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: Philips SXK 55 (MI 80)
Date: Early 1970s - Late 1980s
Dimensions: Length: TBA, Width: TBA, Height: TBA
Light Distibution: Semi Cut-Off (BS 4533:1976)
Lamp: 35W-55W SOX

The early 1970s saw a radical change for Philips in the UK when they started producing their own range of lanterns. A comprehensive advertising campaign (which often saw a series of adverts placed in lighting publications) and contracts to light the UK’s motorway network saw the firm offering a whole range of newly designed lanterns.

The Philips MI80 was one of this new range, replacing the earlier Philips MI8 (another ambiguous lantern as it’s believed it was designed by Phosco and sold by them as the P224). The MI80 featured a distinctive aluminium canopy with the refractor bowl matching the luminarie's characteristic profile. The similar MI50 was designed to take the smaller 35W lamp.

The lantern was also designed as a security lighting option. As it was a compact model (both gear and photo cell were included), the security option simply included a mounting bracket. The catalogue number for this package was SXK 55; this was reflected on the sticker inside the lantern, but this was the only difference between the security SXK 55 and the street MI80.

It remained on the catalogue for over a decade, but was eventually replaced by the Philips MI36. This offered the same optical system (the bowl was the same), gear and photo cell, but the canopy was made of plastic and variable mounting allowed both side-entry and post-top options.

The underside of the gear tray is painted white and acts as a secondary reflector.

The exterior of the bowl is smooth to facilitate easy cleaning.

facing profile I rescued two of these lanterns from a derelict factory in Cambridgeshire. They were oddly installed as they were used to floodlight the delivery yard, mounted low on some metal poles attached to a diesel tank and tilted almost horizontally.

front profile Both lanterns were in good condition although, oddly, the interior wiring had been cut. I repaired this in one lantern and offered the other as a swap.

trailing profile This view clearly shows how the canopy profile matched the bowl profile, presenting a relatively bulky lantern for the early 1970s. The bulge in the canopy was designed for top-entry lanterns but remained undrilled for these side-entry versions.

canopy The photocell was included as standard. The stainless steel hinges both clipped the bowl firmly to the lantern against a rubber gasket and could be easily removed for cleaning.

logo Oddly the lantern doesn’t feature the Philips name and logo embossed on the canopy. This was standard for their lanterns, even the later ones made from GRP.

pedestrian view The refractor prisms were moulded directly into the bowl; there wasn’t the idea of a section of refractor plates which was common in earlier lanterns from the 1950s through to the 1960s.

vertical The refractor pattern continued on the base of the bowl to spread the light beneath the lantern in accordance with the strict BS 1788:1964.

open bowl The bowl swung open on its stainless steel clips and could be easily detached for cleaning (one of the better and easier designs to deal with). The underside of the gear tray can be seen, painted white to act as a secondary reflector. The lantern’s identifying sticker and model number can also be seen.

interior #1 The interior of the canopy featured two locking bolts, cable clamp, terminal strip (with helpful labels) and four screws to hold the gear tray in place.

interior #2 The gear tray held all the gear required to run a 35W or 55W SOX lamp. Included were a Philips BSX 355 L16 ballast, a PED power correction capacitor (rated at 8.0uF and dated 1981) and a Philips SX 72 ignitor.

This was the first streetlight I obtained, rescuing it from the derelict Ross Foods site in Girton, Cambridge before demolition.

The SXK 55 was a commercial security light package; it included a complete lantern (gear-in-head with photocell) with a mounting bracket. Hence they're usually found in car parks and in factory areas.

It's actually more commonly known as a Philips MI 80 lantern.