The investigation of the streamer discharge mechanism by means of a pulse shape analyzing technique and a time-resolved spectroscopy of the escaping light in the 360 to 550 nm range, points to the stability limit of this gas detector mode. Radiative lifetimes are studied for various operation voltages and gas mixtures. Together with limited spatial resolution information obtained by cathode strips, the streamer mode is identified as a fairly localized discharge mode in space and time.