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relite diadem major a42.005/745

Genre: Enclosed Horizontal Transverse Mercury Lantern

The mercury vapour discharge lamp was developed by the GEC in the early 1930s. The first production quality lamps, rated at 400W, were trialled along East Lane, Wembley in June 1932; and the first installation was erected early the next year. The lamp was designed to replace the 1000W GLS lamp which was considered a necessity for good quality street lighting but was unaffordable for most local authorities. The 400W medium-pressure mercury lamp (later called the MA) was therefore marketed as the cost-effective solution for main road lighting.

It was quickly joined by 250W and 150W rated lamps, both tubular, and both designed for road or industrial lighting. The colour was often seen as a problem, and the lamp soon earned the nickname “the cadaver” lamp, so various attempts were made to change its spectral output. Doping the mercury discharge with other metals, running tungsten filaments in series and covering the outer bulb with fluorescent powders were used to modify the lamp’s colour, but all reduced the efficacy.

The lamp’s efficiency and colour output could be improved by increasing the pressure in its enclosed arc-tube. This was initially a problem as the hard-borate based arc tubes couldn’t withstand the increased temperatures and pressures; whilst manufacturing issues with quartz glass tubes prevented their adoption. It wasn’t until 1936 when Philips solved the problems with quartz that a new generation of high-pressure mercury lamps were introduced. Manufacturing costs prevented the quartz based lamps from being made in larger wattages so only lower wattage lamps initially used the quartz technology (and were therefore called MB lamps).

The smaller MB lamps were offered in elliptical glass bulbs and resembled the shape of tungsten filament lamps. To prevent engineers installing MB lamps in unprotected circuits – as they required gear to stabilizethe current – the bayonet connector was fitted with three pins instead of two.

After the Second World War, the mercury lamp was used extensively throughout the UK as part of post-war rebuilding. New installations were often either low-pressure sodium or mercury. The status quo with the MA and MB lamps remained intact until the costs of quartz eventually dropped and higher wattages became financially viable; and so the MA lamp was eventually withdrawn.

Whilst improvements were made with the efficacy and ight quality of the lamps throughout the 1950s and 1960s, the lamp remained less efficient than its sodium counterparts. The introduction of the high-pressure sodium lamp in the early 1960s, which was more efficient, meant that mercury was rapidly becoming a poor option for street lighting. When the energy crisis of the 1970s hit, forcing councils to opt for the cheapest running options, mercury joined fluorescent and tungsten as one of the lamp types to replace.

Despite its disadvantages, mercury still remained popular for residential lighting throughout the 1980s and 1990s, where its bluish-white light was preferred by some authorities. But the high mercury content of the lamps, and their poor efficiency, meant that it became increasingly difficult to support in more environmentally sensitive times. The mercury lamp was eventually removed from the specifications, and its production was stopped in the early 2000s, which resulted in its accelerated demise. By the 2010s, with massive replacement schemes and the adoption of LED, the era of mercury discharge lighting had come to an end.

Name: Relite Diadem Major A42.005/745
Date: Late 1960s - Mid 1970s
Dimensions: Length: 590mm, Width: 260mm, Depth: 180mm
Light Distibution: Semi-Cut-Off
Lamp:80-125W MBF/U


REVO had previously used a traditional pendant top-entry styled lantern for tungsten and mercury vapour lamps. Such designs were fitted to swan-neck style brackets, but the austere and modern movements which followed in the wake of the Second World War, plus the introduction of new materials and manufacturing techniques, meant that a fresh look was taken for this lantern type.

The trend tended towards side-entry lanterns. REVO responded by modifying their existing range of pendant lanterns with side-entry options; but the overall combination of this type of modified pendant lantern, with sleek side-entry bracket and non-decorated column looked ungainly. Therefore the firm designed a new lantern for tungsten and mercury vapour lamps which was smaller, sleeker and designed for side-entry.

The Diadem was first developed for the market in the 1960s. Its design parameters were brief: side-entry lantern with die-cast aluminium canopy; newly designed glass refractor bowl and ring; horizontally mounted lamp; and the absolute minimum of ancillary components. There was no provision for gear, so the lantern's dimensions were kept to a minimum. It was probably the smallest lantern of its type on the market.

