AlGaN-based ultraviolet light detectors with high responsivities in a narrow range of photon energy (approximate to 0.3 eV) were grown by plasma induced molecular beam epitaxy on sapphire substrates. A thin Al0.4Ga0.6N film acting as an optical filter blocking high energy light was separated by an insulating AlGaN barrier from a third light sensitive Al0.3Ga0.7N layer. By optimizing the alloy compositions in the heterostructure and thickness of the filter layer, a peak responsivity of up to 35 A/W was achieved over a narrow range of wavelength with a peak position at 305 nm. The rejection of visible light response with respect to the peak responsivity was 5 orders of magnitude for a photoconductor device with a filter thickness of 1.4 mu m. Decay times of the photoresponse after excimer laser exposure were determined to be between 40 and 330 ns.