“…For an off-axis HOE, where the off-axis angle, Or, is relatively low, it is possible to obtain an approximate solution by simply adding a linear term to the on-axis design to get where a, = sin 0,. Finally it is possible to obtain better approximations for a general two-dimensional grating function, but at the price of added complexity [17]. Note that for a unique solution for a two-dimensional grating function, the condition of V, X K h = 0 must be fulfilled, where the gradient V, denotes (d/dx>P + ( d / d y ) J .…”