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gec Z9532m (nightwatch 35)

Genre: Enclosed Horizontal Traverse Low Pressure Sodium Lantern

The low pressure sodium discharge lamp was developed by Philips in 1932. After two successful trial installations (including the first low pressure sodium installation in the UK along the Purley Way, Croydon) the first commercial installation was installed by Liverpool Council in 1933 using specially commissioned lanterns from Wardle.

The development of lanterns continued through the 1930s and accelerated when it was determined that the lamp’s brightness and its long length made it less susceptible to glare. Lanterns with bare bulbs suspended over an overhead reflector (the so-called "seagull" lanterns) quickly followed. Glass manufacturers were initially slow as the first plate refractors for low pressure sodium lamps didn’t appear until the end of the decade.

The advantages and disadvantages of low pressure sodium were readily debated, especially when an alternative (the medium and high pressure mercury discharge lamp) was also available. The monochromatic light was considered especially useful for arterial and traffic routes, the lamp’s shape cast a wide beam across the road surface, the light was also considered more penetrating in foggy conditions and it was the most efficient light source being manufactured. However, the light was also considered inappropriate for high streets, promenades, civic areas and residential streets and so some lighting engineers restricted its use to traffic routes only. Therefore low pressure sodium became known as "the drivers’ lamp."

The arrival of plate glass refractors resulted in large lanterns made of metal frames enclosing heavy glass sheets. These bulky lanterns continued to be made into the 1950s until being usurped by lanterns with plastic bowls and machined or moulded plastic refractor plates. The lanterns were still large; the size dictated by the bulky control gear, but their design and construction was becoming simpler.

The 1950s and 1960s saw huge improvements in the construction and efficacy of low pressure sodium. Early two-piece designs (dubbed SO) were replaced by the one-piece, more efficient integral design (called the SOI). The development of linear sodium (SLI) broke the one hundred lumens per watt barrier, lead to a radical rewriting of the British Standards of street lighting and prompted the development of new families of streamlined lanterns. But it wasn’t until the arrival of a new heat-reflecting technology (called SOX) that a cheap family of extremely efficient bulbs became available.

The energy crisis of the 1970s saw a rethink in street lighting and lamp efficiency became dominant when fuel was both in short supply and expensive. This saw the large scale removal of colour corrected high pressure mercury, fluorescent and ancient tungsten lamps by low pressure sodium replacements. The old arguments that the smoky-orange lamps were inappropriate for residential areas no longer applied. By the end of the 1980s, low pressure sodium was the dominant street lighting lamp used in the UK.

The use of low pressure sodium came under scrutiny again. High pressure sodium, finally developed as a viable technology in the 1960s, was coming of age and offered a compromise of slightly less efficacy with better colour rendering. Questions were being asked about the physiology of the eye and visual adaptation under low lighting levels; previously the wavelength of low pressure sodium had been deemed the most suitable, but research now suggested that the eye responded better to white light. Concerns were raised about light pollution and the low pressure sodium lamp was seen to be the chief culprit (although it was more to do with older non-cutoff and semi-cutoff optical designs rather than the lamp itself).

By the turn of the century, the age of low pressure sodium was seen as coming to an end. Research in white light technologies, especially metal halide and a renewed interest in compact fluorescent coupled with the advantages of using white light at low lighting levels, saw the end of the low pressure sodium lamp’s dominance. Its use was discouraged in the specifications, lantern manufacturers started to wind down their production and bulb manufacturers followed suit.

By the end of the first decade of the 2000s, low pressure sodium was in stark decline, and less and less of the UK’s streets were being lit by its characteristic orange glow.

Name: GEC Z9532M (Nightwatch 35)
Date: Late 1960s - Late 1980s
Dimensions: Length: 440mm, Width: 190mm, Height: 182mm
Light Distibution: Semi Cut-Off (BS 4533:1976)
Lamp: 35W SOX


The GEC Z9530 family of lanterns was introduced in the late 1960s. It was a complete redesign of the firm's low-pressure sodium (LPS) lighting for side roads and was intended for side road lighting, industrial roadways and/or general security lighting.

