Effects and prediction of stray light produced by diffractive lenses

Correlation between computational models and corresponding experimental data of stray light produced by unwanted diffraction orders of diffractive lenses is described. It is shown that the attractive scalar model under Fresnel approximation does not agree with experimental data. The validity of Fresnel approximation for multi-order focusing optics is derived to ex[plain the limitation of the scalar approach. Geometrical models with both coherent and incoherent summation of each diffraction orders coupled with efficiency estimation is used with success. The model takes into account the local diffraction efficiency and also of the number of diffraction orders supported locally by the structure. This geometrical optics model can be used in standard optical design software. Therefore it may be recommended as an additional tool for stray light analysis at the design stage of hybrid refractive/diffractive optical systems. In a second part, the impact of unwanted diffraction orders when diffractive optics is used in visible imaging system is discussed. Several experimental observations about the use of plastic/diffractive lens in relation to different fabrication process used to build the lens are also discussed. From those observations, we recommend an approach to reduce the stray light produced by the diffractive lens. In conclusion, a serious stray light analysis is mandatory in the design of visible imaging systems using diffractive lenses.