  We show that the leading-power $B$ meson wave function can be extracted
reliably from the photon energy spectrum of the $B\to\gamma l\nu$ decay up to
$O(1/m_b^2)$ and $O(\alpha_s^2)$ uncertainty, $m_b$ being the $b$ quark mass
and $\alpha_s$ the strong coupling constant. The $O(1/m_b)$ corrections from
heavy-quark expansion can be absorbed into a redefined leading-power $B$ meson
wave function. The two-parton $O(1/m_b)$ corrections cancel exactly, and the
three-parton $B$ meson wave functions turn out to contribute at $O(1/m_b^2)$.
The constructive long-distance contribution through the $B\to V\to\gamma$
transition, $V$ being a vector meson, almost cancels the destructive
$O(\alpha_s)$ radiative correction. Using models of the leading-power $B$ meson
wave function available in the literature, we obtain the photon energy spectrum
in the perturbative QCD framework, which is then compared with those from other
approaches.