It replaced the earlier Z9480 range, which dated back to the early 1950s and was now showing its age. The boxy squat profile, designed around the huge leak transformers of the time, was over-sized and over-engineered, and could only accept the lowest wattage LPS lamps.

Therefore the Z9530 family was introduced as its replacement. Two main different sizes of lantern were developed: a compact version which could only accept the 35W SOX lamp, and a longer version which was designed for the 55W SOX lamp. The overall profile of the lantern was also made smaller, now developed around the smaller gear options now available.

The lanterns initially conformed to the new British Standard Code of Practice for street lighting: BS CP 1004:1963.

During the 1970s, new versions of the lantern appeared with Glass Reinforced Plastic (GRP) canopies. These lightweight options were only available as a side entry option.

It remained a mainstay in the firm's portfolio, and its last lantern designed for both the smaller SOX lamps. When the lighting department was sold to Siemens, and then to Whitecroft, the lantern remained on catalogue, and could still be purchased into the 2000s.


This family of lanterns installed in huge numbers and vied with the Thorn Beta 5 series and Philips MI 26/36 as the most popular side-road lanterns in the UK.


The lantern was easily identified by the profile of its canopy, its narrow 'V' shaped bowl, its distinctive small refractor panels, and the optional white over-reflector which could be seen through the bowl.

The plastic bowls were often made of polycarbonate which was non UV-stabilized and so turned yellow over time.

Optical System

The primary optical system comprised of two plate refractors positioned either side of the lamp. As the low-pressure sodium lantern already casts a wide beam in azimuth, the horizontal refractors simply altered the flux elevation by fashioning two main beams in a semi-cut-off distribution (in accordance with BS CP 1004:1963 and BS 4533:1976).

An optional over-reflector was available, used to cover the gear and provide a secondary optical system.

The exterior of the bowl was smooth to facilitate easy cleaning.


Optional gear was available and was mounted into the canopy of the lantern. This could be hidden by a stainless steel over-reflector which was painted white.

the gec Z9532m (nightwatch 35) in my collection

facing profile

This lantern was rescued from the former cold-storage depot in Girton, Cambridgeshire in the late 1980s. It was mounted on the side of the side of the depot where it illuminated a loading bay.

front profile

The lantern was in good condition and only required minimal cleaning.

Unlike many examples of this range, the bowl had not started to crack due to corrosion of the fixings through the bowl.

trailing profile

The lantern had an extremely recognisable side profile with the refractor extending the entire length of the bowl and the over-reflector (which protected the integral gear from the lamp's heat) only covering about 75% of its length.


The Glass Reinforced Plastic (GRP) canopy was in good condition and was not suffering from discolouration and lichen growth. The lantern's later provenance was shown by the “Disconnect from mains before servicing” warning cast into the canopy.


Like many GEC lanterns, it also has the firm's logo cast into the canopy. An additional identification sticker could be found inside.

pedestrian view

The bowl had a narrow 'V' shape, a profile used by the GEC for many years for their main road lanterns. The refractors were moulded into the angled sides, forming the two main beams. The base, being relatively narrow, required no frosting or obscuring as the shape of the bowl helped to diffuse the light below the lantern.


The base of the bowl was clear. This allowed the light from the lamp to directly illuminate the area directly below the lantern. The short over-reflector can also be clearly seen in this shot.

interior #1

The gear was exposed once the bowl and over-reflector is removed. The lantern was fitted with an older-style leak transformer (a GEC Z1614P) with date stamp of 1983. The 8.4uF power correction capacitor was dated 1986. Therefore this lantern was produced near the end of the GEC's run.

The sticker identified the lantern as a GEC Nightwatch 35 (F64335). This was a rebadged GEC Z9532M but intended for the commercial/industrial sectors and not the street lighting sector. GEC offered this lantern as a complete unit (gear-in-head with integral photocell and optional mounting bracket) for the lighting of industrial areas, warehouses, commercial premises etc. This made sense as it was obtained from an old factory site.

As it was the third lantern in my collection, and I didn't have much supporting literature at the time, then it was assumed this range of lanterns were called the "Nightwatch". This name has since stuck amongst collectors and you often hear references to the "Nightwatch" range.