The earlier developed double-modulation chlorphyll (Chl) fluorometer was modified for measurements with intact leaves of higher plants. The Chl fluorometer is based on a non-periodic modulation of both actinic and measuring flashes. In addition, continuous orange actinic and far-red radiation were produced by separate arrays of light-emitting diodes (LEDs). Programmable timing of the flashes allows to cover a wide dynamic range from microseconds to minutes. We have demonstrated that the LEDs can produce single-turnover flashes that saturate QA reduction of intact leaves of Glyceria maxima and shoots of Picea abies. and O. Urban ... [et al.].