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Thorn Beta 79 (QB79-VWNB-1070.4)
Genre: Enclosed Horizontal Traverse High Pressure Sodium Lantern
As soon as the low pressure sodium lamp (LPS) was developed, the race was on to develop a version which would run at high pressure (HPS).
The research was stimulated by the prospect of the spectral broadening of the light emitted from such a bulb and
the improved colour rendering which would result. However the development was hampered by the lack of a translucent material which
could withstand the chemical attack of sodium vapour at extremely high temperatures and pressures.
It wasn’t until the early 1960s that General Electric (GE) cracked the problems,
closely followed by the GEC and Philips. A prototype high pressure sodium lamp was
exhibited at the APLE's annual conference in 1963, but it wasn’t until 1966 that the GEC erected an experimental installation
along East Lane, Wembley (which, incidentally, also saw the first experimental medium-pressure mercury installation over thirty years before).
The first commercial installation in the UK was erected along the Southend ring road, but it was the City Of London who gained the most
recognition by beginning a radial upgrading of all their lighting to high pressure sodium in 1967. As the early HPS bulbs were designed
to be retrofitted into existing mercury installations, the lanterns chosen by the City Of London were slightly modified versions of
existing mercury lanterns.
The first true HPS lanterns (designed from scratch with the new light source in mind) appeared in the late 1960s and early 1970s. All
these lanterns utilised a cross-over design: whilst this was an adaption of existing mercury lanterns, it also moved the lamp up into
the lantern’s canopy (where it could not be directly viewed by drivers, thus minimalising the glare) and allowed designs to cater for
both cut-off and semi-cut-off.
Wider adoption of high pressure sodium was stalled by the energy crisis, as it couldn’t match the efficiency of its low pressure sodium brother.
Councils, believing in the simple maximum lumens per watt paradigm, replaced existing tungsten, mercury and fluorescent schemes by
low pressure sodium. So it wasn’t until the 1990s that HPS started appearing in greater quantities.
With its efficiency between high pressure mercury and low pressure sodium, HPS became even more attractive when LPS was finally exorcised
from the British Standards of street lighting (as it didn't provide an adequate colour rendering). However, it didn't remain as first choice for
new and replacement lighting schemes for long. Renewed interest in fluorescent (from compact fluorescent sources), the emergence of affordable
metal halide and the possible introduction of LED have all questioned the automatic selection of high pressure sodium for schemes.
Therefore by the end of the first decade of the new century, high pressure sodium’s position as the natural choice for street lighting
was coming under pressure.
Name: Thorn Beta 79
Date: Mid 1970s - Early 2000s
Dimensions: Length: 563mm, Width: 242mm, Height: 176mm
Light Distibution: Semi Cut-Off
Lamp: 50-100W SON, 80-125W MBF/U
History
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The Beta 79 was the first lantern explicitly designed for high pressure sodium by Thorn. Although it could take a 70W SON bulb,
it could also be fitted with a 80W or 125W MBF/U bulb. It has been suggested that its name was not based on the year of its introduction
(as these lanterns were being installed in the mid 1970s) but because it was intended to replace both the ageing Beta 7 and Beta 9.
Initial models sported an aluminium canopy but later models were made from plastic. The range of bowl options also increased, giving the
lantern different optics: deep bowls and shallow bowls were available. Different lamp wattages were added to the range with 50W SON and 100W SON
models made available (and the bulbholder was moved in the 100W model from the path side to the road side of the lantern).
The design soon became standard for many Group B lanterns by other manufacturers with its cross-over optics, the separation of the
gear compartment and bulb chamber and the provision of gear.
The lantern continued into the 2000s when it was replaced new Thorn models such as the Riga and Civic (which offered better optical control,
more lighting options, higher IP ratings and were easily recycled).
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Popularity
The lantern was very popular being installed in large numbers throughout the country.
