L/8 Planar Triple Filament Lighthouse Lamp

This is lamp is an unusual design formerly used in English and Welsh lighthouses operated by Trinity House. Its is classified by the LIF as type L/8, also known under its fuller reference BSL8 according to British Standard 1546, and sometimes referenced under the Trinity House Engineering Chief's nomenclature EC101.

Whereas most lighthouse lamps employ filaments intended to deliver a rotationally symmetrial luminance around the lamp axis, so as to ensure uniformity of the apparent signal brightness when viewed from different positions around the horizon, this lamp has a substantially planar filament construction. Although BS1546 indicates that this lamp should have a grid-type filament, it in fact consists of three separate coiled-coil filaments arranged side by side, and connected in series. The reason for this unusual construction is not known, and was possibly unique to GEC's L/8 lamps while equivalents from other manufacturers may indeed have had a grid filament. Alternatively, since it is expected that this lamp would deliver a slightly more uniform distribution than a true planar grid filament, it may have been developed to suit the optical configuration of a particular lighthouse lens that was formerly used with an earlier non-electric light source - for instance a fish-tail shaped petroleum flame.

Owing to the 100V rated voltage it was certainly intended for use on an offshore or remote lighthouse beyond the limits of the 240V mains electricity grid. The filaments are mounted on a special hard glass stem, and again unusually for a lighthouse lamp, sealed into a tubular rather than spherical glass bulb. It has a standard pre-focussed cap.
Manufacturer: Osram-GEC Type L/8 (EC101)
Lamp Power: 375 Watts
Lamp Voltage: 100 Volts
Lamp Current: 3.75 Amperes
Cap Type: P28s/25 Ni plated brass & Vitrite
Bulb Type: T-64 T-20 in eighths/inch
Bulb Finish: Clear Borosilicate glass
Filament Type: 3CC-8 Triple Axial Coiled-Coil
Filament Size: 13mm x 16mm
Atmosphere: Argon-Nitrogen Zirconium-aluminium getter
Luminous Flux: 6,000 Lumens
Luminous Efficacy: 16.0 Lumens per Watt
Luminance: 450 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: 140 mm 51/2 inches
Light Centre Length: 55.5±0.5 mm 23/16 inches
Factory: Wembley England
Date of Manufacture: May 1972, Group 61 Date Code: 61DE
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.
3) Trinity House Engineering Chief's Specification of Electric Lamps for Stores Supplies, 1962/63.