  In this paper, we discuss the problem of determination of light radiation
pressure force upon an anisotropic surface. The optical parameters of such a
surface are considered to have major and minor axes, so the model is called an
orthotropic model. We derive the equations for force components from emission,
absorption, and reflection, utilizing a modified Maxwell's specular-diffuse
model. The proposed model can be used to model a flat solar sail with wrinkles.
By performing Bayesian analysis for example of a wrinkled surface, we show that
there are cases in which an orthotropic model of the optical parameters of a
surface may be more accurate than an isotropic model.
