The spectral dependency of the photophobic step-down reaction is investigated with 8 different wavelengths between 400 and 670 nm through photoaccumulation in light traps. A "living" action spectrum is compiled for Navicula and Nitzschia, and, through a comparison between this and the absorption spectrum, it is shown how the photoreceptors responsible for the step-down reaction can be determined. Time-lapse, comuter animation.