A digital light-flux ratiometer is described in which two beams to be measured are detected by a couple of silicon solar cells. The light signals can then be converted linearly into corresponding electronic signals. By adopting analog-to-digital technique, the data can either be displayed directly or recorded during the measurement. At a ratio of 1:15000 between the minimum and maximum detectable light intensity, the nonlinear deviation of opto-electro conversion is found to be less than 0.05% whereas the resolution is 0.1%.