||This lamp is representative of the first model to have been manufactured with a series of small dimples positioned along the arc tube, to act as sodium reservoirs. It was introduced in 1958 to replace the Bamboo style that had been introduced just a few years earlier in 1955, and was itself superseded by the Integral lamps during 1958-1960.
The dimples serve two purposes. Firstly, they maintain a more even distribution of sodium vapour over the length of the discharge tube. This has a significant effect in improving lumen maintenance during lamp lifetime, by slowing the rate at which some parts of the discharge tube become devoid of sodium and suffer a drop in light output. This is achieved thanks to the dimples being positioned a short distance away from the heat of the discharge, and the resulting cold spots slow the rate of distillation of sodium to the cold U-bend area. As a result, the 4000-hour lumen maintenance was improved from about 60% for the Bamboo design up to 80% for the dimpled model. Secondly, thanks to surface tension effects the dimples serve to concentrate the metallic sodium in globules of smaller diameter than is possible in a plain tube. This has a small effect in reducing the quantity of light that is blocked by the sodium mirrors, and leads to a modest improvement of about 3.5% in initial efficacy.
The chemical composition of the discharge tube in this lamp is similar to bamboo types, and due to its rapid absorption of argon gas the lamp is filled with the same helium-neon-xenon mix - note the characteristic discharge colour. Owing to the fact that the exposed dimples are very fragile and can easily break when inserting into a dewar jacket, these lamps were quickly superseded by the Integral SOI design.