||This early low pressure sodium lamp is representative of the second generation of positive column SO/H lamp produced by Philips. The name SO 400 indicates Sodium vapour with Oxide-coated cathodes, and the number 400 refers to its approximate luminous flux in dekalumens.
The original SO lamps were made using a glass having a particularly high affinity for the argon component of the gas fill, which is required to facilitate ignition of the discharge. In order to ensure that sufficient argon remained until the lamp was in any case useless on account of its failure due to lumen depreciation, a relatively high fill pressure was required. However increased gas pressure leads to a rather low lamp efficacy, due to wastage of energy from electrons colliding with the rare gas instead of sodium vapour.
This lamp uses a superior glass developed in 1937, having a reduced tendency to adsorb argon. It permitted a reduction in the gas filling pressure without life reduction, and brought a consequent increase in luminous efficacy of about 20%. In order to keep the luminous flux of these post-1937 sodium lamps approximately the same as their predecessors, their voltage was adjusted so as to reduce power consumption. It was at this time that the standard ratings of 45W, 60W, 85W and 140W for the SO/H lamps was established.
Note the intense reddish coloration of the discharge in the gas filling, taken immediately after striking the discharge, which is due to the low fill pressure. Sodium lamps made in later years never returned to such low pressures, because as lamp lifetime was further extended, a return to higher gas fill pressures once again became necessary.