Cyanobacteria provide an efficient system for producing H-2 from water using solar energy. The energy conversion efficiency can be defined by the ratio of H-2 produced to the light energy absorbed. An IR and opalescent plate method was used to measure the light energy absorbed. Since cyanobacteria absorb light in the visible range but not in the infrared range, the net amount of light energy absorbed by the cells can be estimated by measuring the fR and visible light intensities transmitted through the biochamber. A rectangular biochamber was used for measuring the conversion efficiency from light energy to H2 energy. A quantum meter and radiometer were used to measure the light intensity transmitted through the chamber. Anabaena variabilis was cultured in a BG11 medium with 3.6 mM NaNO3 and the light intensity was 40-50 mu mol/m(2)/s in the growth phase and 120-140 mu mol/m(2)/s in the H-2 production phase. The maximum H-2 production was 50 ml for 40 h and cell density was 1.2g/l. The H-2 production rate was 4.1 ml H-2/g dry cell weight/h. Based on the light absorbed in the H-2 production phase, the energy conversion efficiency from light to H-2 was 1.5% on average and 3.9% at the maximum. Based on the light energy absorbed in the cell growth and H-2 production phases, the energy conversion efficiency was 1.1% on average. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.