A gear-in-head version was also developed. This enclosed the gear in its own compartment at the back of the lantern, fusing the lantern and gear compartment together and doubling the length of the lantern. The first versions had a bulky gear compartment which was as deep as the bowl itself. But the design was later changed, and the gear compartment was flattened out, giving the lantern a long sleek appearance.

Both lanterns were continued after REVO sold its lighting division and was rebranded Relite. During this period the old REVO casts and model numbers were retained so dating lanterns from this period was difficult. During this period, Relite designed a new plastic bowl, and the old glass refractor bowl and associated gear rings were dropped from the design.

The lantern’s life was further extended when Relite was sold to Simplex. It was kept on catalogue, along with its gearless counterpart for several more years, ensuring the design continued into the 1980s.


Whilst not as popular as its gearless sibling, the Diadem Major was relatively successful, and could be found installed throughout the country.


Identification of the lantern is easy thanks to its unique refractor bowls (both the REVO and Relite versions) and the lantern’s sleek profile. However, identification of the manufacturer and its age is more difficult. Even if the lantern is taken down and examined, some Relite examples use REVO parts and model numbers. Unfortunately, in many cases, identifying stickers inside the canopy, or on the base of the gear compartment, are usually too worn to read.

Optical System

The primary optical system comprised a single glass or plastic refractor bowl. In the case of glass it was specially designed for the lantern and was held in position by a bowl ring. The plastic bowl option simply clipped in place without the requirement of a bowl ring. The profile of the canopy, and its white painted interior, acted as a secondary optical system, reflecting light back which was emitted above the lamp.


The gear was mounted in an integral compartment which was part of the canopy of the lantern. Positioned behind the lamp compartment, it was separated by a bulkhead and accessed via a separate hinged door.

The Relite Diadem Major A42.005/745 In My Collection

facing profile

An eBay win, the provenance of this lantern was completely unknown..

front profile

The lantern was fitted with the Relite/Simplex designed plastic bowl. This had an external black rubber gasket and was held in place by a single stainless steel clip. Hinged at the opposite side, the bowl swung along the axis of the road when released.

trailing profile

The lantern's bowl and lamp canopy was slightly tilted to allow extra throw of light across the road.


The canopy of the lantern was completely flat and showed no differentiation between gear and lamp compartment.


There were no maker's names or manufacturer's logos on the top of the lantern. This was odd as REVO usually branded their lanterns in this way – so this suggests, along with the streamlined design and plastic bowl – that this was the Relite or Simplex version of the lantern.

pedestrian view

The bowl was relatively new and didn’t feature any discolouration. (Most polycarbonate bowls yellowed over the years). The prism design echoed REVO's old design patterns of swapping from vertical to horizontal prisms with smooth arcs and curves. The main beams were formed directly from the lamp itself as both sides of the bowl along the road's axis were blank. The prisms around the rest of the bowl were used to spread the light at high angle around the lantern.


The base of the refractor bowl had simple parallel prisms to spread the light beneath the lantern. This increased the size of the "light spot" or "head" beneath the lantern and prevented the formation of bright spots on the road surface.

Two small screws allowed access to the gear compartment. The bottom of the compartment swung down towards the pavement.

interior #1

The interior of the lantern was extremely simple. The lamp holder was mounted pavement side allowing the connecting leads to simply emerge from a hole in the side of the bulkhead from the gear compartment.

It was painted white to act as a secondary optical system so that light emitted above the lamp would be reflected back down to the refractor bowl. The lamp itself was mounted on its own lamp-holder assembly.

The lantern’s model number was cast into the interior of the canopy: “Pat No A42.005/745. Made In England.” This number suggested it was made either by REVO or Relite.

interior #2

Swinging the bottom door of the gear compartment open revealed the lantern's gear. A black Parmar P885 choke was held in place by gravity and a metal strap. This was rated for 80W HPMV lamps. The A94.090 8uF capacitor had a Relite brand thus confirming the lantern’s Relite provenance. It was dated 11/72, firmly putting it within Relite’s lifespan. Other components included a cable clamp and terminal block.