Compensating pressure variation using automated optical design environmental modeling

The operating environment directly impacts the optical performance of imaging systems. Temperature variation introduces a well-known focus shift and imparts aberrations due to material expansion and changes in refractive index. There are documented solutions to athermalize a system to counteract these changes. A more subtle effect, which can be equally pernicious in high-performance systems, is a variation in pressure. For example, a system may be intended for use at different elevations, which differ in pressure from the nominal optical design at sea level. Pressure variation alters the refractive index of the gas between lens elements, which affects the index differential at all refractive surfaces. Unlike athermalization, ways of counteracting pressure variation (“apressurization”) are less prevalent in the literature. In this work, different means of apressurization are surveyed. Particular focus is paid to using a controlled change in temperature to compensate for the change in pressure. Importantly, the effect of thermal change on an imaging system is different from the effect of pressure change. In order to accurately simulate these coupled environmental effects, automated environmental modeling approaches are used. The automated process simultaneously models different environments for a given system, affecting the refractive indices, element spacings, and surface figures as a function of temperature and pressure. The simulation also accounts for how the elements are mechanically mounted as well as the housing thermal expansion, offering additional degrees of freedom for apressurization. From this framework, an apressurization scheme is demonstrated where the compensating temperature is optimized. Aberration theory is also developed that explains how different aberrations change with environmental conditions.