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philips mi 26

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: Philips MI 26
Date: 1980s - 2000s
Dimensions: Length: 40cm, Width: 16cm, Height: 18cm
Light Distibution: Semi Cut-Off (BS 4533:1976)
Lamp: 35W SOX




history

If a UK lighting engineer was asked about Philips in the early 1960s, then he would've only known them for lamp production and the invention of the low pressure sodium discharge lamp in the early 1930s. By the mid-1960s, the firm had joined forces with ELECO and were advertising their own range of lanterns; but whether these were developed by ELECO or Philips still isn’t known.

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 MI50 was one of this new range, replacing the earlier Philips MI5 (another ambiguous lantern as it's believed it was designed by Phosco and sold by them as the P224). The MI50 featured a distinctive aluminium canopy with the refractor bowl matching the luminarie's characteristic profile. The similar MI80 was designed to take the larger 55W 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 35; this was reflected on the sticker inside the lantern, but this was the only difference between the security SXK 35 and the street MI50.

It remained on the catalogue for over a decade, but was eventually replaced by the Philips MI26. This offered the same optical system (the bowl was the same), gear and photo cell, but the canopy was made of glass reinforced plastic (GRP).




popularity

This lantern was extremely popular for side-road installations in the UK, probably second-only to the hugely popular Thorn Beta 5 and Beta 2 ranges.




identification

The lantern's angular canopy, curved bowl, stainless steel bowl clips and distinctive plastic body make it an easy lantern to identify. The detachable cover to aid mounting by the "U" bolt assembly was unique to this range.




optical system

The primary optical system is the bowl which features moulded refractors. 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 4533:1976).

A sheet of white painted steel acts as a secondary reflector which was fitted above the lamp.

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




gear

The gear is mounted in the canopy of the lantern. This is accessed by loosing two bolts and sliding the over-reflector out of the way. Gear includes a choke, power correction capacitor and ignitor.




the philips mi 26 (mi 26 go*1) in my collection

facing profile

The canopy of the lantern was made from formed glass reinforced plastic (GRP) formed into an angular, chunky shape. This had a detachable rectangular cover at the back which could be removed so the "U" bolt to secure the lantern could be seen. The bowl was curved - the same design as previous models.




front profile

The lantern was in good condition. Only some discolouration of the white plastic canopy and rusting of the transformer showed that it had been in service at some point.




trailing profile

The design of the refractor can be appreciated in this shot. The upper parts of the refractor redirected the flux from the lamp back down towards the street, adding to the main beams. The lower half the bowl was unimpeded allowing as the flux emitted here was at the right angle for lighting the road. The base of the bowl included refractors to spread the flux below the lantern, thus preventing the "bright spot" forming below.

This view clearly shows how the canopy profile matched the bowl profile, continuing the lines of the original metal based lantern (the MI 50). There were some design changes; the bulge for a top-entry option was removed and the cover over the spigot mounting was removable.




canopy

The top of this small lantern was dominated by the large photocell. This was factory fitted and could be specified whilst ordering by adding "*1" to the model name.




logo

Like most Philips lanterns, the top of the canopy featured the company's name and logo.




pedestrian view

The refractor prisms were moulded directly into the bowl; they weren't stuck on as was common with earlier lanterns.




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

Unlike the larger MI 36 which used two stainless steel clips along the side of the lantern, the MI 26 used two clips at either end. The smaller bowl could be held in place, and the lantern sealed to the right IP, by two end clips which saved on manufacturing costs.

It swung open away from the column (a requirement of BS 1788:1964) and could be easily detached for cleaning (one of the better and easier designs to deal with).

The underside of the white-painted over-reflector can also be seen.




interior #1

A sticker on the over-reflector included the lantern's model and lamp information:

MI 26 GO*1
9111 225 604
SOX-E 26W & 35W SOX
240V 50Hz IP 65

The basic model number was "MI 26". The "GO" specified the gear-in-head option, whilst the "*1" specified a NEMA socket for a photoelectric cell.

The ballast was extremely rusty and all identifying marks and stickers had long gone. The power-correction capacitor had faired better (a Cambridge Capacitors 6.5uF) along with the ignitor (a Philips SX 72). All the components were mounted in the body of the canopy.

The almost universal spigot securing assembly of spigot housing along with two grub screws had also been changed. It was replaced by an enclosed compartment (accessed by removing top of the canopy) which housed bolts and a U-shaped spigot clamp.