1972 Philips LL93146E (Hg Cd Zn, 66 W)

For four and half decades Philips produced a range of special electric discharge lamps intended for scientific and technical applications. Many of those were spectral lamps filled with a variety of elements in order to produce specific optical spectra useful in spectroscopy, refractomety, and other applications. Mercury, cadmium, and zinc where possibly the most used elements given their well distributed line spectra that extends from the deep ultraviolet to the near infrared. Mercury, well known for its application in general lighting, is particularly useful because its high volatility enables the realization of compact arcs of high brightness. However, such convenient source characteristics cannot be achieved with cadmium and zinc because of their much lower volatility. Lamps filled with either of these two elements are all of the low-pressure kind whose electrical discharge is diffuse and of low brightness. A solution to this problem consists in filling high-pressure mercury lamps with zinc and/or cadmium. This approach is very effective because it combines the advantages of all three elements, i.e., a high power dissipation, a bright source, and a well distributed emission spectrum (cadmium and zinc add some useful red lines to the red-deficient mercury emission, see emission spectra there). This is the basis for the Philips LL93146E shown here.

The quartz burner inside this lamp is similar to that of the 3rd-gen HP 80W high-pressure mercury lamp made in the 1970s. This vessel features a single tubular body made of fused silica, two full-pressed moly seals, and an auxiliary electrode to facilitate ignition. Each extremity is coated with platinum paint so as to conserve heat and to block off the light radiated from the incandescent electrodes. The most significant departure from the standard HP design is the presence of quartz diaphragms place in front of the electrodes, whose main functions are to stabilize the discharge and prevent flicker. The burner is enclosed in a nitrogen-filled quartz jacket which transmits light down to 180 nm, in the UV-C domain. The lamp construction is of a relatively late design which features a rim-sealed flanged base whose tubular end is pressed onto two molybdenum foils to form gas-tight feedthroughs.



This lamp, like all others from the Philips LL spectral family, was designed for an operation on the commercial 0.9 A leakage-flux transformers ballast intended for SOX low-pressure sodium lamps. The 470 V open-circuit voltage of this ballast is high enough to ensure a reliable ignition of the burner’s argon fill. The discharge in this ternary mixture of metal vapors is characterized by a particularly rich spectrum which covers the whole ultraviolet and visible ranges, a feature particularly useful for many scientific applications such as spectroscopy, refractometry, or fluorescence analysis.

The addition of zinc and cadmium improves the color rendering of the mercury light very significantly thanks to the addition of the red light to the spectrum. The CRI increases from 17 Ra8 to 32 Ra8, a characteristic which led to the use of cadmium in some general-lighting lamps such as early color-corrected medium-pressure mercury lamps and a few metal halide lamps. However, the increased thermal conductivity of cadmium and zinc has a negative impact on lumen efficacy as more energy is dissipated away as heat. This limitation is worsened by the need to reduce the power dissipation so as to mitigate the quartz wall corrosion by the additives. The present LL93146E dissipates 66 W and has an efficacy of 22.4 lm/W only, which is 40 % less than that of the mercury-only lamp variant (Philips LL93110E) which runs at 90 W and has an efficacy of 37.0 lm/W. This issue is the main reason that eventually prevented the use of cadmium and/or zinc in commercial high-pressure mercury lamps (some manufacturers kept on using cadmium in some warm-white metal halide lamps though, but in very small quantity).