Components from the filters described by Klein [Carolina Tips, 27: 25-26] were used in designing simple monochromatic filter sources, adjusted so that equal intensities (20, 10, 5, 2, 1, 0.5, 0.2, and 0.1 [mu]w[center dot]cm-2) can be obtained under blue (400-525 nm), green (500-610 nm), red (600-715 nm), and far-red (690-850 nm) filters. This system is useful in plant physiology studies.