Date Code: GA21
Original Value: Unknown
 
References: 1) BS 1546 - British Standard Specification for Electric Lamps for Lighthouses, 1963.
2) Lighting Industry Federation Schedule of Projection Lamp Types, London, 1982.