In this work, a dual-beam measurement technique ir presented to measure the short-time-scale optical properties of novel materials. It ir shown that the picosecond continuum probe source obtained from nonlinear interaction with H2O/D2O is highly variable and does not permit detection of small changes in absorption/transmission spectra. The dual-beam approach is proposed to allow simultaneous measurement of a sample's properties in both the excited and steady states. As an example, the technique is applied to the picosecond transmission spectrum of light-emitting porous silicon. Photo-induced absorption is observed at a level that could not be detected without the new technique.