Photo-induced nanocomposites are initially transparent polymeric materials having specially designed precursor molecules. UV irradiation leads to the precursor destruction followed by the growth of nanoparticles within the initially homogeneous media. We demonstrate that combining an UV LED and a white LED is a very effective tool for both the UV induced formation of nanoparticles within a polymer film and in situ monitoring of the evolution of the absorbance spectra, which is indicative of the nanoparticle growth.