  When a laser beam passes through a rotating ground glass (RGG), the scattered
light exhibits thermal statistics. This is extensively used in speckle imaging.
This scattering process has not been addressed in photon picture and is
especially relevant if non-classical light is scattered by the RGG. We develop
the photon picture for the scattering process using the Bose statistics for
distributing $N$ photons in $M$ pixels. We obtain analytical form for the
P-distribution of the output field in terms of the P-distribution of the input
field. In particular we obtain a general relation for the $n$-th order
correlation function of the scattered light, i.e., $g_{\text{out}}^{(n)}\simeq
n!\,g_{\text{in}}^{(n)}$, which holds for any order-$n$ and for arbitrary input
states. This result immediately recovers the classical transformation of
coherent light to pseudo-thermal light by RGG.
