Genre: Enclosed Horizontal Transverse Mercury Lantern
The mercury vapour discharge lamp was developed by the GEC in the early 1930s. The first production
quality lamps, rated at 400W, were trialled along East Lane, Wembley in June 1932; and the first installation was erected
early the next year. The lamp was designed to replace the 1000W GLS lamp which was considered a necessity for good quality street
lighting but was unaffordable for most local authorities. The 400W medium-pressure mercury lamp (later called the MA) was
therefore marketed as the cost-effective solution for main road lighting.
It was quickly joined by 250W and 150W rated lamps, both tubular, and both designed for road or industrial lighting.
The colour was often seen as a problem, and the lamp soon earned the nickname “the cadaver” lamp, so various
attempts were made to change its spectral output. Doping the mercury discharge with other metals, running tungsten filaments
in series and covering the outer bulb with fluorescent powders were used to modify the lamp’s colour, but all reduced
The lamp’s efficiency and colour output could be improved by increasing the pressure in its enclosed arc-tube. This was
initially a problem as the hard-borate based arc tubes couldn’t withstand the increased temperatures and pressures; whilst
manufacturing issues with quartz glass tubes prevented their adoption. It wasn’t until 1936 when Philips solved
the problems with quartz that a new generation of high-pressure mercury lamps were introduced. Manufacturing costs prevented
the quartz based lamps from being made in larger wattages so only lower wattage lamps initially used the quartz technology
(and were therefore called MB lamps).
The smaller MB lamps were offered in elliptical glass bulbs and resembled the shape of tungsten filament lamps. To prevent
engineers installing MB lamps in unprotected circuits – as they required gear to stabilizethe current – the bayonet connector
was fitted with three pins instead of two.
After the Second World War, the mercury lamp was used extensively throughout the UK as part of post-war rebuilding.
New installations were often either low-pressure sodium or mercury. The status quo with the MA and MB lamps remained intact
until the costs of quartz eventually dropped and higher wattages became financially viable; and so the MA lamp was eventually withdrawn.
Whilst improvements were made with the efficacy and ight quality of the lamps throughout the 1950s and 1960s, the lamp remained
less efficient than its sodium counterparts. The introduction of the high-pressure sodium lamp in the early 1960s, which was more
efficient, meant that mercury was rapidly becoming a poor option for street lighting. When the energy crisis of the 1970s hit,
forcing councils to opt for the cheapest running options, mercury joined fluorescent and tungsten as one of the lamp types to
Despite its disadvantages, mercury still remained popular for residential lighting throughout the 1980s and 1990s, where its
bluish-white light was preferred by some authorities. But the high mercury content of the lamps, and their poor efficiency, meant
that it became increasingly difficult to support in more environmentally sensitive times. The mercury lamp was eventually
removed from the specifications, and its production was stopped in the early 2000s, which resulted in its accelerated demise.
By the 2010s, with massive replacement schemes and the adoption of LED, the era of mercury discharge lighting had come to an end.
Name: GEC Z8691
Date: Late 1960s - Mid 1970s
Dimensions: Length: 580mm, Width: 267mm, Depth: 203mm
Light Distibution: Semi-Cut-Off
The GEC had previously used a traditional pendant top-entry styled lantern for tungsten and mercury vapour
lamps. Such designs were fitted to swan-neck style brackets, but the austere and modern movements which followed in the
wake of the Second World War, plus the introduction of new materials and manufacturing techniques, meant that a fresh
look was taken for this lantern type.
The trend tended towards side-entry lanterns. The GEC responded by modifying their existing range of pendant
lanterns with side-entry options; but the overall combination of this type of modified pendant lantern, with sleek side-entry bracket and non-decorated
column looked ungainly. Therefore the firm designed a new lantern for tungsten and mercury vapour lamps which was smaller, sleeker
and designed for side-entry.
The Z8490 range was the first developed for the market in the 1950s. Its design parameters were brief: side-entry
lantern with die-cast aluminium canopy; newly designed glass refractor bowl and ring; horizontally mounted lamp; and the
absolute minimum of ancillary components. There was no provision for gear, so the lantern's dimensions were kept to a minimum.
