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: Thorn Gamma 6
Date: Late 1960s - Early 2000s
Dimensions: Diameter: 10½", Height: 19¾", Spigot: 3" dia.
Light Distibution: Symmetrical
Lamp: 100-200W GLS, 160W MBTF, 80-125W MBF/U, 35W SOX

facing profile The lantern was a swap with Peter so I believe it was probably installed in Surrey.

canopy This version of the lantern includes the smaller canopy which simply forms a deep lip over the top of the lantern. The aluminium spun hood is painted black and held in place by one knurled plastic screw. This is designed to also accommodate a photocell.

logo There's no manufacturer's mark or logo anywhere on the exterior of the lantern. The paint is scratched and battered after years of service.

pedestrian view The one-piece diffuser bowl is complete and in good condition. It features internal vertical ribbing to also further spread and scatter the light. The bowl doesn't show any signs of becoming brittle or discolouring. The spigot features the smaller base which was originally designed to accomodate gear for a high-pressure mercury lamp (MB, MBF).

interior #1 The body of the lantern is supported by two vertical ribs which support the top assembly onto which the canopy sits. The photocell NEMA adapter is an integral part of the top assembly, onto which the canopy sits, and is secured by the large knurled screw. It's clear how the lantern was designed around the dimensions of the 35W SOX lamp. A hooked tie around the mid-section of the lantern acts as a lamp steady.

interior #2 The gear for later low-pressure sodium lamps could fit in the smaller space originally designed for high-pressure mercury lamps and this is the configuration used here. The base of the lantern also includes the cable entry, cable clamp and terminal block.

interior #3 The lantern's had a long life in service as shown by the rusting and discolouration of the base. The gear includes a Thorn G53332.4 Transformer, a Thorn G53311 Ignitor and a LE2EC capacitor. The capacitor is marked 9/93 so the lantern was probably manufacturered in late 1993 or early 1994.

This is probably the most popular version of the Gamma6, with its small canopy rather than the wide brimmed version.

This lantern is fitted with a 35W SOX. And was obtained as a swap with Peter.