1970 Westinghouse SAH 250A

Introduced in the late 1950s, the Westinghouse SAH 250A is a jacketed, single-ended compact mercury short-arc lamp for AC circuits, intended for the illumination of optical comparators. A second model, the B type designed for a flicker-free operation on DC circuits, was released in 1961. Both lamps feature a thick-walled three-piece quartz burner with a barrel configuration, sealed with pressed molybdenum feedthroughs. This design is rather unusual for a mercury arc lamp, which are usually formed with an spherical or an elliptical shape from a single quartz tube. This is even more peculiar since Westinghouse employed the latter standard technique to produce larger short-arc sources. They resorted to the present compound burner design in the 250 W SAH lamps probably because they could not produce small shaped single-piece burners within properly tight tolerances and/or the accurate placement of the electrodes was too problematic with their production equipment and methods.

The arc tube is not the only unique feature of the lamp, its electrodes are quite unusual too. Those are of the rod-coil type to ensure a proper heat dissipation via thermal radiation. In the present type A lamp, the electrodes are symmetrical as they have to act both as cathodes and anodes depending on the phase of the current. The tip is molten into a bead, merging the rod and coil into a tungsten ball onto which the mercury arc attaches. The size of the ball tip is precisely adjusted so the ratio between current and cross section lies between 70 and 75 A/cm², ensuring an optimum balance between arc stability (i.e., as little arc spot attachment wandering as possible) and tungsten evaporation (i.e., minimum burner blackening rate). As this balance is also affected by the amount of energy radiated by the coil, its length is therefore carefully and precisely adjusted for the best possible operation of the lamp. In the DC lamp, i.e., the type B, only one electrode has its tip fused, the anode, while the cathode retains its full rod-coil design with a protruding rod tip. In all cases the back of the electrode rod has a slot where a sliver of thorium or barium zirconate is inserted in order to lower the cathode temperature by way of a reduced material electron workfunction, which also improves the arc stability.

The burner assembly is mounted inside a borosilicate glass jacket attached to a P28s prefocus end cap which allows a precise positioning of the light source in the optical system and makes re-alignment superfluous, thus simplifying maintenance. The nitrogen-filled outer jacket serves several important functions, such as the protection of the moly end seals of the burner from oxidation, which thus enables a very compact design operating at high temperature. Double-ended lamps require much longer end seals to ensure a low enough temperature at their extremities, which is not required here. Other benefits of the jacket include the filtering of short-wave radiation, the mitigation of non-passive failures, and the protection of the burner against dirt and physical contact. The lamp’s gaseous atmosphere ensures a good thermal dissipation of the arc tube, whose lower cap-side extremity is coated with platinum paint to prevent the condensation of mercury there. Such design, combined with this burner shape and dimensions, thus ensure a relatively homogeneous temperature distribution with a maximum temperature not too far above 1000 K, critical to limit the rate of quartz devitrification.

The SAH 250A has an electrode gap length of 2.1 mm with a total light output of 10 klm, which results in a source brightness of about 40 kcd/cm². The operating pressure (~30 bars) is relatively low for this kind of lamp, a limitation set by the compound design of its quartz burner and its seal shape in particular. This, combined with a very short arc, results in a rather low operating voltage of 37 V for a high drive current of 8.7 A. The latter causes high electrodes losses (~90 W in total), which caps the lamp efficacy to 40 lm/W, about 16 % lower than that of 200 W double-ended mercury arc lamps of similar brightness and which operate at a higher pressure (~50 bars).