  We performed light optical diffraction experiments on a nanoparticle-polymer
volume holographic grating in an angular range including also far off-Bragg
replay. A comparison of three diffraction theories - on the same level of
complexity - with our experimental results shows that the dynamical theory of
diffraction and the first-order two-wave coupling theory using the beta-value
method fit the data very well. In contrast, the prevalent two-wave coupling
theory using the K-vector closure method yields a poor fit with an order of
magnitude worse mean squared error. These findings must be considered for
accurate determination of coupling strength and grating thickness.
