The background energy content of a linear cw CPM rhodamine 6G-DODCI femtosecond dye laser is determined by time- resolved measurement of the Rayleigh scattered signal in a colloidal silicon dioxid suspension with an ungated inverse time- correlated single photon counting system. A constant background intensity signal is observed outside the temporal region of the instrument response function of the detection system. Assuming the same background intensity level within the temporal region of the instrument response function, an average background intensity level of approximately 2.5 × 10-8 of the peak pulse intensity, and a background energy content of approximately 2 × 10-3 of the pulse energy are found.