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 Z9484
Date: Early 1960s - Early 1980s
Dimensions: Length: 23½", Width: 5¾", Height: 5¾"
Light Distibution: Semi Cut-Off (BSCP 1004:1963)
Lamp: 35-55W SOX

GEC 9494/5 catalogue

The GEC Z9494 family of lanterns was introduced in the early 1960s. It was part of a range of new lanterns which were designed to utilise the new SLI low pressure sodium bulb and conform to the new British Standard Code of Practice for street lighting: BS CP 1004:1963. The lantern’s sleek lines were also made possible by the decision to forgo a gear-in-head option which had made previous designs so bulky and boxy. The design was effectively a thin elongated extruded oval with the line of the perspex bowl matching the curve of the aluminium alloy canopy. A singular clip was provided at the spigot end of the lantern and the bowl carried the rubber gasket. The optical system was designed for the tubular 60W SLI/H bulb and comprised of two Perspex refractor plates glued to the Perspex refractor bowl to give a semi-cut off light distribution (in accordance with CP 1004:1963). As the popularity of the SLI/H bulb started to decline, the GEC offered a version of the lantern for SOX lamps (the Z9484). This simply required the removal of the two bi-pin connectors and the addition of one porcelain lamp holder and a lamp steady. The design of the bowl also changed and a polycarbonate version was offered with the refractor plates machined or moulded into sides. The lantern was discontinued in the early 1980s. The GEC already had an extremely popular low pressure sodium gear-in-head range of lanterns (the Z9530 range) and the Z9494 range was probably considered an expensive, historical and redundant luxury.

The two bowl options (Perspex with glued refractor plates and Polycarbonate with moulded refractor plates) seem to be equally common.

The interior of the canopy is painted white. This secondary optical system reflects any flux emitted above the horizontal back toward the ground.

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

This shot shows the lantern after it was removed from its bracket - the grease around the grub screws can still be seen. I'd also removed the P42 two-part photocell in this shot.