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Thorn Riga

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 Riga (QR42F2APT)
Date: Mid 2000s -
Dimensions: Length: 510mm, Width: ???, Height: 210mm
Light Distibution: Clear cut-off (street), Soft cut-off (residential)
Lamp: 50W HPS, 70W HPS, 57W CFL


The Riga is the successor of the popular Beta 79/99 lantern. Its design was prompted by the removal of larger lamp types from the specifications (most typically LPS), the need of lanterns with a higher IP rating, a universal mounting system (either side or post-top) and the requirements of better light control.

Smaller lamps allowed a corresponding decrease in the size of the optical component of the lantern. This was formed by a complex reflector and a choice of bowl types which allowed the lantern to be used for pure street lighting or for the lighting of residential areas.


The Riga is a popular lantern and has been installed in many districts throughout the UK.


The lantern is easily identified by its size and profile. Its curved canopy with central strengthening rib, black Thorn bowl clips, shallow bowl and separate mounting cover are key traits.

Optical System

The flux is controlled by a modern reflector in which the lamp is centrally placed. (This differentiates it from previous lanterns which only had side reflectors and an over reflector.) Two bowl options allow the cut-off to be modified allowing for clear cut-off (for street lighting) and soft cut-off (for residential areas).


The gear is mounted in the mid-section of the lantern and is easily replaceable with a plug-and-socket connection.

The Thorn Riga (QR42F2APT) In My Collection

facing profile

I obtained this Thorn Riga from a fellow collector in the Burton area. It was used as a trial lantern and had since been removed. Therefore it had seen little service life and was in excellent condition.

front profile

The lantern is easily recognisable thanks to its distinctive curved profile, shallow bowl, large black Thorn bowl clips and a strengthening rib over the centre of the canopy.

trailing profile

The exterior of the lantern gives some clues of its internal partitioning. The back half of the lantern allows either post-top or side-entry mounting – the large U-bolt and securing nuts being housed under the separate back section of the lantern. The forward half includes the gear and the optical compartment – both accessible when the bowl is opened.


The top of the lantern is relatively plain – there are no logos or identifying marks. The factory-fitted photocell is located in the centre, underneath the gear tray.


The universal mounting U-bolt is accessible when the back cover is removed by undoing the securing screw. The U-bolt can be positioned vertically or horizontally allowing side or post-top mounting. Two securing lock-nuts can then be tightened to fix the lantern in place.

pedestrian view

Access to the lamp and the control gear is easily achieved by simply undoing the bowl clips.


This shot shows how the bowl encloses both the gear compartment and the reflector. It’s fitted with a 57W CFL lamp.

interior #1

The interior of the lantern shows how small the optical system is (as it only has to cater for 50-70W SON and 57W CFL lamps). The gear fitted is a Philips BSN 50L427 ITS ballast and 9uF power correction capacitor (which is dated 2005).

Thorn Riga: As Aquired

A contemporary lantern from Thorn, the Riga is a sleek, well designed lantern for post-top and/or side-entry mounting. It's fitted with a 57W compact fluorescent bulb and was originally installed somewhere in the Burton area.