||Following the development of the 18W Mini-SOX lamp by Philips in the late 1970s, whose outer bulb design was improved so as to provide better thermal insulation of the U-bend, the same principle was progressively introduced across the rest of the Philips SOX range. The design of the discharge tube in this lamp is substantially unchanged vs the original Philips 90W SOX, and it is only the outer bulb and its support mechanism that has been upgraded.
Most noticeable is the change of shape of the glass dome at the end of the outer jacket, which has evolved from the former hemispherical section to the dimpled shape that was introduced for the 18W model. The discharge tube is supported at the U-bend by a stamped metal sheet, whose form closely follows the shape of the outer surface of the bend so as to provide improved thermal insulation at that point. This is important because there is no IR coating around the dome of the outer jacket, and normally the reduced thermal insulation at that point leads to progressive migration of sodium out of the dimples and towards the cold end. That process leads to a gradual loss of efficacy over time, along with an increase in lamp voltage and hence a reduction in lifetime. The new design brought a modest performance increase in both aspects.
At the same time, the lower support mechanism was changed to an intricate assembly of curved wires which reduced cold spots arising from metal-to-glass contact thanks to small ceramic insulating tubes at the necessary locations. This style of SOX lamp was manufactured from the late 1970s to the mid 1990s, when, to achieve a cost saving, the 3rd generation Philips SOX lamps with full mica supports were introduced.