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Thorn Beta 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 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 Beta 5
Date: Mid 1950s - Mid 1960s
Dimensions: Length: TBA", Width: TBA", Height: 7 ¾"
Light Distibution: Semi Cut-Off (BSCP 1004 Part Two:1956)
Lantern Specification: BS 1788:1964
Lamp: 35W SOX (originally 40W SOX / 60W SOI)
History
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It isn’t known which company originally designed this lantern although evidence suggests it was Associated Electrical Industries (AEI)
in the late 1950s. Like many luminaries produced by the company, the Amber Minor (as it was originally called)
became embroiled in the merges, shared companies and takeovers which defined the electrical industry in the 1960s, eventually
emerging as the Beta Five after a successful purchase by Thorn.
During this tumultuous decade the same lantern could be purchased from AEI as the Amber Minor,
from Atlas as the Beta Five, from British Lighting Industries as the
Beta Five (a company formed between AEI and Thorn with Thorn holding
the majority share) and finally as the Thorn Beta Five (when Thorn eventually
purchased the entire company). Identifying labels stuck inside the lantern during this period (and for years afterwards) reflected
this uncertainty by carrying two distinct names: "Amber Minor" and "Beta Five."
After this period, the lanterns schizoid identity started to rub off on its appearance. By the 1970s and throughout the next decades,
various modifications changed its design to such an extent that it was almost unrecognisable from its original guise. Therefore,
collectors were forced to quote the lantern’s "Mark Number" to differentiate between the different versions.
The energy crisis of the 1970s saw the Thorn Beta Five as a natural contender for Group-B replacement
schemes. It was taken up in huge numbers and is arguably the most common low pressure sodium lantern installed throughout
the UK. Its demise was only caused by a change in the specifications which saw low pressure sodium discouraged as a light
source for street lighting in the late 2000s.
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Popularity
This was the second known version of the Amber Minor/Beta Five, produced during the dual
ownership days of AEI and Thorn. The first version included opal end-caps on the bowl; a feature
this lantern is lacking.
It wasn’t installed in huge numbers. Lighting engineers still preferred to install more natural light sources (such as tungsten,
fluorescent and high-pressure mercury) for Group-B schemes, despite the inefficiency of such light sources.
By the time the Thorn Beta Five was being installed in huge number during the energy
crisis (and installing all those inefficient group-B schemes), the lantern had been largely redesigned.
Therefore this version remained comparatively rare.
Identification
The lantern has a distinctive profile which aids identification. However, the bowl, secured by a separate bowl
ring and with its broad, course refractor grooves is a unique feature. (Note that vertical refractor grooves on the bowl-ends
was also a characteristic of early Phosware lanterns; and this is the only known case where another manufacturer
has adopted the same refractor pattern).
Optical System
The primary optical system comprised of two plate refractors positioned either side of the bulb. As the low
pressure sodium lantern already casts a wide beam in azimuth, the horizontal refractors simply alter the flux
elevation by fashioning two main beams in a semi-cut-off distribution (in accordance with BS 1788:1964).
The underside of the gear tray is painted white and acts as a secondary reflector.
The vertical refractors at the end of the lantern are designed to spread the flux in azimuth.
Gear
This lantern doesn’t have gear. It isn’t known if the gear was removed whilst in service or if the lantern
was originally purchased without gear. Interestingly AEI offered this option whilst Thorn didn’t.
The Thorn Beta 5 In My Collection
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facing profile
This model of the Thorn Beta Five (or AEI Amber Minor)
was installed throughout the 1960s. It wasn’t the most popular lantern as lighting engineers preferred a more natural
light for Group-B installations during that decade.
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front profile
This shot clearly shows the vertical refractor grooves at the end of the lantern. These were designed to spread
the flux in azimuth; but the grooves also continue along the base of the bowl, which allowed the light to be
spread further beneath the lantern. This was a consequence of BS 1788:1964 which was strict on the amount of
light which could be emitted from 0° to 30° from the vertical.
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trailing profile
The lantern was in good condition despite its years in service. A P42 two-part photocell had been added at
some point, but that was the only alteration. The Diakon bowl remained clean and translucent.
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canopy
The same style canopy was used by the subsequent, more popular, model of the Thorn Beta Five.
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logo
The lantern's canopy didn’t display the maker's name. In fact, the manufacturer's name wasn’t featured anywhere on the lantern
(except a logo on a sticker). This was due to the lantern being sold by both AEI and Thorn
at the time; it allowed both to use the same canopy. (It was also a trait of both companies, as neither
Thorn nor AEI marked the exterior of their lanterns with their company names).
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pedestrian view
This shot shows the unique refractor bowl used by this lantern. It was held in place by a separate bowl ring; subsequent versions of the lantern got rid of this feature.
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vertical
The bowl was held in place by a single stainless steel toggle clip at the road-end of the lantern.
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interior #1
This shot clearly shows the secondary optical system of the reflective base of the gear tray. The bowl ring can also be
clearly seen in this shot.
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interior #2
The interior of the lantern reveals two interesting design features: the lantern is locked to the bracket spigot
by normal bolts instead of grub screws; and the gear tray fastening screw engages with a curved piece of metal
which also held the power correction capacitor in place. Unfortunately this lantern doesn’t have any gear.
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interior #3
This pre-restoration shot shows the frayed, original sticker which was the only identification within the lantern.
The Thorn logo to the left has disappeared with only a rust patch showing where it once was. All that’s
left is the lantern’s two names: Beta Five and Amber Minor.
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Thorn Beta 5: Night Burning
Thorn Beta 5: As Aquired
I obtained this lantern from fellow collector Phil. It was installed somewhere in Staffordshire.
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