We evaluate the most important tree-level contributions connected with the
b-> u \bar{u} d gamma transition to the inclusive radiative decay \bar{B}-> X_d
gamma using fragmentation functions. In this framework the singularities
arising from collinear photon emission from the light quarks (u, \bar{u} and d)
can be absorbed into the (bare) quark-to-photon fragmentation function. We use
as input the fragmentation function extracted by the ALEPH group from the
two-jet cross section measured at LEP, where one of the jets is required to
contain a photon. To get the quark-to-photon fragmentation function at the
fragmentation scale \mu_F \sim m_b, we use the evolution equation, which we
solve numerically. We then calculate the (integrated) photon energy spectrum
for b-> u \bar{u} d gamma related to the operators P^u_{1,2}. For comparison,
we also give the corresponding results when using nonzero (constituent) masses
for the light quarks.Comment: 13 pages, 4 figure
