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Phosco P225
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: Phosco P225
Date: Early 1970s - Late 1980s
Dimensions: Length: TBA, Width: TBA, Height: TBA
Light Distibution: Semi Cut-Off (BS 4533:1976)
Lamp: 35W SOX
History
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In 1971, Phosco unveiled a new range of lanterns for the lighting of Group B roads. The designs were
immediately striking with their linear canopies broken by sharp angles and by their equally geometrically chopped bowls.
These made this family of lanterns (options included gear and mounting choices) extremely striking, especially when compared
with the "smoother" lanterns produced by other manufacturers.
The new range was prompted by a reduction in the size of control gear and the impending new British Standard lighting specifications.
It also allowed Phosco to replace their older ranges of low pressure sodium lanterns; this fresh approach also allowed
them to dispense with other unique traits such as Oddie locks for bowl securing and unique vertical refractor patterning at the ends of the bowl.
The Phosco P223 was the the second-smallest lantern in the new range. It could only take the 35W SOX lamp and
but did have provision for gear.
The lantern remained on catalogue for the next couple of decades but dwindling requirement for low pressure sodium lanterns saw it obsoleted.
However, its bigger brother, the Phosco P224 (which could accept 55W SOX) is still in Phosco’s catalogue,
the last low pressure sodium lantern still made by the company.
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Popularity
The range was popular enough for Phosco to continue manufacture them over the years, but the lantern was never installed
in such numbers as its major competitors (such as similar wattage lanterns made by the GEC or Thorn). Interestingly the
lantern was also sold by Philips as the MI5.
Identification
The lantern is easily identified by its size and profile. There are no external
manufacturer’s markings although "P225" is cast inside the canopy. A silver sticker placed inside the canopy states:
Phosco Ltd.
To BS 1788
Made In UK
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 base of the gear track is painted white. This secondary optical system reflects any flux emitted above
the horizontal back toward the ground. The interior of the canopy is also painted white to reflect flux.
The exterior of the bowl is smooth to facilitate easy cleaning.
Gear
The gear (comprising ballast, ignitor and power-correction capacitor) is secured to a gear tray
housed in the canopy of the lantern. The gear tray also functions as the secondary optical system acting as the
bulb's over-reflector.
The Phosco P225 In My Collection
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facing profile
I never kept any records concerning this lantern so I'm uncertain of where it was originally installed.
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front profile
This view clearly shows the angular shapes of the canopy and bowl. The ends of the bowl were left clear; previous Phosco
designs employed vertical refractors, but this feature has been omitted.
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trailing profile
Despite being in service for years (during which time a photocell was added), the lantern is still in good condition. Unfortunately the polycarbonate
bowl is just starting to go yellow with age.
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canopy
Oddly there are two screw heads on either side of the lantern. These fix a rod upon which the gear tray rotates when released. (Obviously
these screws aren't included on the P223 nor P224 which don't have gear, and have sleeker canopies).
The bowl is secured by two stainless steel hinge clips. This was another new design for Phosco,
who used the distinctive Oddie key for their previous lanterns.
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logo
The canopy of the lantern is entirely smooth and doesn’t have any maker’s mark or logos.
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pedestrian view
The bowl’s angled sizes were designed to position the refractor plates in the ideal position with respect to the bulb.
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vertical
Further refractors were positioned in the base of the bowl to disperse the light in plan and prevent
a bright spot appearing below the lantern. (Other manufacturers used diffused plastic and/or white paint to achieve the same result).
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open bowl
This view of the lantern with the bowl open clearly shows how the underside of the gear tray also acts as the bulb over-reflector.
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interior
The interior of the lantern, with the gear tray hinged open, shows a compact, yet accessible design. The gear fitted, which appears
to be original, are a SB 53312/4 ballast, a Thorn G.53311 ignitor,
and a GC2273 capacitor (rated at 6uF and dated 1973).
The canopy includes the usual earthing screw, grub screws, bulb holder and terminal strip block. The canopy also includes the
lantern's catalogue number P225 cast into the interior whilst a Phosco sticker identifies the
manufacturer.
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Phosco P225: As Aquired
Unfortunately I never kept records for this lantern. So I can't recall where it was obtained nor where it
was installed.
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