||The Philips Polarimetry lamp is perhaps the smallest low pressure sodium discharge lamp produced. Rated just four watts and drawing a minimal 82mA, it provides a tiny source of monochromatic radiation in polarimetry equipment. It also finds a niche in microscope illumination, for improved discrimination of interference fringes in micro-measurement of surface anomalies in polished metallurgical samples.
The lamp consists of a 6mm diameter discharge tube approximately 40mm long, with a gap between the electrodes of around 20mm. The electrodes take the form of small tungsten U-shaped coils coated with triple carbonate emitter, both lead wires of each being pinch-sealed through either end of the discharge tube. The exhaust tubulation is attached as a side T-piece just below the mid point of the discharge tube. The lead wires from each electrode are shorted together and spot welded to the mount frame. The discharge tube is made from a soft 2-ply borate glass as used in SOX lamps, and the seals are made to unborated dumet wire. The entire assembly is stem-sealed into a small 16mm diameter tubular bulb which has been exhausted to high vacuum and is equipped with a candelabra screw base. The outer bulb is sprayed black apart from a 10mm wide clear band around the centre of the discharge tube. This provided a compact and flicker-free light area.
Also shown here is an earlier version of the lamp, as it was made in its original form after development in Eindhoven. The design is substantially the same except that the electrode lead wires are shrink-sealed into a glass tube, which is then rim-sealed to the discharge tube. The outer bulb is internally aluminised rather than sprayed black.