Identification
The Thorn Beta 79 can usually be identified by its deep bowl, which in turn has a unique
refractor pattern. However, a shallow bowl was also available, making identification a little more difficult.
When faced with comparisons between two popular competitors (the GEC Z8832 and DAVIS GR70)
the Thorn Beta 79 has a flat canopy, uniform lower rim around the canopy, relatively thin, streamlined canopy
(which accentuates the deep bowl option) and a black Thorn clip securing the bowl.
Optical System
Two curved polished metal reflectors comprise the primary optical system which produce a cross-over
flux distribution (which, in turn, gives better control of the degree of cut-off of the light produced).
In some models, these reflectors are smooth, whilst in others they have an irregular stamped appearance.
The secondary optical system comprises a polished over-reflector mounted above the bulb (and has an irregular stamped appearance
to partially diffuse and scatter the light reflected) and a bowl. The shallow bowl simply acts to enclose the lantern, but the
deeper bowl features a refractor plate below the bulb which is designed to spread the light emitted below the lantern.
Gear
The gear is housed in its own gear compartment. This has a lower IP rating than the lamp compartment and access is
achieved by unscrewing a single screw.
When opened, the cover plate swings open and all the gear is mounted on its opposite site. Three components are usually
screwed to the cover plate: the main lamp choke, a power correction capacitor and an ignitor.
The Thorn Beta 79 (QB79-VWNB-1070.4) In My Collection
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facing profile
This lantern originally illuminated part of the car-park at Cambridge University Library. When part of the car-park was removed for an extension, I
was able to save the lantern which had been placed in a skip.
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front profile
This view shows the classic large black Thorn clip; this is used to open and close the bowl.
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trailing profile
The lantern was generally clean and in good condition. Despite some scuffs on the canopy (probably when the lantern was put in the skip) and some
dirt inside the gear compartment, it scrubbed up well.
This example is fitted with the common deep bowl option with refractor.
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canopy
Like the vast majority of Thorn lanterns, no external logos, makers name or other identification are present on the canopy. The four bolt heads
engage with the spigot mounting plate inside the gear compartment; this allows the lantern to clamp to various sized spigots.
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logo
The only marks on this lantern are the various scuffs and scratches on the canopy; the rest is clean and tidy.
Note the raised area for a photocell. The lantern could be provided with or without a photocell.
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pedestrian view
This is a classic view of a classic lantern. The black clip securing the bulb was also matched by the black
plate covering the gear compartment.
The 100W SON models all seem to have the bulb holder mounted on the road side of the lantern instead of against the
gear compartment (which is the more common configuration).
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vertical
This shot clearly shows the refractor plate in the base of the bowl. This refractor spreads the light directly beneath the lantern.
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interior #1
This view of the lantern clearly shows the two curved textured reflectors on either side of the canopy which form the main beam and the textured over-reflector.
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interior #2
When opened (by removing a Philips screw), the gear cover hinges open to reveal the interior of the gear compartment.
The SON choke (Thorn GF53520.4 for 100W SON), power correction capacitor (Cambridge Capacitors, dated 1995) and ignitor
(Thorn G53504) are all fixed to the other side of the gear cover. In theory, this could be removed for maintenance but would
require several wires to be disconnected both within the gear compartment and bulb compartment.
A photocell was a standard option for the lantern; this hasn't been fitted. (As this lantern was used for car park lighting
then I assume they were all group switched from inside the building).
The other components comprised a cable clamp and the spigot clamp; the latter is adjusted via four nuts rather than two grub screws.
This allowed the Beta 79 to accommodate several different spigot sizes.
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Thorn Beta 79 (QB79-VWNB-1070.4): Night Burning
Thorn Beta 79 (QB79-VWNB-1070.4): As Aquired
This lantern is now a classic, but I can still recall it appearing in the early 1970s, and being impressed by
its new clean lines. Thorn have stopped making it now, thus making these even more collectable.
I found this example in a skip at Cambridge Library when they were extending and rebuilding the car park.
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