Three different optical diagnostic techniques have been used to measure the density of boron atoms in a microwave generated H2/B2H6 plasma of a diamond deposition reactor. These techniques are: optical emission intensity ratio of doublet 249.677 and 249.772 nm boron lines (OEIRD), laser induced fluorescence on 208.96 nm line (LIF) and resonance absorption on 249.772 nm line (RA) with a boron hollow cathode lamp (HCL) as light source. LIF results point to an important variation of the boron atom density with the distance from the substrate, in contradiction with OEIRD which indicates an almost constant density. RA measurements show a stronger absorption when the probe light from the HCL crosses the reactor outside the bright plasma core region than when it goes through the plasma core. This indicates a larger boron atom density in the volume surrounding the plasma core than inside it. As the plasma is not homogeneous, in the sense that the plasma induced emission is confined inside the plasma core but the ground state boron atoms are also present outside this region, the OEIRD method cannot be applied to this plasma. These experimental results question the simulation results of H2/B2H6 microwave plasma which predicted a fast decay of the boron atom density outside the plasma core.