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GEC Z8611

Genre: Enclosed Horizontal Traverse High Pressure Sodium Lantern

As soon as the low pressure sodium lamp (LPS) was developed, the race was on to develop a version which would run at high pressure (HPS). The research was stimulated by the prospect of the spectral broadening of the light emitted from such a bulb and the improved colour rendering which would result. However the development was hampered by the lack of a translucent material which could withstand the chemical attack of sodium vapour at extremely high temperatures and pressures.

It wasnít until the early 1960s that General Electric (GE) cracked the problems, closely followed by the GEC and Philips. A prototype high pressure sodium lamp was exhibited at the APLE's annual conference in 1963, but it wasnít until 1966 that the GEC erected an experimental installation along East Lane, Wembley (which, incidentally, also saw the first experimental medium-pressure mercury installation over thirty years before).

The first commercial installation in the UK was erected along the Southend ring road, but it was the City Of London who gained the most recognition by beginning a radial upgrading of all their lighting to high pressure sodium in 1967. As the early HPS bulbs were designed to be retrofitted into existing mercury installations, the lanterns chosen by the City Of London were slightly modified versions of existing mercury lanterns.

The first true HPS lanterns (designed from scratch with the new light source in mind) appeared in the late 1960s and early 1970s. All these lanterns utilised a cross-over design: whilst this was an adaption of existing mercury lanterns, it also moved the lamp up into the lanternís canopy (where it could not be directly viewed by drivers, thus minimalising the glare) and allowed designs to cater for both cut-off and semi-cut-off.

Wider adoption of high pressure sodium was stalled by the energy crisis, as it couldnít match the efficiency of its low pressure sodium brother. Councils, believing in the simple maximum lumens per watt paradigm, replaced existing tungsten, mercury and fluorescent schemes by low pressure sodium. So it wasnít until the 1990s that HPS started appearing in greater quantities.

With its efficiency between high pressure mercury and low pressure sodium, HPS became even more attractive when LPS was finally exorcised from the British Standards of street lighting (as it didn't provide an adequate colour rendering). However, it didn't remain as first choice for new and replacement lighting schemes for long. Renewed interest in fluorescent (from compact fluorescent sources), the emergence of affordable metal halide and the possible introduction of LED have all questioned the automatic selection of high pressure sodium for schemes.

Therefore by the end of the first decade of the new century, high pressure sodiumís position as the natural choice for street lighting was coming under pressure.

Name: GEC Z8611
Date: Circa Mid 1970s -
Dimensions: Length: 75cm, Width: 32cm, Height: 26cm
Light Distibution: Conforms with BS 5489-1 / EN 13201.
Lamp: 150-250W MBF/U, 250W SON.


The GEC Z8600 range of lanterns were the first main-road lanterns produced by the GEC specifically designed for the newly introduced High Pressure Sodium (HPS) lamp. Various existing lanterns originally designed for high pressure mercury lamps had been adapted for HPS, but the Z8600 range was the first to take full advantage of the new lamp.

It shared many design traits with its competitors (such as the Thorn Alpha 8). The lamp and gear were housed in two separate compartments, the optics were based on a cross-over reflection system, and the canopy was made from moulded GRP.

It could also take high pressure mercury (MBF/U) therefore allowing the lantern to be installed within an existing scheme.

Oddly, however, the lantern could only take up to lamp sizes of 250W. And it was also one of the last lanterns designed by the GEC.

By the end of the 1980s, the street lighting section was moved to the companyís OSRAM division before being sold to Siemens and subsequently purchased by Whitecroft (now Indal WRTL). Each company manufactured the lantern with little change: so examples included the GEC 8600 family, the OSRAM Z8600 family, the Siemens Z8600 family and finally the WRTL MRL6.


It was a popular lantern and could be found installed throughout the country. Some examples do look rather the worst for wear now: the GRP canopy attracts lichens and moss and some of the bowls tend to yellow. (In some cases the bowls have cracked around the refractor).


The curved body of the GEC Z8600 series make them easy to identify. It's a large lantern with an equally large bowl and its profile is unique. Cut-off versions have flat-glass bowls but, again, the profile of the canopy makes identification easy.

Optical System

Two curved polished textured metal reflectors comprise the primary optical system which produce a cross-over flux distribution (which, in turn, gives better control of the degree of cut-off of the light produced).

The secondary optical system comprises a white plastic over-reflector mounted above the lamp which reflects light emitted above the lamp back towards the street surface.


The gear is housed in its own gear compartment. This has a lower IP rating than the lamp compartment and access is achieved by unscrewing a single screw.

When opened, the cover plate swings open and all the gear is mounted on the base of the lantern near the lamp holder wall. Three components are usually screwed in position: the main lamp choke, a power correction capacitor and the ignitor.

The GEC Z8611 In My Collection

facing profile

I can't recall the provenance of this lantern. I suspect it was never in service as it's in such good condition for its age.

front profile

The bowl is secured by a stainless steel clip mounted at the front of the lantern. Given the smaller area of the bowl (as compared with the earlier larger HMPV and SON lanterns), the clip and gasket are able to seal the lamp compartment to IP 54.

trailing profile

The profile of the lantern is easily identifiable thanks to the sweeping curves of the GRP moulded body and the smooth curve of the bowl itself. The bowl has not yellowed which is a problem with lanterns which have seen a long service. (In some cases, the bowl also cracks around the refractor).


The lines and style of the lantern also matches the GEC Z8832 which was designed for smaller wattage lamps.


No logos were cast into the canopy, but the GEC tended not to do this with their GRP lanterns. Note the photocell which had to be completed covered in black insulating tape in order to switch on the lantern.

pedestrian view

The lantern was designed for the 150-250W SON lamp and not the original, larger MBF/U lamp. This allowed the GEC designers to decrease the lanternís size and create a smaller lamp compartment which could be sealed to a higher IP rating. Two curved reflectors either side of lamp formed a cross-over distribution and created the main beams.


A white plastic over-reflector in the lamp compartment also reflected flux emitted above the lamp back down onto the road surface. An oval-shaped spreader refractor further directed this flux Ė and flux emitted directly below the lamp Ė up and down the road.

interior #1

The entire optical system within the lamp compartment can be seen with the bowl open. Given the short length of the lamp as compared with the lamp compartment, then I suspect flux was also directed towards the end of the curved mirrors before being reflected onto the street surface. This avoided the cross-over distribution crossing through the lamp and being further refracted or absorbed.

interior #2

The gear compartment was separate, sealed to IP 44, and opened by undoing a Philips screw. The lanternís identification sticker can be found on the inside of the lid and it states the catalogue number and IP ratings of the two compartments. The gear appears to be all original: a GEC Z1607P choke for 250W MH/SON, a Parmar PB404 ignitor (for 250-400W SON) and a Cambridge Capacitors PFC 30S rated at 30uF and stamped with December 1989.

GEC Z8611 As Aquired

An extremely popular lantern, the GEC Z8601 outlived its parent company, being made by both Siemens and WRTL. This is the original GEC version fitted with its common bowl option.