Variation of efficacy with arc tube bore and current in a high-pressure sodium lamp

The total wattage and voltage drop across a high-pressure sodium discharge in an arc tube constructed with three sections of different bores (4 mm, 5.7 mm and 7.4 mm) have been measured for three different currents (3A, 4A and 5A). The light intensity and visible region spectra were measured with a spectro-radiometer. The sodium and mercury vapour pressures were held constant throughout the experiment. This was achieved by using a large quantity of amalgam contained in a reservoir appended to the arc tube and submerging the reservoir in a bath of molten indium whose temperature could be independently fixed. The wattages and voltages across each section have been calculated, and a model used to calculate the spectra. There was good agreement between the calculated and measured total wattages, voltages and spectra for the nine combinations of current and bore. It has been determined that the efficacy increases by between 2% and 4.8% per mm of bore at constant current and by between 2.3% and 7.5% per A at constant bore. Throughout the experiment the wall temperatures varied between 1296 K and 1720 K. If the wall temperatures could have been held constant at 1500 K, it is predicted that the efficacy would have increased at rates between 8.8% and 11.7% per mm of bore and between ¿1% and 4% per A.