The design suffered from an extruded spigot which was mounted over the top of the lantern. This gave the lanterns an unfortunate
drooping appearance when mounted on the bracket. Therefore the Z8495 improved its aesthetic appeal by moving the spigot into
the canopy and angling the lantern’s body slightly to throw more light across the road.
Unfortunately this was still not adequate as the thin spigot presented an obvious break in the flow of the lines of the whole
unit from column, bracket and into the lantern. Therefore the Z8591 range was developed which bulked out the spigot
entry, making it more of an integral part of the lantern, and offering a wide flat area onto which it would abut a bracket.
This design also changed the optical system of the lantern slightly by moving the lamp assembly from the pavement side to the road
side of the carriageway. This entry in the family also included a gear-in-head option, which extended the lines the lantern through the gear compartment
itself giving the lantern a unified feel.
The final design in the range was the Z8895 which restyled the canopy of the lantern, giving it a flatter, streamlined
appearance. But with the size of gear decreasing, and new lamp options appearing then this style of
lantern came to an end; and it was eventually superseded by the GEC Z8832 range.
It was a popular lantern, second only to the Z8895 range. It was installed throughout the country.
Identification of the lantern is relatively easy. Other manufacturers made similar designs but the GEC version
was the bulkiest with the deepest style bowl.
The primary optical system comprised of a single glass or plastic refractor bowl. In the case of glass it was specially designed
for the lantern and was held in position by a bowl ring. The plastic bowl option simply clipped in place without the requirement of a
bowl ring. The curved profile of the canopy, and its white painted interior, acted as a secondary optical system, reflecting light back which was emitted above the lamp.
The gear was mounted in an integral compartment which was part of the canopy of the lantern. Positioned behind the lamp compartment,
it was separated by a bulkhead and accessed via a separate hinged door.
The GEC Z8691 In My Collection
This lantern was originally installed somewhere in the York area. The only change, after many decades of service, was
the yellowing of the polycarbonate bowl.
The bowl was held in position by a stainless steel clip, bowl ring and simple hinge. The bowl swung backwards towards
the pavement when opened. The position of the bowl hinge was a stipulation of BS 1788:1964 and it required
bowls to swing towards the road to prevent hitting the bracket. There is little chance of that happening with this lantern
due to the side of the gear compartment.
The lantern’s bowl and lamp canopy was tilted to allow extra throw of light across the road. The gear compartment featured a deep
mounting area which abutted against the bracket – especially concrete brackets – and so continued the line of the bracket.
The canopy of the lantern was slightly rounded. This ensured a large elliptical interior which aided the secondary optical system
of the lantern.
Strangely, and unlike many GEC lanterns, the firm’s name wasn't embossed into the top of the canopy.
The polycarbonate bowl was a GEC Z6818. Its function could be determined by “reading” the refractor pattern;
horizontal prisms on the sides to elevate and control the main beams; vertical prisms on the rest of the sides to spread the
light onto the road surface and back behind the lantern for roads and front gardens.
The base of the refractor bowl had simple parallel prisms to spread the light beneath the lantern. This
increased the size of the “light spot” or “head” beneath the lantern and prevented the formation of bright
spots on the road surface.
Two small screws allowed access to the gear compartment. The bottom of the compartment swung down towards the
pavement. The two larger screws held the ballast which was mounted on the other side.
The interior of the lantern was extremely simple. The lamp holder was mounted pavement side allowing the connecting leads to simply
emerge from a hole in the side of the bulkhead from the gear compartment.
It was painted white to act as a secondary optical system so that light emitted above the lamp would be reflected back
down to the refractor bowl. It was slightly compromised by the extrusion of the top of the gear compartment which
created a large slot in the elliptical canopy.
Swinging the bottom door of the gear compartment open revealed the lantern's gear. A standard GEC
ballast was mounted to the top of the door, but it was now too rusted to read its model number. The power correction capacitor
was also by the GEC but it was too difficult to read any other information printed on it. Other components
included a terminal block and cable clamp.