Particle number and pressure determination in high-pressure lamps

An experimental setup is described which makes it possible to determine the particle number per unit length and the pressure in high-pressure mercury lamps. For this purpose the absorption of the frequency-doubled 514.5-nm line of an Ar+ laser by the mercury ground state is used. The method has been applied to measure the spatial variation of the particle number along the axial direction of a high-pressure arc and also to determine the time-dependent particle number during the warm-up and cool-down phases of a lamp. Furthermore the frequency dependence of the pressure fluctuations in a high-pressure lamp during an ac cycle has been measured for sinusoidal as well as pulsed supply voltage. The results are in agreement with calculations from a numerical model.