Patent ID: 7157867

Claim:
An operating method for acoustic resonant operation of a metal halide lamp using a radio frequency carrier frequency which is frequency-modulated (FM) by means of a sweep signal and which is amplitude-modulated (AM) at the same time, with a fundamental frequency first of all being set for the AM signal, which is derived from an i th , preferably the second, longitudinal resonance, wherein at least the following steps are carried out successively after starting of the lamp and after waiting for a stabilization time: a) Preparation step: during this step, a first resonance of the impedance value (Z) of the lamp, that is to say (ZEXT 1 ), is first of all determined theoretically or experimentally during this step with a very low AM level (AMI) of 5 to 15% in such a way that a large frequency interval is covered for (f(AM)) which will certainly include an extreme; a frequency point (FM 1 ) associated with (ZEXT 1 ) is used to define a measurement interval (MI) which has a width of 8 to 25 kHz with cut-off points (ST) (upper cut-off frequency) and (SM) (lower cut-off frequency), and in which (FM 1 ) is arranged such that (FM 1 ) is located at approximately 55 to 80%, preferably 60 to 70%, of the interval between (SM) and (ST), where (FM 1 ) must always be closer to (ST); b) Step 1: the measurement interval (MI) is passed through during this step; a family of characteristics is thus determined which is obtained by passing through different AM levels, which are changed in steps, in a maximum range (AMI)=5 to 45%, in particular 10 to 40%, with the measurement interval (MI) in each case always being passed through from top to bottom, that is to say from (SM) to (ST); c) Step 2: determination derived from the family of characteristics, of an optimum AM level (AMI)=(AMopt) by comparison of a change in the lamp impedance as a function of the frequency for various AM levels; determination of a maximum of a change in the lamp impedance for an associated value of (AMI) equal to (AMmd); definition of (AMopt) by the rule (AMopt)=(AMmd)+(W) where (W)=3 to 10%; d) Step 3: search for a frequency (FMmin) associated with (AMopt); e) Step 4: definition of the operating frequency (FMopt) by passing through a frequency interval (OPT) between (FMmin) and a value which is 20% of (FM 2 ) away in the interval between (FMmin) and a lower frequency end point (FM 2 ).