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: AEI Amber IV (51/74007)
Date: Late 1950s - Mid 1960s
Dimensions: Length: 28", Width: 14", Height: 10"
Light Distibution: Non Cut-Off Medium Beam (BSCP 1004 Part One:1952)
Lamp: 140W (later 90W) SOX

The Amber was the name given to the family of BTH's low-pressure sodium lanterns. BTH gave their lantern’s purely functional names (a trait which continued to successor companies AEI and Atlas), so the name continued for over a decade as the lantern evolved. The range followed the classic design of post-war lantern design: starting with a relatively sleek lantern with an aluminium canopy and Perspex bowl with refractor plates, evolving into much larger lantern designed to carry the heavy gear of the time. The Amber MK III and MK IV became classic designs, installed in large numbers across the country in the second half of the 1950s. The MK IV became especially successful, being accepted by the Council of Industrial Design for Design Review, and successfully retained the lantern's handsome profile and increased its ease of use (by replacing the large knurled screw which held the bowl with far more user-friendly toggle catches). The lantern was retained in the range during the "great reorganisation" of BTH with its sister companies, as AEI consolidated and rebadged its disparate companies. It was eventually discontinued in the early 1960s, replaced by another lantern called Amber, which radically altered all aspects of the design, and removed any family resemblance (this lantern eventually becoming known as the Alpha 9).

All lugs and bosses for interior mounting of interior secondary lighting system - such as a white over-reflector - were removed. Likewise there was no provision for gear either.

facing profile The AEI Amber MK IV was a popular lantern and was installed in many towns and cities throughout the UK. They were installed around the south-eastern parts of Cambridge on conventional Stewarts And Lloyds columns and brackets. I believe the relighting of Cambridge took place in the mid-1950s. Two BTH Amber MK III designs were probably trialled (these ended up in two installations) and then BTH Amber MK IV lanterns were used for the main schemes in the centre, north, south and west of the City. As the council continued the relighting scheme into the late 1950s, BTH effectively disappeared, and so the council's lighting engineer used REVO Silvergolds on various streets. The AEI rep must've visited Cambridge, and explained that the Amber was still available - albeit in slightly modified form - and so the rest of the City, including the south-east areas, were lit with the AEI Amber IV.

front profile The bowl was large and deep to accommodate the large leak transformers and condensers of the 1950s. The base and ends of the bowl were slightly textured to facilitate extra diffusion of the light. Interestingly the bowl had the same curved base of the BTH Amber MK III rather than the flatter bowl of the BTH Amber MK IV.

trailing profile Despite many years of service, the lantern was still in good condition. The bowl and refractor plates were made of Perspex and had slowly become milky due to the action of ultra-violet light. The Perspex of the bowl was especially thick and this probably explained why the bowl had no damage after being installed for 50 years.

canopy The canopy was a heavy casting of aluminium alloy. At some point, the canopy had been drilled for a new photocell, but this was the only alteration.

logo All logos and identifying marks had been removed from the surface of the canopy.

pedestrian view The two large brass toggle catches engaged with two moulded Perspex blocks which were stuck onto the bowl separately. This was another modification of the previous BTH Amber MK III design which used a metal bowl ring to hold the bowl.

vertical The bottom of the bowl was unadorned allowing the flux from the lamp to illuminate the area around the base of the column.

interior #1 The interior of the lantern was incredibly simple and consisted of a lampholder assembly, lamp steady, asbestos wiring, terminal block, cable clamp and earthing point. All lugs and screw holes for optional gear and/or overreflector had been removed. All logos and identifying marks had also been removed. All that remained was "MADE IN ENGLAND" which was cast into the street-end of the lantern. The original neoprene gasket had mostly perished and was replaced with a new one.

This lantern was originally installed along the High Street, Cherry Hinton, Cambridge. It was rescued when the road was relit in 2016 as part of the Cambridgeshire-Northamptonshire PFI.

The lantern can be seen on the right.