Blue glass lens elements used as IR cut filter in a camera design and the impact of inner quality onto lens performance

摘要

Every digital camera has an IR cut filter to adapt the spectral sensivity of the sensor to that of the human eye. The trend to backside illuminated CMOS chips enable higher "light gathering", especially under higher angles of incidence (field of view). Due to the virtually angle-independent transmittance characteristics of absorptive filter glasses (e.g. IR blocking so-called "blue glass"), blue glass is more and more used. Pure interference filters have a high angle dependency and in addition reflect the IR light which can cause ghost images. A typical design of a smart phone camera will be used to design and analyze the quality of a blue glass absorption filter. Blue glass as plano-plano filter plate in front of a CMOS chip will be examined. Furthermore, a lens made out of blue glass (substituting the plano-plano filter) will be designed and analyzed. It turns out that the blue glass lens can be used as a crown glass in an achromatic lens. The required transmittance (filter) curve will be elaborated. Such a blue glass lens can shrink down the size of the digital camera significantly. The blue glass lens needs to have a certain inner glass quality, e.g. striae, and thus the effects of wavefront distortion due to inner glass quality will be investigated. As a result striae of blue glass used lens or as plano-plano filter plate needs to be at a certain level. The blue glass lens has tighter restrictions on striae. For both cases a recommendation of inner glass quality level in terms of wavefront distortion will be given.