Luminous Efficiency of the High-Pressure Sodium Lamp

Abstract

The luminous efficiency of the high-pressure sodium lamps (one sodium-xenon lamp and four sodium-mercury-xenon lamps) is measured as a function of the vapor pressure of sodium and of the discharge current. The vapor pressure of sodium is determined from the self-reversal width, which is defined as the distance between both lateral peaks of the self-reversed resonance line of sodium. The self-reversal width becomes large when the vapor pressure increases at a constant discharge current, and when the discharge current decreases at a constant vapor pressure. It is attributed to elevation of the plasma temperature that the luminous efficiency becomes high when the discharge current increases at a constant vapor pressure and when mercury is added to the sodium-xenon plasma. When the discharge current is kept constant, there is an optimum pressure at which the luminous efficiency takes its maximum value. The optimum pressure depends on the discharge current, and it corresponds to the self-reversal width of about 100 Å for every discharge current.