Quantitative evaluation of refractive index inhomogeneity and its effect on the imaging performance of high-precision injection molded optical lens

Abstract

High-precision injection molded plastic lenses are gradually substituting the application of glass lenses in advanced imaging and illumination optical systems for their high efficiency and reproducibility. Despite the surface deviation of the plastic lenses having been reduced to micron dimension, the imaging performance of the systems could not be guaranteed. More and more investigations demonstrated that the inhomogeneity of refractive index (RI) variation of plastic lenses was much higher than that of glass lenses. However, the effect of refractive index inhomogeneity on imaging performance is still not well understood, and is not fully considered in the optical design and injection molding process. Thus, an integrated analytical approach was proposed to quantitatively evaluate the inhomogeneity of the refractive index and to reveal its effect on the imaging performance of the optical system. It was found that the inhomogeneity reached 3.44 × 10−3, which was three orders of magnitude higher than that of the glass blank (2.80 × 10−6). Both the magnitude and distribution of refractive index variation significantly degraded the quality and homogeneity of the modulation transfer function (MTF) and spot diagram, from calculated imaging performance considering the inhomogeneity. The maximum MTF was reduced by 60 % compared with the design value. Comparative investigation also demonstrated that refractive index inhomogeneity had an equivalent effect on the imaging performance in comparison with the micron order surface deviation. Reducing the degradation of refractive index inhomogeneity provides an efficient way to improve the imaging performance of advanced optical systems.