Suspended particle devices (SPDs) are able to rapidly switch from a dark bluish-black state to a clear greyish appearance when an AC electric field is applied. Two-flux and four-flux models were used to derive refractive indices and extinction coefficients, as well as scattering and absorption coefficients, of the particle-containing active layer. These entities were used in model calculations to predict direct, total and diffuse components of transmittance and reflectance, along with color appearance and haze, as a function of the thickness of the active layer. An optimum thickness for optical contrast of the SPD was determined in this way and was found to be in the range of 200-300 nm. The devices exhibit significant haze particularly in reflection. (C) 2015 Elsevier B.V. All rights reserved.