Investigations on the replacement of mercury in high-pressure discharge lamps by metallic zinc

The replacement of mercury in high-pressure discharge lamps by metallic zinc is studied. Pure zinc/argon discharges as well as lamps including zinc or mercury and metal halide additives are investigated. The studies focus on the physical properties as a function of the discharge geometries and lamp power. Experimental data are compared with model calculations of the energy balance involving the transport of heat and radiation. Since the excitation energies of relevant zinc transitions are lower than for mercury, axis temperatures of zinc lamps are about 300 K below the value of mercury arcs. In addition, the thermal conductivity of zinc, including the contribution of radiation diffusion, is larger than that of mercury. From lamp voltage measurements, it is found that the cross section for elastic electron scattering by zinc atoms is about the same as that for mercury. This fact needs to be verified by measurements of the momentum transfer cross section of zinc due to the lack of experimental data in the low-energy region below 1 eV. When adding metal halides, i.e. NaI, TlI, DyI 3 , the axis temperatures decrease to about 5100 K due to strong radiation cooling. In these lamps the coldest spot and the mean wall temperatures must be increased when replacing mercury by zinc in order to obtain sufficiently large electrical fields. Temperatures of about 1350 K are adjusted by means of polycrystalline alumina oxide (Al 2 O 3 ) as a wall material. In addition, the electrode distance of metal halide lamps containing zinc must be slightly increased by 25% in order to obtain lamp voltages of typically 80-90 V, of mercury metal halide lamps. The light technical data of the discharges are very close, since mercury and zinc do not contribute significantly to the radiation in the visible range. Efficacies of up to 93 lm W -1 and 100 lm W -1 are found in metal halide lamps with zinc and mercury, respectively. Consequently, zinc turns out to be an attractive replacement for mercury in this type of lamp, and not only from an environmental point of view.