1984 National K-HICA 50W

Despite their high efficacy, high-pressure sodium lamps have been applied to a few applications only due to the poor color rendering of their emitted light. K. Schmidt, one of the inventors of the technology at GE (USA), found in the early 1960s that increasing the sodium vapor pressure results in a broader emission spectrum which significantly improves the light quality. Unfortunately this change results also in a lower lumen efficacy as a result of a characteristic increase in infrared output from the sodium plasma. The development of sodium lamps having an improved color nevertheless started in the late 1960s following various strategies. The most common and simplest approach consisted in increasing the operating temperature and pressure, but the ceramic burners of the time were not resilient enough to contain the superheated alkali vapor, which led to service lives too short for any applications.

Matsushita, in Japan, is the first company to work on the development of a practical discharge vessel designed for such an extreme operation. The Japanese recognized that a homogeneous burner temperature was needed for an improved lamp operation, which they achieved by optimizing the burner shape and by limiting its end losses. GE was the first company to wrap the burner extremities with metal foils in order to limit thermal losses there, a design it introduced in 1969 to limit seal corrosion in its newly released 250 W Lucalox. In 1969, Matsushita was the first to use these foil shields specifically to raise the cold-spot temperature and the sodium vapor pressure without impacting on the maximal wall temperature, which is critical to maintain a stable lamp chemistry over time. After some further work and improvements on the end seals and electrode designs during the first half of the 1970s, Matsushita introduced its K-HICA white sodium lamps on the Japanese market. The lamps released during the second half of the 1970s were relatively large models rated from 150 to 400 W, which found a use primarily in outdoor lighting applications.

In order to adress the indoor lighting market which called for smaller lamps, Matsushita developed special shaped monolithic burners optimized for a lower power operation. The Japanese recognized that the burner’s temperature profile must be as homogenous as possible, and to this end they devised arc tube shapes with tapered ends, manufactured using a slip-casting method. Such design effectively prevents the condensation of liquid sodium over the metal-ceramic end seals, were severe corrosion normally occurs at high temperature.

Matsushita introduced its first low-wattage K-HICA white sodium lamp using this burner design in 1984 in the form of the present miniature 50 W model aimed at shop lighting applications. This particular lamp is designed with a short electrode distance for a 50 V discharge operation, which permitted the use of a simple series-connected choke ballast on 100 V mains. An additional benefit of such compact discharge is its high luminance which improves the optical control of light. The combination of short electrode distance with an evenly loaded burner ensures a stable operation and color temperature at all positions, an important feature in retail lighting. Finally, Matsushita introduced a new electrode made of a central protruding rod with a first coil coated with an electron emitter, protected from the discharge by a second coaxial coil. This improvement combined with a maximal burner temperature of 1100 °C despite a wall load raised from 20 W/cm² to about 35 W/cm², are the factors which enabled a service life of 6000 h with an efficacy of 45 lm/W.

The K-HICA 50W has an unusual compact construction which features an aluminosilicate glass jacket mounted on an EX19s base, designed to withstand high voltage ignition pulses. Interestingly, the barium flash getter, used to maintain a high level of vacuum inside the lamp, is located at the side opposite to the base so as act as a glare shield. However clever this design is, it did not prove very successful and was soon superseded by a larger type of lamp with a wider borosilicate bulb mounted on an E26 base. The burner construction was also changed in the late 1980s into a straight tubular design provided with external amalgam reservoirs designed to better define the sodium vapor pressure in the discharge.