with ase detection in accord with a preferred embodiment includes electronics for temporally discriminating all but the leading edge of a detected portion of the main beam ase photons are generated by the discharge and not by stimulated emission facilitated by the presence of other photons thus the ase photons are the first ones to appear from the discharge chamber after the discharge as such the leading edge which substantially contains the ase signal portion of the detected portion is separated from the entire signal which may then be analyzed in accord with the present invention a fourth apparatus for use with ase detection in accord with a preferred embodiment is illustrated at fig2 c and includes an excimer or molecular fluorine laser having a shutter 20 for blocking the resonator the resonator may be blocked e g using a shutter or other mechanism placed inside the resonator between the discharge chamber 1 and one of the resonator reflectors 2 3 such as the outcoupling mirror 3 and or a reflecting surface of the rear optics module 2 a large portion of the stimulated emission signal i e that arising from photons reflecting back into the discharge chamber from the resonator reflectors 2 or 3 that are blocked by the shutter s 20 and otherwise appearing at the detector 10 is thus avoided while practically none of the ase signal is hampered only one of the resonator reflectors need be blocked to avoid stimulated.