ABSTRACT
Searching for working gases for substituting mercury in metal-halide lamps, the rare gases came into focus. Besides the high vapor pressure, the main property which makes mercury so unique for the filling of highpressure lamps is its large transport cross section. It allows lamp operation with low current due to the high electric field strength which can be established in mercury discharges. Therefore, as a necessary condition the substitute must allow a high field strength. This is of course not sufficient for defining a substitute: finally the resulting lamp must have a high luminous efficacy. As a first step, however, the field strength is a criterion for the selection of substitute candidates. The discharge voltage alone, taken under otherwise the same conditions, is not convincing enough because it consists of the voltage of the positive column (field strength times column length) and of the electrode sheath voltage (ESV), and the latter does not participate in light production. Therefore, we have determined the electric field strengths in Hg, Ne, Ar, and Xe, and the corresponding ESVs were obtained as a by-product.
