||The so-called ML "Mixed Light" lamp was Philips' improved version of the original mercury blended MAT lamps that were developed by British Siemens a few years earlier. Whereas the Siemens originals employed hard glass medium pressure discharge tubes, Philips created its ML lamps around the compact and more efficient quartz arc tubes.
This presents a light source of much more compact dimensions, in which it is possible to mount both the filament and discharge tube at the same height, facilitating optical control. Furthermore, to minimise colour separation effects, the bulb is internally etched to give it the 'pearl' finish, which thoroughly homogenises the light. The resulting lamp is equipped with an E40 screw base and offered in the same dimensions as a comparable GLS incandescent lamp. This lamp is the ML500 version (the 500 signifying its luminous flux in dekalumens, the traditional unit of measure for Dutch and German lamps years ago).
The arc tube form is one of the earliest types employed by Philips, in fact the second type that was commercially employed. First the molybdenum foils to which the main electrodes are welded were sleeved in a layer of quartz, which was shrunk down under vacuum, over this area only. Then the assemblies were loaded into the arc tube itself, which was again shrunk down over the inner quartz sleeves. At the base end of the lamp, a third foil for the auxiliary electrode is sandwiched between the arc tube and this inner quartz sleeve. The auxiliary itself is made from a flat sliver of tungsten sheet, presumably to make it thin enough to easily pass through the gap into the seal. The precise construction is visible in the X-Ray shadowgraph to the left.