thorn alpha 9 (qa9 d 1090)
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: Thorn Alpha 9 (QA9 D 1090)
Date: 1960s - 1990s
Dimensions: Length: 25½", Width: 11 7/8", Height: 9"
Light Distibution: Non Cut-Off Medium Beam (BSCP 1004:1963, BS 1788:1964)
Lamp: 140W (later 90W) SOX
The Amber lantern range was first developed by British Thompson Houston (BTH) in the late
1940s for the low-pressure sodium (LPS) discharge lamp. BTH's naming was industrial and pragmatic, and
Amber perfectly described the orange light emitted by these lanterns. Another of their straight-forwardly-named
lanterns, the Residential, was essentially an Amber lantern fitted with fluorescent tubes for
residential and side-road lighting. These fluorescent lanterns were often tilted slightly to increase the
throw of light across the road.
The fully history of the final Amber lantern has yet to be discovered, but it was a curious design, with
the lamp held in a slanted position under the canopy, with the bowl following the same sloped angle. It is
thought that this design started off life in 1961 as a new version of the Residential with in-built
angled lamps, obviating the need for an angled bracket. The designers at Associated Electrical Industries (AEI) Lamps And Lighting – a
holding company that subsumed BTH and others in the late 1950s – simply reused the design for their new
low-pressure sodium lantern. Hence the last Amber had an extremely angular and tilted appearance – unlike
lanterns from other firms.
Thorn Lighting purchased a number of shares of AEI Lamps And Lighting in the early 1960s, allowing
them to make AEI's products under their own Atlas trademark. So, in the early part of the decade, the
AEI Amber was also sold as the Atlas Alpha 9. When Thorn became a majority shareholder in the
mid 1960s, they formed British Lighting Industries (BLI) to promote and sell all their products, and
the lantern was branded the BLI Alpha 9. By the end of the 1960s, Thorn bought
AEI Lamps And Lighting outright and the lantern was kept in production.
It was still sold as the BLI Alpha 9 for a short time, before Thorn reverted back to its Atlas trademark
in the 1970s and so the lantern was distributed with Atlas Alpha 9 stickers. By the next decade, the company
dropped the Atlas brand, and the lantern's final name became the Thorn Alpha 9 in the 1980s and beyond.
The result being that this quirky lantern had several different names and brandings:
AEI Amber, Atlas Alpha 9, BLI Alpha 9, or finally, the Thorn Alpha 9.
A version with gear was also made. This included a different canopy with was fitted with the necessary mounting points
for a small gear tray on which all the components were mounted. The bulky gear necessitated an elongated bowl
to be fitted. It had the angular lines as the more streamlined counterpart but with a much deeper bowl.
Despite its quirky design, it outlived Thorn's own homegrown medium-wattage LPS lantern, the
Thorn Alpha 1, and went on to become the firm's LPS lantern for main road use. It continued on catalogue into
the 2000s until demand for such lanterns – especially old bulky 1960s designs – started to wain and the lantern
The lantern missed the major relighting schemes of the immediate post-war decades so was not initially
used in large numbers. However, by the end of the 1960s, it became the lantern of choice for casual
replacements, lighting upgrade schemes and new roads. Therefore it became an extremely popular lantern
throughout the country, although often found sporadically installed in established installations.
The lantern's distinctive sloped profile and angular bowl made it an easy lantern to identify. The canopy
was never marked with the manufacturer's name or logo (both AEI and Thorn dropped this form of lantern identification)
but a sticker inside gave the lantern's name, maker, wattage and light distribution.
The light flux was controlled by two large Perspex refractor plates which were stuck on
either side of the bowl. These produced a medium-angle beam in accordance with BSCP 1004:1963.
The interior of the canopy of the lantern was either painted a very light blue (Atlas) or white
(Thorn) to act as the secondary lighting system. No lugs or bosses were provided for the fitting of an
Two styles of canopy were produced: one without the necessary fixing points for gear and one with the gear
mounting points. Lanterns fitted with gear often had deeper bowls to accomodate the gear.
the thorn alpha 9 (qa9 d 1090) in my collection
The Thorn Alpha 9 was a popular lantern of conventional design (canopy, plastic bowl with
refractors, easy access via a clip) but of unconventional appearance (angled canopy, sharp angled bowl, lamp
positioned with lamp holder at the roadside).
I obtained this lantern a few years ago from a fellow collector. He said it had been in service and hadn't alerted it
since it was removed.
Despite its sharp profile, the lantern actually had gently sloping sides and a reflectively
large base area. The angled reflectors created the main beams for illuminating the road, whilst the gently
curved base spread the flux below the lantern, ensuring the road had a uniform luminated appearance.
After many years of service, the lantern was still in good condition, but the interior of the canopy was dirty and
discoloured. Therefore it was stripped down, cleaned and then repainted.
The canopy was a heavy casting aluminium alloy. Like all later AEI/Thorn lanterns, it
did not have the manufacturer's name cast into the canopy.
The bowl was angled by a hinge at the roadside of the lantern, being secured by a clip at the pavement end.
This ensured the bowl swung out over the road – a design choice emphasised by BS 1788:1964. (If the bowl was hinged
at the pavement side of the lantern then there was a change that it would become unclipped and smash into the column).
The bowl could be easily removed by pulling it upwards from the hinge. This helped cleaning and/or replacement.
The tensioned clip which held the bowl securely at the pavement side of the lantern, was positioned at the pavement side
of the lantern.
The base of the bowl wasn't obscured in any way – the gentle curve the bowl's base spread the flux sufficiently. The
odd positioning of the lamp-holder at the roadside of the lantern can clearly be seen here. Also the silver
identification sticker – another requirement of by BS 1788:1964 – can also be seen.
Also clearly seen is the gear. This includes a metal plate which is bolted to the canopy of the lantern onto which is
mounted (clockwise from top-left): Ignitor, leak transformer and power correction capacitor.
The interior of the canopy was painted white to act as a secondary reflector.
The canopy was the standard Alpha 9 canopy, but this did include two large lugs for extra gear provision. Onto
which is bolted a metal plate which holds all the other components.
Thorn did provide a cut-off version of the Alpha 9. But the larger gear of the 1960s and 1970s took up
a large amount of space in the canopy so the bowl was deepened and the lamp and lampholder moved downwards. This resulted
in the 'deep bowl' Alpha 9.
This lantern used smaller gear - a combination of new technology and the introduction of the ignitor. Due to its
reduced size, this could be fitted into the canopy of the standard Alpha 9 without requiring the deeper bowl.
Hence the mystery surrounding this lantern. Was it a legitimate Alpha 9 option, produced by Thorn? Or was
it a local council bodge, converting previously gearless lanterns into geared versions?
BS 1788:1964 required every lantern to have some form of identification. Many manufacturers used internal
stickers. The label included the following information:
Lantern's name: Thorn Alpha 9
Optical system: Semi Cut Off Type
Lamp: 90W SOX
Ingress level: IP23