||This lamp is representative of the 3rd generation produced by the Leicester factory of Thorn. The only significant change vs the 2nd generation is in the form of the U-bend of the discharge tube, which has been modified to achieve a small improvement in lifetime and luminous efficacy.
The dimensions and thermal insulation of the U-bend of SOX lamps are crucial in terms of determining lamp voltage (which has a knock-on effect on lifetime), as well as the luminous efficacy. The temperature at this point is always somewhat lower than the straight limbs of the discharge tube, because it is not possible to apply the indium heat-reflective film of the outer jacket around the domed end region. Consequently the sodium vapour pressure in that region is reduced, leading to some ionisation of the neon gas filling, which causes an increase in discharge voltage and hence lamp power. In an effort to raise the temperature at the U-bend Thorn's original lamps had a constricted tube diameter at that point, to bring the glass closer to the discharge. However that led to an increase in current density in the discharge, which has a negative effect on efficacy. The constriction itself also led to an increase in lamp voltage and power consumption. To make matters worse, it also led to some ionisation of the argon starting gas filling, which was absorbed more rapidly, leading to reduced lamp life due to inability to start the discharge.
By the mid 1980s it was determined that constricting the glass at the U-bend was in fact not particularly beneficial, and the practice was abandoned. Thereafter, Thorn and the subsequent GE lamps were produced with much larger diameter bends, as in this example.