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Thorn Beta 2
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 2
Date: Mid 1980s - Mid 2000s
Dimensions: Length: TBA", Width: TBA", Height: TBA"
Light Distibution: Semi Cut-Off (BS 4533:1976)
Lantern Specification: BS 1788:1964
Lamp: 35-55W SOX
History
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The Beta 2 was designed by Thorn as a companion lantern for the successful Beta 5. The
Beta 5 was limited to 35W SOX; the Beta 2 was an elongated version of the basic design of
the Beta 5 and could take up to 55W SOX.
It took a while to appear as Thorn already produced the 55W SOX Beta 8 for Group-B installations. However,
it was probably one of the redesigns of the Beta 5 that prompted Thorn to
produce a scaled version of the Beta 5 for larger wattage lamps rather than redesign the Beta 8.
(It was named the Beta 2 as there was a gap in Thorn's catalogue numbering. The original Beta 2
was a long-obsoleted fluorescent lantern).
The Beta 2 appeared in the 1980s, and followed the various later design changes made to the Beta 5.
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
The Thorn Beta 5 wasn't never as popular as it's baby brother as most lighting engineers preferred to standardise
on 35W installations rather than 55W. Therefore it only saw fairly limited use.
Identification
The lantern is easily to identify as it has the elongated lines of the classic, and very recognisable, Thorn Beta 5.
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.
Gear
This lantern was available as a geared option.
The Thorn Beta 2 In My Collection
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facing profile
This was the last version of the Thorn Beta 2. The canopy is fairly slim when compared to the
depth of the bowl, and the bowl is secured by one, large black clip on the street side of the lantern.
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front profile
This shot clearly shows the large black plastic clip which secures the bowl. This was standard for the last models of the
Beta 2 and Beta 5.
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trailing profile
The bowl is in good condition which isn't surprising given how new this lantern is.
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canopy
The canopy for this version of the Beta 2 (and the same version of the Beta 5) is flatter than the
canopies used for previous models.
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logo
The lantern's canopy doesn't display the maker's name. In fact, the manufacturer's name isn’t featured anywhere on the lantern
except for a sticker on the gear tray.
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pedestrian view
This shot clearly shows the main refractors moudled into the bowl. The side refractors fashion the main beam whilst the refractors
in the base spread the light below the lantern (this prevents bright spots forming under the lantern).
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vertical
The refractors in the base clearly extend the length of a 55W SOX bulb. The lamp steady is positioned so the lantern can
also support the smaller 35W SOX bulb.
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interior #1
This shot clearly shows the secondary optical system of the reflective base of the gear tray. The lantern's identifying sticker can
also be seen.
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interior #2
The interior of the canopy has a simple, functional design. Grub screws, cable clamp, terminal strip (with terminal marking), photocell
and earthing screw are all screwed into the canopy.
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interior #3
And like many lanterns of this period, the gear is mounted on the top of the gear tray (the underside of which doubles up as secondary
reflector). A Thorn G5333.2 choke acts as the ballast for either a 35-55W SOX lamp,
the power correction capacitor is a Cambridge Capacitors GC2435/S (rated at
6uF and dated September 1993), and the ignitor is a Philips SX 72 for 35-55W SOX.
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Thorn Beta 2: As Aquired
I never kept a record for this lantern so I don't know where it was originally installed.
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