||Osram (formerly GEC) Low Pressure Sodium SOX lamps underwent a number of design changes over the years, and this lamp is representative of the 6th generation. It was introduced following the unfortunate decision of Osram to close its Shaw factory in 2000. That was the company's only site having the capability to make low pressure sodium lamps. Since the SOX lamp was beginning to decline in sales, and its manufacture is rather complex and depends in great measure on the skill and experience of the production operators, Osram decided to abandon its manufacture rather than relocate the SOX production to another factory.
The market was however still sufficiently large that Osram did not wish to delete the products from its range. Rather than simply sourcing in the lamps from a competitor under its own name, Osram wished to maintain a visually different design, and came to arrangements with Philips to ensure a continued supply of a hybrid Osram-Philips design.
The key difference between Philips and all other SOX lamps is that the IR coating of the outer jacket on Philips lamps is of constant thickness - and since there is a thermal gradient across the discharge tube from the hot electrodes end to the cool U-bend, Philips lamps require sodium-retaining dimples to maintain a distribution of sodium vapour all along the lamp. For many years Osram had preached the advantages of its dimple-free construction, in which sodium uniformity is maintained thanks to a special IR coating having a graded film thickness. The resulting lamp is thus made according to Philips' design, but with graded IR film made on the old Osram equipment, no dimples, and ring-shaped sodium reservoirs at the periphery of the U-bend.