A polychromatic irradiation system for the outdoor exposure of plants or other samples to additional UV and longer-wavelength radiation has been constructed and tested. The system provides a range of wavebands in the ultraviolet-B radiation (280–315 nm) and ultraviolet-A radiation (315–400 nm) spectral regions, and the irradiance is fully adjustable. The performance of the lamp and filter system after over 4000 h of use is described; a brief description of filter degradation after over 30 000 h of use is also presented. Cellulose acetate filters were found to be adequate for some purposes in the UV waveband, but rigid plastic and glass-based filters were superior in terms of stability and the range of spectral combinations available. They also exhibited relatively small changes in transmittance with time. The system is also convenient for the measurement of differential polychromatic action spectra.