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Thorn Precinct (Mark XIII)

Genre: Diffuser Lantern

Since the earliest days of lighting, the glare from a point light source could always be softened and reduced by means of a diffuser. Although the gentle light produced by the early gas lanterns never merited any diffusing glass, the arrival of the arc lamps necessitated huge opaque glass globes required to soften and spread the piercing light of the electric arc.

When the incandescent bulb arrived, some exterior lanterns offered a choice of either clear flint glass or opaloid glass, but the function of the lantern was still to protect the lamps(s) and provide an even symmetrical distribution of light. The opaloid glass could be used to further spread the light, soften glare, hide the light source(s), and provide a decorative finish to the lantern.

The Projected Area Principle was used to further control the flux; the area of the flashed area corresponded directly to the amount of flux emitted from that area. Therefore the shape of the diffuser directly controlled the distribution of the light. However, due to high losses (especially when compared to optical systems based on reflection or refraction), the use of diffusers was not widely employed. Some manufacturers (especially Thorn) offered street lighting units based on the Projected Area Principle but such units were not widely taken up.

The use of diffusers found special application in areas where symmetrical lighting was required, especially when lighting areas such as car parks, promenades, walkways, shopping centres etc. Such lanterns were especially popular from the 1960s through to the 1980s, especially globe and post-top designs, which were installed in large numbers in open areas, but were rarely used for street lighting.




Name: Thorn Precinct
Date: Late 1960s - Early 1990s
Dimensions: Length: 331mm, Width: 193mm, Height: 139mm
Light Distibution: Symmetrical
Lamp: 80-125W MBF/U




History

The Thorn Precinct was designed primarily for the lighting of town centres and densely populated areas where one-way and pedestrian-only areas were being created. As it was designed for area lighting, and not street lighting, Thorn used a diffuser-based lighting system, styling the bowl from opalescent plastic.

Its design was probably based on the earlier AEI Oadby which had a similar lighting system, gear mounting position and styling. The Precinct was developed by reducing and squashing the dimensions of the long slender Oadby, a simple task as the Precinct was designed for the much smaller MBF/U lamp.

It was also designed for a variety of mounting positions including wall, post-top and back-to-back mounting.




Popularity

The lantern was very popular being installed in large numbers throughout the country. It could be found lighting car parks, pedestrian walkways, town centres and other locations where non-directional lighting was required. It was rarely used for street lighting.




Identification

The lantern had classic 1970s styling and is easily identified by its shape and distinctive black-and-white colour scheme.




Optical System

The lighting system was based on the Projected Area Principle of opalescent diffusers. Therefore the majority of the light was directed downwards and outwards, with a smaller amount projected to either side. The interior of the lamp compartment was painted white to act as a secondary reflector.




Gear

The gear was housed in its own gear compartment to the rear of the lantern (which was a design characteristic of later lanterns). A single choke for either 80W or 125W lamp and a power correction capacitor were squeezed into the back of the housing.




The Thorn Precinct (Mark XIII) (OC 1125.4) In My Collection

facing profile

This shot shows the classic, instantly recognisable, side-profile of the Thorn Precinct.




front profile

The rectangular diffusing bowl was held in position by a simple screw. Although an irritant for the maintenance engineer, clips and/or toggles wouldíve ruined the lanternís appearance.




trailing profile

Oddly the lantern was part of several which were used to light the interior of a church. Itís difficult to imagine why it was selected for this task, where its angular 1970s styling wouldíve clashed with the older architecture.




canopy

The rest of the lantern was smooth with the canopy and body formed from one piece of metal. Like the bowl, it was an angular, simple design.




logo

The top of the canopy was smooth and unadorned. Thorn didnít press or cast any company names or logos into their lanterns, and the Precinct followed this pattern.




pedestrian view

The pedestrian view revealed the top of the hinges which allowed the bowl to open for access to the lamp compartment.




vertical

The base of the bowl had the largest surface area; therefore the largest amount of light was emitted in this direction. This revealed how the lantern was designed for general area lighting rather than for more specialised street lighting.




interior #1

Undoing the screw allowed the bowl to fall open revealing the lamp compartment. The lantern was fitted with a 125W MBF/U lamp. Identification stickers revealed the manufacturer, name and model number of the lantern. The whole canopy and bowl could be removed by undoing the two fixing screws on either side of the lamp.




interior #2

Removing the canopy and bowl revealed the gear compartment and back fixing plate. Undoing two further screws allowed the lamp holder assembly to be removed, revealing the gear, fixing screws and terminal strip.




interior #3

The gear was mounted back-to-back behind the lamp holder. The choke was a Thorn EMI G53439.1 unit which was designed for a 125W MBF/U lamp whist the capacitor was manufactured by Cambridge Capacitors, rated at 8uF and date stamped 1991. (The fact that the gear was by Thorn EMI also dated the lantern to the 1990s).