  A quantitative analysis of the excitonic luminescence efficiency in hexagonal
boron nitride (hBN) is carried out by cathodoluminescence in the ultraviolet
range and compared with zinc oxide and diamond single crystals. A high quantum
yield value of ~50% is found for hBN at 10 K comparable to that of direct
bandgap semiconductors. This bright luminescence at 215 nm remains stable up to
room temperature, evidencing the strongly bound character of excitons in bulk
hBN. Ab initio calculations of the exciton dispersion confirm the indirect
nature of the lowest-energy exciton whose binding energy is found equal to 300
meV, in agreement with the thermal stability observed in luminescence. The
direct exciton is found at a higher energy but very close to the indirect one,
which solves the long debated Stokes shift in bulk hBN.
