A once common sight on UK roads, the orange glow of the low pressure sodium (SOX) lamp will soon be no more. The last SOX factory at Philips in Hamilton, Scotland is due to cease production in 2020. Installed in large swathes during the energy crisis of the 1970s and 1980s the largest lamps can reach an efficacy of 200 lumens per watt, which is still unsurpassed by most LED installations today. The colour is a single monochromatic orange line on the spectrum at 589 nanometres, known as the Sodium D line. No colour is rendered under these lamps; everything instead appearing a shade of orange or black.
Not all SOX is dead just yet though. This installation on the M602 road from Eccles to Manchester centre is in fine working condition:
A row of unbroken 180W SOX lights the way towards Manchester. The lights of the city can be seen in the background.
Another installation, featuring an impressive loop into the distance is this row of 180W SOX lighting the M60 motorway northbound just outside Stockport.
It's unusual to see such an unbroken installation nowadays, so it appears lamp maintenance is still occurring and the council must have some spares yet to use up. The 180W lamps are around 4 foot in length and the lanterns are around 5.5 feet in length. The lights on the side of the road are high pressure sodium, which don't have a monochromatic yellow colour and are a little whiter.
