mailing list
site map

Thorn Alpha 5

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 lampís 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 didnít 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 lampís 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 driversí 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 wasnít 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 lampís 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 UKís streets were being lit by its characteristic orange glow.

Name: AEI Amberline / Thorn Alpha 5
Date: Late 1950s - Mid 1980s
Dimensions: Length: 41.25", Width: 10.25", Height: 7.5"
Light Distibution: Semi Cut-Off DLOR 75% (BS 1788:1964)
Lamp: 140W-200W SLI/H


By the end of the 1950s, the country's growing road network was putting new pressures on the public lighting engineer. The new dual-carriageways, bypasses and motorways required lighting but existing lamp technologies, and the (now) restrictive specifications were effectively preventing their economical illumination. Therefore the development of the 200W SLI lamp at the end of the 1950s with its wide beam and hitherto unprecedented efficacy (over 100 lm/W) was seen as a timely saviour.

Manufacturing firms quickly responded with a range of new slim-line lanterns to exploit the thin, linear lamp. The lighting engineers at AEI created the Amberline, a long, sleek lantern which utilised a reflector-refractor optical system. The lantern was adopted by Thorn when the two firms pooled their lighting products and the lantern was rebranded the Alpha 5 (replacing an existing design from Thorn which was quietly retired).

The lantern was more efficient than its competitors thanks to the provision of the over-reflector. The design was simple and rugged which allowed it considerable longevity; it outlived the lifespan of the expensive SLI lamp and was readily converted to SOX, therefore remaining on catalogue as the high-wattage low-pressure sodium lantern within Thorn's range.

Later models had much smaller over-reflectors, being reduced to a small section within the centre of the canopy. Such a price-saving modification was probably prompted by a change in the specifications, probably the introduction of BS 4533 which was less stringent than its predecessor.

Yet it was oversized and somewhat bulky by the 1980s and in a streamlining and efficiency operation, Thorn replaced the semi-cut-off Alpha 5 (and its cut-off counterpart the Alpha 6) by the newly designed Alpha 4 in the 1980s.


The lantern was fairly popular and found widespread use on motorway slip roads, motorway intersections, dual carriageways and even traffic routes within towns and cities. The obsolescence of the SLI lamp in the 1980s saw many retired and removed; whilst others were converted to SOX using a variety of ad-hoc measures. But by this point the lantern was in decline and numbers gradually reduced during the 1990s and 2000s. Hardly any now survive.


The lantern is easily identified due to its unique profile, sleek linear lines and the use of double refractor plates along the bowl. An internal sticker provided lantern information but this is usually missing.

Optical System

The lantern was unusual, and fairly unique within the SLI lantern range, due to its use of a seagull reflector. This concentrated flux emitted above the lamp into two rudimentary main beams before being further refined by the refractor plates. This gave the lantern an advantage over the other crude refractor only lanterns being produced by competitors (such as the GEC Z9504 and the REVO Hyperion).

Unfortunately this also presented problems when the larger diameter SOX lamp was introduced in the 1960s. The designers had to move the lamp further down for the overhead reflector to work correctly (as its focal distance wouldíve changed); this required the provision of a deeper bowl. Therefore the lantern had two distinct bowl and lamp mounting options for different lamp types. (Note that the SOX optical system reflected the flux into two beams above the refractor plates as the sectional diagram shows.)

This system also saved the lantern from modification in 1964 when the stricter BS 1788 was introduced and severely curtailed the amount of light which could be emitted directly below the lantern. Other manufacturers responded with "V" grooved bases in the bowls, or even adopted a low-tech solution with white paint, but as upwardly emitted light was directed towards the main beams instead of straight down, then the Thorn Alpha 5 escaped these changes.


The lantern was never equipped with gear.

The Thorn Alpha 5 In My Collection

facing profile

This Thorn Alpha 5 was one of around eight which were installed at the start of Newmarket Road, Cambridge. This was probably intended as a trial to evaluate the lantern's performance. However, given the evidence of other installations around Cambridge, then the lighting engineer probably wasn't impressed, and used the REVO Hyperion instead.

The lantern was removed in 2012 as part of the PFI replacement scheme. My thanks to Cambridge County Council and Balfour Beatty for saving it for me.

front profile

It was originally an SLI lantern but was crudely converted to SOX whilst in service. This can be seen by the poor way the SOX lamp is supported; originally the SLI/H lamp would've been supported by bi-clip connectors at either end of the lantern.

trailing profile

The trailing profile clearly shows how two, separate refractor plates were stuck onto either ides of the bowl. The sleek profile was also designed to minimise construction costs; there was no provision for the big, bulky gear of the period.


The bowl was held in place by two spring clips which hooked over the lip of bowl. This was a standard AEI design which Thorn continued to use.


There were no identification marks or embellishments on the canopy of the lantern. This was standard practice for both AEI and Thorn who included stickers within the lantern's canopy instead. (Unfortunately, in this case, the sticker had long gone.)

pedestrian view

It's clear that the lantern was originally designed for the longer SLI lamp given the empty space at either end of the lantern. Luckily the SOX lamp was long enough to utilise the full length of the refractor plates but little else.

across street view

The bowl had a relatively wide base which allowed a small amount of the flux to illuminate the area directly below the lantern. The overhead seagull reflector also directed flux emitted above the lamp into the two main beams rather than reflect it back down through the lantern, so the Alpha 5 didn't require any modifications (such as "V" shaped bases or the application of white paint) when the overly stringent BS 1788:1964 was introduced.


This view again shows how the SOX lamp was just the right length to utilise the entire optical system Ė in this case, the overhead seagull reflector.

open bowl #1

The entire bowl could be easily and quickly removed. This was to facilitate a bowl changing regime when cleaning and relamping i.e. dirty bowls could be removed and thoroughly cleaned at the depot rather than in-situ.

open bowl #2

The conversion to SOX was extremely haphazard. The bi-pin connectors were removed and a single porcelain BC lamp-holder was attached to the spigot end of the lantern. The SOX lamp was supported by a single piece of wire which was looped around the lamp and then threaded through a hole in the over-reflector before being folded into the edge of the over-reflector itself. This was a poor bodge; the lamp hung crazily at a drunken angle, but it extended the life of the lantern.

open bowl #3

The wiring provisions supplied within the lantern was also very basic. One porcelain terminal block was provided along with an earth connection and cable grip. At some point in its lifetime, a photocell was added and the additional porcelain terminal block added to support the additional connections.

Thorn Alpha 5: As Aquired

This lantern was originally installed in Newmarket Road, Cambridge when the road was widened as part of a road scheme in the late 1960s/early 1970s. The lighting engineer used a mixture of Relite Hyperion B and Thorn Alpha 5 lanterns.