A biased optical probe (BOP) method, i.e. novel technique for measurements of the electron energy distribution function (EEDF) in a range of such high energies as 10-50 eV has been developed. This method is based on the optical emission induced by electrons passing through a negatively biased mesh in a plasma. The BOP method was used to measure the EEDF in a DC glow discharge in He/Xe mixture while the low-energy part (<10 eV) of the EEDF was measured using the conventional Druyvesteyn method. The measured distribution function considerably deviates from an ideal Maxwellian distribution, because of abundant high-energy electrons generated in a cathode fall. On the contrary, the EEDF measured in a low-pressure inductive RF discharge was found to follow a single Maxwellian distribution over a wide energy range.