Sodium Vapour Lamps.

Sodium vapour lamps appeared relatively early on in the history of electric lighting, as the vapour pressure is helpfully low and it was quickly realised that they could be made into a very efficient light source.  Low pressure sodium lamps took a place well ahead of their competition right back at the time of their inception as the most efficient electric light source in the world, a position it still holds today - though the market position has all but disappeared with the advancement of LED technology.  It wasn't until the 1930s though that makers figured out how to build a lamp which both had a usefully long lifetime and provided decent efficacy.  This was due to two major inherent drawbacks.  The first is that at the operating temperatures these lamps run at that sodium is very corrosive to the glass the arc tube is fabricated of.  So coatings had to be developed to prevent the lamp literally eating itself from the inside out.  These coatings couldn't easily be applied to the areas behind the lamp electrodes though, so the lamp geometry had to be carefully designed to prevent the sodium from accumulating in those unprotected areas.  The second challenge is that to achieve the optimum vapour pressure (hence lamp efficacy), the wall temperature of the arc tube has to be maintained as close as possible to a target temperature of 260 degrees centigrade.  It wasn't until the mid 1960s that this was really cracked when the technology to create reliable, relatively cheap, extremely thin semiconductor coating on the inside of the outer bulb were developed.  Since then the technology has been refined incrementally, but the designs from the late 60s/early 70s didn't really change hugely through to the turn of the millennium.  The principle drawbacks of the technology as a whole mainly are that the output of low pressure sodium lamps is an almost completely monochromatic deep amber colour, precluding it from any general lighting application and limiting it to industrial and street lighting applications.  The other single big drawback is that the lamps have always been somewhat expensive due to their relative complexity and requirement for special coatings on the glass components.  The lamps are also physically quite large, which can make precise optical control quite difficult.

High pressure sodium lamps provided a far whiter light (I'd describe it as a "golden white"), and are of a less sophisticated construction (even if some of the materials involved were quite exotic when the technology was new), so tend to be cheaper.  The lamps are also physically far more compact so it's easier to design fixtures around them to provide more accurate optical control.  They are not as efficient as their low pressure counterparts, but the whiter light, longer life spans, simpler control gear and cheaper lamps saw this technology become hugely popular.  It's only in the last ten years or so that its position in the fields of area lighting, street lighting and exterior architectural illumination have started to be poached by the relentless march of LED technology. 

While the light produced by high pressure sodium (SON) lamps is far more pleasant than that of the low pressure sodium variants, it's still far too far off white to be suitable for most interior applications (unless blended with other sources as some clever architects did especially in the late 70s).  This shortcoming was addressed with a very particular variant where the vapour pressure was raised even further.  While this did reduce the lamp efficacy somewhat, the result was a lamp whose light was almost indistinguishable from that of a tungsten incandescent lamp, but with a very long lifespan, around 5 times better efficacy than the tungsten lamp, and because the lamps were kept very compact supremely good optical control was possible.  The big downside there was that specialised and quite expensive control gear was required to precisely control the operating conditions of the lamp to maintain the colour where it should be.  It's a shame that this technology never became as popular as I think it deserved to as the light produced is of exceptionally good quality.

Low Pressure:

Osram SOX/H 35W

Osram SOX/H 55W
Osram SOX/H 55W (Newer example)

Osram SuperSOX 18W

Philips SOX-E 18W

High Pressure:

Philips 250W SON-T E40

White SON:

Iwasaki Eye NHT-150 SDX

Page change log:

11th June 2023: Page order rationalised to make it easier to navigate, some changes to the code have been made in the background as well.

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